1.1 A Brief History of Measurement System

Weights and measures are among the earliest tools invented by man. Man understandably turned first to parts of his body and his natural surroundings  measuring instruments. Early Babylonian and Egyptian records, and the Bible, indicate that length was first measured with the forearm, hand, or finger and time was measured by the periods of the sun, moon, and other heavenly bodies.

The ancient English system “digit”, “palm”, “span” and “cubic” units of length slowly lost preference to the length units “inch”, “foot” and “yard”.

Roman contributions include the use of 12 as a base number (the foot is divided into 12 inches) and the words from which we derive many of our present measurement unit names. For example, the 12 divisions of the Roman “pes” or foot were called unciae. Our words “inch” and “ounce” are both derived from that Latin word.

The “yard” as a measure of length can be traced back to early Saxon kings. They wore a sash or girdle around the waist that could be removed and used as a convenient measuring device. The word “yard” comes from the Saxon word “gird” meaning the circumference of a person’s waist.

Tradition holds that King Henry I decreed that a yard should be the distance from the tip of his nose to the end of his outstretched thumb. The length of a furlong (or furrow-long) was established by early Tudor rulers as 220 yards. This led Queen Elizabeth I to declare in the 16th century, that henceforth the traditional Roman mile of 5000 feet would be replaced by one of 5280 feet, making the mile exactly eight furlongs and providing a convenient relationship between the furlong and the mile.

The need for a single worldwide coordinated measurement system was recognized over 300 years ago.

In 1790, the Commission appointed by the French Academy created a system that was, at once, simple and scientific. The unit of length was to be a portion of the Earth’s circumference. Measures for capacity (volume) and mass were to be derived from the unit of length, thus relating the basic units of the system to each other and to nature. Furthermore, larger and smaller multiples of each unit were to be created by multiplying or dividing the basic units by 10 and its powers. This feature provided a great convenience to users of the system, by eliminating the need for such calculations as dividing by 16 (to convert ounces to pounds) or by 12 (to convert inches to feet). Similar calculations in the metric system could be performed simply by shifting the decimal point Thus, the metric system is a “base-10” or “decimal” system.

The initial metric unit of mass, the “gram,” was defined as the mass of one cubic centimeter (a cube that is 0.01 meter on each side) of water at its temperature of maximum density. The cubic decimeter (a cube 0. 1 meter on each side) was chosen as the unit for capacity. The fluid volume measurement for the cubic decimeter was given the name “liter.”

By 1900 a total of 35 nations – including the major nations of continental Europe and most of South America – had officially accepted the metric system.

In 1960, the General Conference on Weights and Measures, the diplomatic organization made up of the signatory nations to the Meter Convention, adopted an extensive revision and simplification of the system. Seven units — the meter (for length), the kilogram (for mass), the second (for time), the ampere (for electric current), the Kelvin (for thermodynamic temperature), the mole (for amount of substance), and the candela (for luminous intensity) were established as the base units for the system. The name System International d’unites (International System of Units), with the international abbreviation SI, was adopted for this modern metric system.

The Bangladesh Government has promulgated an ordinance named “The Standards of Weights and Measures Ordinance, 1982” (Ordinance No. XII of 1982) and under this Ordinance a rules named “The Bangladesh Standards of Weights and Measures Rules, 1982” and empowered the authority to Bangladesh Standards and Testing Institution (BSTI) to introduction and implementation of Metric System in Bangladesh.

1.2  Methodology

Collection of data from Bangladesh Standards and Testing Institution (BSTI).

Collection of information from books and internet.

Visit of Temperature and Mass Measurement Laboratory of National Metrology Laboratory (NML), BSTI.

Calibration of Electronic Balance, Standard Platinum Resistance Thermometer (SPRT) and Thermocouples.

2.1  Bangladesh Standards and Testing Institution (BSTI)

Bangladesh Standards and Testing Institute came into being in 1985 through an Ordinance (Ordinance XXXVII of 1985) with the merger of Bangladesh Standards Institution and the Central Testing Laboratories. The BSTI is a body corporate and its administrative Ministry is the Ministry of Industries. BSTI Council is the supreme policy making body which consist of 33 members.

2.2  Functions of BSTI

BSTI is entrusted with the responsibility of formulation of National Standards of industrial, food and chemical products. Quality control of these products is ensure as per specific national standards made by the technical committees. BSTI is also responsible for the introduction & implementation of metric system of weight and measures in the country.

2.3  Wings of BSTI

2.3.1  Standards Wing

Standards wing is responsible for the adoption/ formulation/ revision/ amendment/ reaffirm of standards for different kinds of products, commodities, structures, practices and operation. So standards wing has formulated more than 1900 National Standards of various products & services. Among these 185 international Standards (ISO/IEC/CAC etc.) have been adopted as National Standards.

In preparing National Standards, the Standards wing is assisted by 6 (six) Divisional Committees & 71 Technical Committees consisting of eminent Scientists, Engineers, Professors and Experts in the relevant field.

2.3.2  Certification Marks Wing (CM)

Function and responsibilities of this wing includes

  • Promotion of quality control.
  • Ensuring compliance of products with the Bangladesh Standards.
  • Implementation of Bangladesh Standards through the administration of National Certification Marks scheme or inspection of goods or both and
  • Certify quality of commodities, materials, produces, and other things including food items for local consumption, export or import.

2.3.3  Chemical Testing Wing

The functions of Chemical Testing Wing is to perform the task of chemical Testing and analysis of different types of samples received from the following sources:

  • Samples received from govt., semi- govt., and autonomous bodies;
  • Finished products of industries;
  • Raw materials used in the industries;
  • Imported and, exported goods.
  • Samples seized by police department.
  • Samples received from courts in connection with arbitration of cases on disputes of quality.
  • Samples of compulsory items under Certification Marks Scheme.

2.3.4  Physical Testing Wing

The functions of physical testing wing is to perform the task of physical and engineering testing/analysis of different types of samples received from the following sources:

  • Samples received from govt., semi- govt., and autonomous bodies.
  • Finished products of various industries.
  • Raw materials used in the industries.
  • Imported and, exported goods.
  • Samples seized by police department.
  • Sample received from courts in connection with arbitration of cases on disputes of quality.
  • Samples of compulsory items under Certification Marks Scheme.

2.3.5  Metrology Wing

The legal enforcement of accurate weight and measures in industrial and commercial practices to ensure national and international fair trading and consumer protection was enforced by promulgating “The Standards of Weight and Measures Act. 1982 and the Standards of Weight and Measures (Amendment) Act. 2001” by the Government.

Metrology wing is responsible for-

Implementation of Metric system/SI of weights and measures throughout the country.

Maintenance of standards of weights and measures with international traceability.

For verification and calibration of weight and measures and measuring instruments used in industries and commercial transaction.

BSTI has been maintaining secondary reference standards of mass, Length and volume. Calibration of weights and measures used in pharmaceutical industries including other small industries are being done in the central metrology laboratory. Working standards of weights & measures those are being used by BSTI for verification are also calibrated in the laboratory.

2.3.6  Administration Wing

Administration Wing provides the logistic and support services for the technical wings. These include; general services and logistic, accounting, financial management, legal personnel, store & purchases, transports and other matters related to establishment including planning & development.

2.4  Organogram of BSTI

Council (Supreme Policy making Body)

Director-General (Chief Executive)

2.5  List of products brought under compulsory Certification Marks (CM) scheme

Agriculture and Food Products –      (64 items)

Chemical Products –                            (39 items)

Textile Products –                                (11 items)

Electrical & Electronic Products –    (25 items)

Engineering Products –                       (14 items)


Total                                                      153 items

2.6  National Metrology Laboratory (NML) of BSTI

National Metrology Laboratory (NML) of Bangladesh was established in BSTI in 2009 spending Tk. 3200 Lakh under a TA  Project Quality Management System and Conformity Assessment Activity for Bangladesh Quality Support Programme(Post MFA) with the financial and technical support EU, UNIDO and NORAD. The NML had come under operation in 2009. But, formally NML  was inaugurated by hon’ble Minister, Mr. Dilip Baurua MoI and Dr. Kandeh K Yumkela on 6th June 2010.

2.6.1  Main Function of NML

  • It is the primary metrology laboratory; as such it develops national measurement standards and disseminates their exactitude to industry and users in the country,
  • It establishes and maintains the national measurements system, giving technical support to the  network of secondary and tertiary laboratories,
  • It provides traceability to the national system and  through it to the international system,
  • It offers technical support to industry in everything related to measurements, reference materials, calibrations and data to establish traceability of their measurements,
  • It participates in modernization and technology transfer between academia, industry and government, contributing to reinforce the scientific and technical infrastructure required by industry to compete in the present global markets,
  • It supports development of reference standards and the national system of standards,
  • It facilitates international harmonization and compatibility of measurements,
  • It represents the country in the Regional Metrology Organization RMO and the worldwide metrology system coordinated by BIPM,
  • It participates in internationally organized inter-comparison measurements,

Together with the national accreditation body it organizes national inter-comparison measurements for calibration laboratories in the country.

2.6.2  Laboratories of National Metrology Laboratory

  • Mass Measurement Laboratory.
  • Length & Dimension Measurement Laboratory.
  • Temperature Measurement Laboratory.
  • Force and Pressure Measurement Laboratory.
  • Volume, Viscosity and Density Measurement Laboratory.
  • Electrical, Time & Frequency Measurement Laboratory.
  • Hierarchy of Standards

2.7  Important Definitions Related to Measurement System

2.7.1  Verification

Verification, with its grammatical variations and cognate expressions, includes, in relation to any weight or measure, the process of comparing, checking, testing or adjusting such weight or measure with a view to ensuring that such weight or measure conforms to the standards established by or under this Ordinance and also includes, re-verification and calibration.

2.7.2  Calibration

The set of operations that establish, under specific conditions, the relationship between values for quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or a reference material, and corresponding values realized by a standards.

2.7.3  Acclimatization

Acclimatization or acclimation is the process of an individual organism adjusting to a gradual change in its environment, (such as a change in temperature, humidity, photoperiod, or pH) allowing it to maintain performance across a range of environmental conditions

2.7.4  Accuracy and Precision

In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity’s actual (true) value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which repeated measurements under unchanged conditions show the same results.

2.8  Uncertainty

Parameter associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measured.

2.8.1  Measurement Uncertainties

Measurement uncertainties are caused by a number of  factors influencing  a measurement process.

2.8.2  Major factors

  • Accuracy and repeatability of the measuring instrument
  • Environmental conditions, e.g . Temperature, Humidity
  • Operator   errors
  • Computational  errors

2.8.3  Uncertainties due to random effects

Random effects are those that vary continuously e.g. temperature, pressure humidity

2.8.4  Uncertainties due to systematic effects

Systematic effects are relatively constant. e.g zero error of an instrument, resolution of  an instrument

2.8.5  Expanded Uncertainty & Coverage Factor

U  = k .  uc(y)

U- Expanded Uncertainty

uc(y)- Combined Standard Uncertainty

k- Coverage factor, obtained from the t-distribution corresponding to the level of confidence desired (95 %)

2.9  International Cooperation of BSTI

BSTI-NML has maintained close ties with many international organizations and has always been an active part in activities of world metrological community. BSTI-NML is a full member of Asia Pacific Metrology Programmed (APMP), corresponding member of OIML (International Organization of Legal Metrology), associate member of BIPM (International Bureau of Weights and Measures) BSTI also member of International Organization for Standardization (ISO), International Electro-technical Committee (IEC), CODEX, SARSO. BSTI has also signed CIPM-MRA.

The BSTl-NML also maintains sound cooperative relations with NMLs worldwide. Bilateral cooperation activities such as the technical and personnel exchange are prosperous.

2.9.1  Asia Pacific Metrology Programme (APMP)

In 1980, Asia Pacific Metrology Programme (APMP) was established to improve the level of measurement standards in the Asia Pacific region. In the early 80s, APMP mainly focused on supporting developing economies. From the 90s, APMP has the nature of a regional organization with a view to establish the international equivalence in measurement standards.

In November 1999, Japan assumed the Chairpersonship of APMP, and the Secretariat was also set up in NRLM (NMIJ from 2001) to provide with various useful information to the member economies and other international organizations. APMP is playing a major role as one of the largest regional metrology organizations in the world. In October 2004, the Chairpersonship was transferred to New Zealand.  Full Members of APMP

23 countries are the full member of APMP. In favour of Bangladesh, BSTI became full member of APMP in 1977.

Figure 2.3 : Members of APMP

2.9.2  International Organization of Legal Metrology (OIML)

The International Organization of Legal Metrology (OIML) is an intergovernmental treaty organization whose membership includes Member States, countries which participate actively in technical activities, and Corresponding Members, countries which join the OIML as observers. It was established in 1955 (see the Convention) in order to promote the global harmonization of legal metrology procedures. Since that time, the OIML has developed a worldwide technical structure that provides its Members with metrological guidelines for the elaboration of national and regional requirements concerning the manufacture and use of measuring instruments for legal metrology applications.

According to 2007 World Bank figures, OIML Members cover in total an astounding 86 % of the world’s population and 96 % of its economy.

2.9.3  General Conference on Weights and Measures

The General Conference on Weights and Measures is made up of delegates of the governments of the Member States and observers from the Associates of the CGPM.

The General Conference receives the report of the International Committee for Weights and Measures (CIPM) on work accomplished; it discusses and examines the arrangements required to ensure the propagation and improvement of the International System of Units (SI); it endorses the results of new fundamental metrological determinations and various scientific resolutions of international scope; and it decides all major issues concerning the organization and development of the BIPM, including the donation of the BIPM for the next four-year period.

The CGPM currently meets in Paris once every four years; the 23rd meeting of the CGPM was held in November 2007, and the 24th will be held in 2011.

2.9.4  The International Bureau of Weights and Measures (BIPM)

The BIPM was created on 20 May 1875, following the signing of the Metre Convention, a treaty among 51 nations (as of August 2008[update]). It is based at the Pavilion de Britoil in Sevres, France, granted to the Bureau in 1876.

The International Bureau of Weights and Measures (French: Bureau international des poids et mesures), is an international standards organization, one of three such organizations established to maintain the International System of Units (SI) under the terms of the Metre Convention (Convention due Metre). The organization is usually referred to by its French initialize, BIPM.

The other organizations that maintain the SI system, also known by their French initializes are the General Conference on Weights and Measures (French: Conférence générale des poids et mesures) (CGPM) and the International Committee for Weights and Measures (French: Comité international des poids et mesures) (CIPM).  Member States and Associates

As of 24 May 2011, there are 55 member states of the BIPM, and 33 associate states and economies of the General Conference. Bangladesh is the associate member of BIPM.

2.9.5  International Organization for Standardization (ISO)

ISO is the world’s largest developer and publisher of International Standards.

ISO is a network of the national standards institutes of 162 countries, one member per country, with a Central Secretariat in Geneva, Switzerland, that coordinates the system.

ISO is a non-governmental organization that forms a bridge between the public and private sectors. On the one hand, many of its member institutes are part of the governmental structure of their countries, or are mandated by their government. On the other hand, other members have their roots uniquely in the private sector, having been set up by national partnerships of industry associations.

Therefore, ISO enables a consensus to be reached on solutions that meet both the requirements of business and the broader needs of society. ISO is responsible for formulation of International Standards of different products, raw materials, services etc.

There are 162 members which are divided into three categories:

  1. Member bodies.
  2. Correspondent members.
  3. Subscriber members.

BSTI is the full member body in favor of Bangladesh.

3.1  Bangladesh Accreditation Board (BAB)

Bangladesh Accreditation Board (BAB) is the national authority with responsibility of the accreditation in Bangladesh. It offers accreditation programs for various types of conformity assessment bodies, such as laboratories, certification bodies, inspection bodies, training institutions or persons in accordance with the relevant International Organization for Standardization (ISO), International Electro technical Commission (IEC), and other regulatory standards and national standards.

BAB is the statutory body established in 2006 as an autonomous organization responsible for upgrading the quality assurance infrastructure and conformity assessment procedures in Bangladesh and enhancing the recognition and acceptance of products and services in international, regional and domestic markets.

3.1.1  Functions

  • Accreditation of Testing & Calibration and Medical Laboratories accrediting to ISO/IEC 17025, ISO 15189
  • Accreditation of Certification Bodies IS0/IEC 17021, IS0/IEC 17024, ISO/IEC 17065
  • Accreditation of Inspection Bodies ISO/IEC 17020
  • Establishing MRA and MLA with Regional and International Forums, and cooperate with relevant national, regional and international organizations in accreditation.
  • Arrange Training Programs, seminar-symposium, and Proficiency Testing
  • Harmonization of Standards & Requirements and Exchange of Information

3.1.2  Roles

  • To identify centers of competence in all areas of conformity assessment, minimize unnecessary duplication and gives users confidence in the results of that conformity assessment.
  • To make arrangements at the national level to increase the acceptance of nationally manufactured products, to promote product safety and efficiency and to ensure product/service quality.
  • To improve the competitiveness of products and services.
  • To enhance the protection of consumers, manufactures and the broad community in terms of security, safety, health and environment.
  • To formulate its criteria, standards, policies and practices, based on inputs from relevant academia and institutions.

3.1.3  BAB Organizational Chart


3.2  Asia Pacific Laboratory Accreditation Cooperation (APLAC)

APLAC is a cooperation of accreditation bodies in the Asia Pacific region that accredit laboratories, inspection bodies and reference material producers.  It is recognized by the Asia Pacific Economic Cooperation (APEC) as one of five Specialist Regional Bodies (SRBs).

3.2.1  International Laboratory Accreditation Cooperation (ILAC)

ILAC is an international cooperation of laboratory and inspection accreditation bodies formed more than 30 years ago to help remove technical barriers to trade. Accreditation bodies around the world, which have been evaluated by peers as competent, have signed an arrangement that enhances the acceptance of products and services across national borders. The purpose of this arrangement, the ILAC Arrangement, is to create an international framework to support international trade through the removal of technical barriers.

ILAC counts as its members laboratory and inspection accreditation bodies representing more than 70 economies and regional organizations

The ultimate aim of the ILAC Arrangement is the increased use and acceptance by industry as well as regulators of the results from accredited laboratories and inspection bodies, including results from laboratories in other countries. In this way, the free-trade goal of  ‘product tested once and accepted everywhere’ can be realized.

4.1  Measurement System

4.2  Metrology may be put into following Four  Categories

4.2.1  Scientific Metrology:  development of primary measurement standards and their maintenance (highest level).

4.2.2 Industrial Metrology: proper maintenance and control of industrial measurement equipment including calibration of instruments, measurement standards and production and testing processes.

4.2.3  Legal Metrology: verification of instruments used in commercial transactions, according to criteria defined in technical regulations.

4.2.4 Chemical Metrology:  Metrology in Chemistry, commonly known as Chemical Metrology, is the science concerned with studying and providing the basis for comparability of chemical measurements and their traceability.

4.3  Base Units of System International (SI)

4.3.1  SI Base Units

The International System of Units (SI) defines seven units of measure as a basic set from which all other SI units are derived. These SI base units and their physical quantities are:

  • meter for length
  • kilogram for mass
  • second for time
  • ampere for electric current
  • Kelvin for temperature
  • candela for luminous intensity
  • mole for the amount of substance.

The SI base quantities form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology.  However, in a given realization of these units they may well be interdependent, i.e. defined in terms of each other:

The names of all SI units are written in lowercase characters (e.g., the meter has the symbol m), except that the symbols of units named after persons are written with an initial capital letter (e.g., the ampere has the uppercase symbol A).

Table 4.1 :  SI base units


Name Symbol Measure Definition Historical origin/ justification
meter m length “The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second.” 1⁄10,000,000 of the distance from the Earth’s equator to the North Pole measured on the circumference through Paris.
kilogram kg mass “The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.” The mass of one liter of water. A liter is one thousandth of a cubic meter.
second s time “The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.” The day is divided in 24 hours, each hour divided in 60 minutes, each minute divided in 60 seconds.
A second is 1⁄(24 × 60 × 60) of the day
ampere A electric current “The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10−7 Newton per meter of length.” The original “International Ampere” was defined electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of silver nitrate. Compared to the SI ampere, the difference is 0.015%.
Kelvin K thermodynamic temperature “The Kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.” The Celsius scale: the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0 K is absolute zero).
mole mol amount of substance The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is “mol.” Atomic weight or molecular weight divided by the molar mass constant, 1 g/mol.
candela cd luminous intensity “The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.” The candlepower, which is based on the light emitted from a burning candle of standard properties.

4.3.2  Symbols of SI Base Units at a Glance

4.3.3  SI Derived Units

Other quantities, called derived quantities, are defined in terms of the seven base quantities via a system of quantity equations. The SI derived units for these derived quantities are obtained from these equations and the seven SI base units. Examples of such SI derived units are given in Table 4.2, where it should be noted that the symbol 1 for quantities of dimension 1 such as mass fraction is generally omitted.

Table 4.2 :  Examples of SI derived units


SI derived unit


Derived quantity Name Symbol
area square meter m2
volume cubic meter m3
speed, velocity meter per second m/s
acceleration meter per second squared m/s2
wave number reciprocal meter m-1
mass density kilogram per cubic meter kg/m3
specific volume cubic meter per kilogram m3/kg
current density ampere per square meter A/m2
magnetic field strength ampere per meter A/m
amount-of-substance concentration mole per cubic meter mol/m3
luminance candela per square meter cd/m2
mass fraction kilogram per kilogram, which may be represented by the number 1 kg/kg = 1
volume cubic meter m3

For ease of understanding and convenience, 22 SI derived units have been given special names and symbols, as shown in Table 4.3.

Table 4.3 :  SI derived units with special names and symbols


Name Symbol Expression
in terms of
other SI units
in terms of
SI base units
plane angle radian (a) rad   – m·m-1 = 1 (b)
solid angle steradian (a) sr (c)   – m2·m-2 = 1 (b)
frequency hertz Hz   – s-1
force newton N   – m·kg·s-2
pressure, stress pascal Pa N/m2 m-1·kg·s-2
energy, work, quantity of heat joule J N·m m2·kg·s-2
power, radiant flux watt W J/s m2·kg·s-3
electric charge, quantity of electricity coulomb C   – s·A
electric potential difference,
electromotive force
volt V W/A m2·kg·s-3·A-1
capacitance farad F C/V m-2·kg-1·s4·A2
electric resistance ohm V/A m2·kg·s-3·A-2
electric conductance siemens S A/V m-2·kg-1·s3·A2
magnetic flux weber Wb V·s m2·kg·s-2·A-1
magnetic flux density tesla T Wb/m2 kg·s-2·A-1
inductance henry H Wb/A m2·kg·s-2·A-2
Celsius temperature degree Celsius °C   – K
luminous flux lumen lm cd·sr (c) m2·m-2·cd = cd
illuminance lux lx lm/m2 m2·m-4·cd = m-2·cd
activity (of a radionuclide) becquerel Bq   – s-1

absorbed dose, specific energy (imparted), kerma









dose equivalent (d) sievert Sv J/kg m2·s-2
catalytic activity katal kat s-1·mol
(a) The radian and steradian may be used advantageously in expressions for derived units to distinguish between quantities of a different nature but of the same dimension; some examples are given in Table 4.

(b) In practice, the symbols rad and sr are used where appropriate, but the derived unit “1” is generally omitted.

(c) In photometry, the unit name steradian and the unit symbol sr are usually retained in expressions for derived units.

(d) Other quantities expressed in sieverts are ambient dose equivalent, directional dose equivalent, personal dose equivalent, and organ equivalent dose.


Table 4.4 :  Examples of SI derived units whose names and symbols include SI derived units with special names and symbols


SI derived unit
Derived quantity






dynamic viscosity pascal second Pa·s
moment of force newton meter N·m
surface tension newton per meter N/m
angular velocity radian per second rad/s
angular acceleration radian per second squared rad/s2
heat flux density, irradiance watt per square meter W/m2
heat capacity, entropy joule per kelvin J/K
specific heat capacity, specific entropy joule per kilogram kelvin J/(kg·K)
specific energy joule per kilogram J/kg
thermal conductivity watt per meter kelvin W/(m·K)
energy density joule per cubic meter J/m3
electric field strength volt per meter V/m
electric charge density coulomb per cubic meter C/m3
electric flux density coulomb per square meter C/m2
permittivity farad per meter F/m
permeability henry per meter H/m
molar energy joule per mole J/mol
molar entropy, molar heat capacity joule per mole kelvin J/(mol·K)
exposure (x and rays) coulomb per kilogram C/kg
absorbed dose rate gray per second Gy/s
radiant intensity watt per steradian W/sr
radiance watt per square meter steradian W/(m2·sr)
catalytic (activity) concentration katal per cubic meter kat/m3

4.3.4  SI Prefixes

The 20 SI prefixes used to form decimal multiples and submultiples of SI units are given in Table 4.5.

Table 4.5 :  SI prefixes


Factor Name Symbol Factor Name Symbol
1024 yotta Y 10-1 deci d
1021 zetta Z 10-2 centi c
1018 exa E 10-3 milli m
1015 peta P 10-6 micro µ
1012 tera T 10-9 nano n
109 giga G 10-12 pico p
106 mega M 10-15 femto f
103 kilo k 10-18 atto a
102 hecto h 10-21 zepto z
101 deka da 10-24 yocto y

5.1  Electronic Balance

The modern electronic balance is a deceptively simple device. To use it well and get good results, we must understand how it works, and what precautions we must take when handling samples to get the precision we need for good laboratory practice.

When we press Tare, the balance readout is simply set to zero. When we use Calibrate, a known weight is internally placed on the balance, and the electronics are recalibrated to provide the exact magnetic current to the electromagnet to balance the weight as displayed.

5.2  Working Standards

A weight is a piece of material, usually metal, of known mass and usually of known uncertainty. The OIML definition of a weigh is ‘‘A material measure of mass, regulated in regard to its physical and metrological characteristics: shape, dimensions, material, surface quality, nominal value and maximum permissible error’’.

According to the OIML R111 publication, there are seven classes of weights (Classes: E1, E2, F1, F2, M1, M2 and M3) in tiers of uncertainty, with E1 as the highest class. The uncertainty of calibration and change in mass are assigned to each class and are included in the uncertainty budget of the task. A summary chart appears in the following table:

Table 5.1 :       Maximum Permissible Errors

± δm in mg
Nominal Value Class E1 Class E2 Class F1 Class F2 Class M1 Class M2 Class M3
50 kg 25 75 250 750 2 500 7 500 25 000
20 kg 10 30 100 300 1 000 3 000  10 000
10 kg 5 15 50 150 500 1 500 5 000
5 kg 2.5 7.5 25 75 250 750 2 500
2 kg 1.0 3.0 10 30 100 300 1 000
1 kg 0.5 1.5 5 15 50 150 500
500 g 0.25 0.75 2.5 7.5 25 75 250
200 g 0.10 0.30 1.0 3.0 10 30 100
100 g 0.05 0.15 0.5 1.5 5 15 50
50 g 0.030 0.10 0.30 1.0 3.0 10 30
20 g 0.025 0.080 0.25 0.8 2.5 8 25
10 g 0.020 0.060 0.20 0.6 2 6 20
5 g 0.015 0.050 0.15 0.5 1.5 5 15
2 g 0.012 0.040 0.12 0.4 1.2 4 12
1 g 0.010 0.030 0.10 0.3 1.0 3 10
500 mg 0.008 0.025 0.08 0.25 0.8 2.5
200 mg 0.006 0.020 0.06 0.20 0.6 2.0
100 mg 0.005 0.015 0.05 0.15 0.5 1.5
50 mg 0.004 0.010 0.04 0.12 0.4
20 mg 0.003 0.008 0.03 0.10 0.3
10 mg 0.002 0.006 0.025 0.08 0.25
5 mg 0.002 0.006 0.020 0.06 0.20
2 mg 0.002 0.006 0.020 0.06 0.20
1 mg 0.002 0.006 0.020 0.06 0.20

5.3  Environmental Conditions, Suitability for Calibration

The environmental conditions (air currents, vibrations, stability of the weighing site) shall be suitable for the instrument to be calibrated. In particular, operational disturbances due to contamination or damage must be avoided. The weight values must be unequivocally indicated and indications, where given, shall be easily readable.

The user of the instrument shall be asked to ensure that the customary working conditions prevail during the calibration. In this way the interference effects of air currents, vibrations and inclination of the measuring platform will be inherent to the measured values and will therefore be incorporated in the determined uncertainty.

5.4  Preparation for the Calibration of the Weighing Instrument

All details of the operation manual regarding the setting up, the environmental conditions, the technical specifications (Max, d, linearity and temperature coefficient), as well as the adjustment and calibration process should be taken into account.

The most important thing for the reliable operation of a weighing instrument is a location free from vibrations and draughts. If the required conditions are not fulfilled, the calibration process must not be carried out. Before the calibration of the weighing instrument, the following preliminary checks should be realized:

Visual Check.

Cleaning Check.

  • Functionality Check.
  • Leveling Check.
  • Pre-Load.
  • Adjustment.
  • Temperature.

5.5  Acclimatization Time

Reaching the operating temperature is a prerequisite for every test. Therefore the warm-up time should be looked up in the operating instruction manual.

If details of the warm-up time are missing, choose the right warm-up time within the following table:

Table: 5.2 : Acclimatization Time
Max/d ≥ 1.000.000 At least 12 hours
1.000.000 > Max/d ≥ 300.000 At least 4 hours
300.000 > Max/d ≥ 30.000 At least 2 hours
30.000 > Max/d ≥ 6.000 At least 30 min
6.000 < Max/d At least 10 min

5.6  Off-Centre Loading

The test load is applied at the positions quoted. These positions mark the centre of gravity of the load for the appropriate measurement.

Central measurement

Front left measurement

Back left measurement

Back right measurement

Front right measurement

After the first measurement, tare setting may be done when the instrument is loaded.

For the test load P, 0,3 Max £ P £ Max .

A one-piece test load should preferably be used.

E = the greatest difference between off-centre and central loading indicationsThe variance ve is given by


5.7  Expanded Uncertainty

The expanded uncertainty is given by


5.8  Experimental Data Sheet for Balance Calibration

Name of the instrument: Digital Balance.

Place of calibration: Lab of the customer.

Manufacturer: AND corporation.

Brand: AND.

Maximum capacity: 600 g.

Minimum capacity: 0.01 g.

Readability: 0.01 g.

Drift: 0.01 g.

S/N: 63

Environmental condition :

Temp 260C
R. H. 70%
Air pressure 1012 mbar
Reference weight F1


Preloading 600 g
Indication 599.95 g 599.94 g 599.95 g

Indication Test :

Min 0.01 0.01g
25% 150 149.98
50% 300 299.96
75% 450 449.98
25% 150 149.98
Max 600 599.94


U= ±(0.00931 + 1.39 x 10-04 x M) g

6.1  Thermometer

Thermometers measure temperature, by using materials that change in some way when they are heated or cooled. Modern thermometers are calibrated in standard temperature units such as Fahrenheit or Celsius and Kelvin.

6.2  Resistance Thermometer

Resistance thermometers are usually made using platinum, because of its linear resistance-temperature relationship and its chemical inertness. The platinum detecting wire needs to be kept free of contamination to remain stable. A platinum wire or film is supported on a former in such a way that it gets minimal differential expansion or other strains from its former, yet is reasonably resistant to vibration. RTD (Resistance Temperature Detector) assemblies made from iron or copper are also used in some applications.

Commercial platinum grades are produced which exhibit a temperature coefficient of resistance 0.00385/°C (0.385%/°C) The sensor is usually made to have a resistance of 100 Ω at 0 °C. This is defined in BS EN 60751:1996 (taken from IEC 60751:1995). The American Fundamental Interval is 0.00392/°C, based on using a purer grade of platinum than the European standard. The American standard is from the Scientific Apparatus Manufacturers Association (SAMA), who are no longer in this standards field. As a result the “American standard” is hardly the standard even in the US.

Measurement of resistance requires a small current to be passed through the device under test. This can cause resistive heating, causing significant loss of accuracy if manufacturers’ limits are not respected, or the design does not properly consider the heat path. Mechanical strain on the resistance thermometer can also cause inaccuracy. Lead wire resistance can also be a factor; adopting three- and four-wire, instead of two-wire, connections can eliminate connection lead resistance effects from measurements  three-wire connection is sufficient for most purposes and almost universal industrial practice. Four-wire connections are used for the most precise applications.

6.3  Standard Platinum Resistance Thermometer (SPRT)

SPRT consists of spectrally pure platinum wire which is wound free from mechanical stresses. Resistance thermometers use electrical resistance and require a power source to operate. The resistance ideally varies linearly with temperature. This thermometer can be used up to 6600 C. RTD (Resistance Temperature Detector) assemblies made from iron or copper are also used in some applications. SPRT ensures high accuracy, low drift and wide operating range and it is suitable for precession applications.


6.4  Environmental Conditions

Calibration can be carried out in laboratory. The calibration performed in ambient conditions under normal atmospheric pressure, temperature 23°C ± 5°C and a relative humidity 50% ± 20%.

6.5  Preparation for the Calibration of Thermometers

Calibration of the thermometer is performed before any adjustments take place. The calibrator is operated in the vertical position.

All measurements are to be carried out with the top of the block or bath exposed or insulated, as recommended by the manufacturer and described in the calibration certificate of the temperature block or bath calibrator.

All measurements are to be carried out in such a way that the standard and test thermometers sensors are placed in the homogeneous zone of the temperature calibrator.

6.5.1  Visual Check

Visually examine the thermometers of obvious defects that would affect the accuracy. Furthermore, if the thermometer consists of thermocouples or other probes as sensors look for obvious signs of mechanical defects, contamination, etc. which shall be recorded and the client informed if the laboratory feels that the validity or uncertainty of measurement in the calibration could be impaired.

6.5.2  Initial Zero Point Check

Before the placement of the thermometer under calibration in the temperature bath zero point is  checked in the ice bath.

The low uncertainty of the reference standard thermometers requires a carefully prepared ice bath to maintain the temperature of the reference probe at 0 °C. A properly prepared ice bath will have an expanded uncertainty of 2 mK.

The ice for the ice bath should be finely-crushed or shaved ice that has been prepared from distilled water. The ice should be saturated with distilled water, and then packed gently into an insulating Dewar flask, such that ice fully fills the volume of the flask with no large voids. A cylindrical flask (7 cm inner diameter and 30 cm deep) having a polyethylene-foam cover, 2.5 cm thick, is used. Other flask geometries are allowable, provided the flask is at least 6 cm inner diameter and 30 cm deep, and the thickness of the cover is in the range 1.5 cm to 3 cm. The level of the ice-water mixture should be within 5 mm of the bottom surface of the cover. The cover should have a hole of adequate diameter in the center, to allow insertion of the temperature probe into the ice point. The probe should be inserted until the stop on the probe is butted against the flask cover. Since the ice in the Dewar flask will tend to float as the ice melts, a rubber band should be used to secure the cover onto the flask.

The use of electronic ice-point compensators, extension wires, and automated ice points cooled with thermoelectric modules is not recommended unless a careful analysis of the additional uncertainties is performed. These devices, in general, contribute additional errors to the measurements.

When it is possible use a Triple Point of Water (TPW) cell to maintain the reference thermometer at a temperature of 0.01 °C.

6.6  Calibration Procedure of Standard Platinum Resistance Thermometer

The calibration of the thermometer is achieved by comparison against a standard thermometer when both are placed in a temperature calibrator. Both the standard thermometer and the test thermometer are maintained in an isothermal region so comparisons can be possible.

The temperature calibrator shall have a zone of sufficient temperature homogeneity (referred to as measurement zone). The homogeneous zone will in general be at the lower end of the boring or bath. If the homogeneous zone is situated at another place, this is explicitly stated in the calibration certificate of the temperature calibrator.

Thermometers are calibrated by comparison with standard thermometer, in thermally stabilized temperature calibrators suitable for the calibration by comparison techniques. The standard thermometer and the temperature calibrator shall be traceable to national standards.

6.6.1  Immersion of Thermometers in the Temperature Bath

Immersion of the standard Pt 100 and the thermometer under calibration in the temperature bath is realized as follows:

  • The bath is powered ON.
  • The reference Pt 100 is sunk inside the bath and it is stabilized via stanchion. The immersion depth is the largest possible and it is placed near the side from where the operator takes the measurements.
  • The under calibration thermometer is also sunk inside the bath and is stabilized via stanchion.
  • The two sensors are placed the nearest possible to one each other. The maximum number of thermometers that can be calibrated simultaneously is three (for liquid in glass the maximum number is two).
  • The reference sensor is suitably connected with its digital indicator for the running temperature to be recorded.
  • The two sensors remain immersed for all the duration of measurement.

6.6.2  Measuring Points

The bath is set at the proper temperatures placing its set point at the corresponding values.

The number of temperature points (from 3 to 5) are selected in such a way for to correspond in roughly equal intervals in the measuring range.

In each reference temperature and after the stabilization of the bath, we wait 15 more minutes  in order thermal balance to be restored.

Under stabilization conditions the measurements of temperature from the reference thermometer are recorded (A circle: 10 values with minimum time interval between them 5 sec).

The measurements of temperature from the thermometer under calibration are recorded (B circle: 10 values with minimum time interval between them 5 sec).

The measurements of temperature from the reference thermometer are repeated again and recorded (A circle: 10 values with minimum time interval between them 5 sec).

The mean values of the two measuring cycles from the reference thermometer must not be different more than two times the bath stability (0.02 0C).

Calibration of the thermometer at other measuring points is realized with the same methodology in increasing only sequence.

6.6.3  Final Zero Point Check

After the check of the thermometer in the measuring points check in zero point is repeated as follows:

Afterwards the end of calibration the reference sensory and the thermometer under calibration are maintained at environmental temperature until thermal balance.

The two thermometers are cleaned with distilled or de ionized water.

The two sensors are immersed in the ice bath in the Dewar vessel.

Indications are recorded as in the initial control of zero.

6.7  Evaluation

The values measured in series at increasing temperatures are averaged for each calibration point. The calibration result (deviation of the standard thermometer from the indication of the test thermometer) is documented in the certificate after the necessary corrections.

6.8  Estimation of the Uncertainty of Measurement

The uncertainty to be stated as the uncertainty of the calibration of the thermometer is the measurement uncertainty with which the standard temperature can be stated.

The following contributions to the uncertainty of measurement shall be taken into account.

6.8.1  Reference Temperature

ti + δti-std-mean + δti-cal + δti-drift + δti-immersion + δti-meter-cal + δti-meter-drift + δti-meter-resolution + δtbath-homogeneity + δtbath-stability                                                                                                           (1)

ti: mean value of the reference temperature (of the 20 measurements with the reference thermometer).

δti-std-mean: The standard uncertainty is the standard deviation of the mean value of the 20 measurements with the reference thermometer.

δti-cal: correction from the calibration certificate of the reference temperature sensor. The standard uncertainty is the half value of that expanded uncertainty U(k=2) reported in the calibration certificate.

δti-drift: correction due to the drift of the reference temperature sensor. It is estimated from the calibration history of the sensor. The corresponding standard uncertainty is estimated from rectangular distribution as Δ(ti+1-ti)/2∙√3.

δti-immersion: correction due to the inadequate immersion of the reference thermometer sensor (it is estimated via 20mm erases of the sensor from its position and record of the change of the temperature indication Δt: u(δti-immersion)= Δtimm/(2∙√3).

δti-meter-cal: correction due to the calibration of the indicator of the reference sensor.

δti-meter-drift: correction due to the drift of the indicator of reference temperature.

δti-meter-resolution: correction due to the resolution of the reference temperature. The estimated correction value is zero with corresponding standard uncertainty d/(2√3).

δtbath-homogeneity: correction due to bath in homogeneity. The estimated correction value is zero with corresponding standard uncertainty Δthomog/(2∙√3), where Δthomog is the maximum difference of the single values of the temperature indications taken from the reference thermometer at the two positions where the standard sensor and the under calibration thermometer are situated. Alternatively the bath homogeneity can be estimated through the procedure described in WI-T03.

δtbath-stability: correction due to bath instability. The estimated correction value is zero with corresponding standard uncertainty Δtbath-stability/(2∙√3), where Δtbath-stability is the difference between maximum and minimum temperature indications during 20 min. Alternatively the bath stability can be estimated through the procedure described in WI-T03.

In case where the indicator and sensor have been calibrated as one system the terms δti-cal and δti-meter-cal must be replaced by one term (the same for δti-drift and δti-meter-drift).

The estimated value of most corrections factors is zero with a corresponding however standard uncertainty (with no zero value). The combined uncertainty is estimated via the rule of RSS (Root of Sum of Squares) of all the terms:


The uncertainty budget of each reference temperature is presented in the following table (where Ci is the sensitivity factor and equals to 1 cause the mathematical model is linear to all the influenced parameters):

Table 6.1 : Uncertainty Budget of the Reference Temperature

Term Description Esti-

mated value

Standard uncertainty Type of Distribution Ci Uncerta-

inty contrib-


.ti Mean value of temperature measured with the reference thermometer .{δti-mean .{δti-std-mean normal 1 {(δti-std-mean)2
δti-cal Calibration of the reference sensor Corre-

ction from certifi-


U(2σ)/2 normal 1 (U(2σ)/2)2
δti-drift Drift of the reference sensor 0 Δ(ti+1-ti)/(2∙√3) rectangular 1 (Δ(ti+1-ti)/(2∙√3))2
δti-immersion Immersion depth of reference sensor 0 Δtimm/(2∙√3) rectangular 1 (Δtimm/(2∙√3))2
δti-meter-cal Calibration of indicator normal 1
δti-meter-drift Drift of indicator rectangular 1
δti-meter-resolution Resolution of indicator 0 d/(2∙√3) rectangular 1 (d/(2∙√3))2
δtbath-homogeneity Bath homogeneity 0 Δthomog/(2∙√3) rectangular 1 (Δthomog/(2∙√3))2
δtbath-stability Bath stability 0 Δtbathsta/(2∙√3) rectangular 1 (Δtbathsta/(2∙√3))2
Reference temperature: δti-mean + correction .u(ti)  k= 1 √ (Σuti2)

6.8.2  Temperature of the Thermometer under Calibration

R(ti) + δR(ti)std-mean + δR(ti)immersion +  δR(ti)resolution + δR(ti)hysteresis


R(ti): mean value of the temperature measured with the under calibration         thermometer (of the 10 measurements with the reference thermometer).

δR(ti)std-mean: standard deviation of the mean of the estimation of R(ti)

δR(ti)immersion: correction due to the inadequate immersion of the thermometer under calibration (it is estimated via 20mm erases of the sensor from its position and record of the change of the temperature indication ΔR: u[δR(ti)immersion]=ΔRtimm/2∙√3). It is a common practice to be taken as the half of the standard uncertainty due to resolution: u[δR(ti)immersion]= dR/(2∙2∙√3).

δR(ti)resolution: correction due to the resolution of the thermometer under calibration. The estimated correction value is zero with corresponding standard uncertainty dR/2√3.

δR(ti)hysteresis: correction due to the hysteresis of the thermometer due to its drift during the procedure which is estimated from the difference of the indication of the thermometer in the ice bath  at the initial and final stages.

The estimated value of most corrections factors is zero with a corresponding however standard uncertainty (with no zero value). The combined uncertainty is estimated via the rule of RSS (Root of Sum of Squares) of all the terms:


The uncertainty budget of each temperature of the under calibration thermometer is presented in the following table (where Ci is the sensitivity factor and equals to 1 cause the mathematical model is linear to all the influenced parameters):

Table 6.2 : Uncertainty Budget of the Temperature under calibration

Term Description Estimated value Standard uncertainty Type of Distribution Ci Uncertainty contribution
R(ti) Mean value of temperature measured with the thermometer under calibration δR(ti)mean δR(ti)std-mean normal 1 (δR(ti)std-mean)2
δR(ti)immersion Immersion depth 0 ΔRtimm/(2∙√3) rectangular 1 (dR/(2∙2∙√3))2


Resolution 0 dR/(2∙√3) rectangular 1 (dR/(2∙√3))2
δR(ti)hysteresis Repeatability 0 ΔRt(00C)/(2∙√3) rectangular 1 (ΔRt(00C)/(2∙√3))2
Temperature under calibration: δR(ti)mean .u(R(ti)) k=1 √ (ΣuRti2)

6.8.3  Combined Standard Uncertainty

The combined standard uncertainty for a nominal reference temperature t, is estimated by:


6.8.4  Expanded Uncertainty

The uncertainty of the thermometer is expressed as the expanded uncertainty ± Uexp which corresponds to a probability of approximately 95 %. This is k times the combined uncertainty, where k is the coverage factor:

Uexp = k.Ucomb


The coverage factor k can be estimated from t- student distribution for specific effective degrees of freedom νeff and confidence level. In case where νeff ≥ 100, then k = 2 can be used.

By estimating the effective degrees of freedom from Veff of the combined standard uncertainty Uc(y) from the Welch-Satterthwaite equation


where  we can take the proper coverage factor k from Table E.1 of the EA-4/02 Expression of the Uncertainty of Measurements in Calibrations.

In our case all the uncertainties estimated as Type B have infinite degrees of freedom. The type A uncertainties have n-1 degrees of freedom where n the number of measurements.

Veff =


6.8.5  Deviation of Indications

The deviation δI of the indication Ι, of the under calibration thermometer from the reference indication Εref, is estimated by:

δI = Ι – Eref + ΣδΤi


where ΣδΤi is the sum of all the correction factors.

6.8.6  Best Measurement Capability

The best measurement capability of the lab is deduced from the lowest uncertainty of the temperature measurement that the lab can achieve in every day operation. It depends on the uncertainty contributions of the standard thermometers, the calibration procedure and the environmental conditions (the contribution of the thermometer under test is not taken into account). The best measurement capability has been evaluated in the excel file.

6.9  Experimental Data Sheet for SPRT Calibration




Scale interval reference 0.001 0C Scale interval 0.01 0C
δtimmersion 0.001 0C Resolution of item 0.01 0C
δbath homogenity (uniformity) 0.01 0C Range of measurements 0-50 0C
δbath stability 0.01 0C


0 30 50
-0.092 0.02 -0.092 30.003 30.01 30.008 50.004 50.02 50.008
-0.092 0.02 -0.092 30.005 30.01 30.009 50.005 50.02 50.008
-0.092 0.02 -0.091 30.006 30.02 30.009 50.005 50.02 50.008
-0.092 0.02 -0.090 30.005 30.01 30.008 50.006 50.02 50.008
-0.092 0.02 -0.090 30.004 30.03 30.009 50.006 50.02 50.009
-0.092 0.02 -0.089 30.004 30.04 30.009 50.007 50.02 50.008
-0.092 0.02 -0.089 30.007 30.05 30.008 50.006 50.02 50.008
-0.092 0.02 -0.089 30.008 30.05 30.009 50.007 50.02 50.009
-0.092 0.02 -0.089 30.008 30.06 30.009 50.007 50.02 50.008
-0.092 0.02 -0.089 30.008 30.01 30.009 50.007 50.02 50.009

6.10  Results of SPRT Calibration



Standard ( 0C ) Indication (UUT) ( 0C ) Uexp ( 0C )
0.011 0.02 0.068
30.082 30.03 0.069
50.082 50.02 0.070

7.1  Thermocouple Thermometer

Thermocouple Thermometers are the most widely used of all temperature sensors. Their basic simplicity and reliability have an obvious appeal for many industrial applications. However, when accuracies greater than normal industrial requirements are called for, their simplicity in use is lost and their reliability cannot be assumed.

For example, a major manufacturer of Type K thermocouple wire advises: ‘Once a thermocouple has been used at a high temperature, it is not good practice to use it later at a lower temperature’. Yet commercial hand-held electronic thermometers using Type K thermocouple thermometers are sold for use over the range -200 °C to 1400 °C and at an accuracy far exceeding that claimed by the wire manufacturer!

Such misuse of thermocouple thermometers arises in large part from a lack of understanding of how thermocouples work. Thermocouple thermometer literature often mistakenly states that the thermocouple junction is the source of the voltage, whereas in a well-designed measurement the junction does not contribute to the signal at all! Instead, the signal is generated along the length of the thermocouple thermometer wire. This small piece of knowledge tells us that conventional calibration techniques applied to thermocouple thermometers are often futile, and has a profound effect on the way traceability must be established.

William Thomson (Lord Kelvin) outlined the principles of thermocouple thermometer in the 1850s. He explained the relationship between the thermoelectric effects discovered by See beck in 1821 and Pettier in 1834, and predicted and verified the effect now known as the Thomson effect. Unfortunately, in most manufacturers’ literature and texts this understanding has been replaced by three empirical observations that have come to be known as the ‘Three Laws of Thermoelectricity’. These laws have the appeal of simplicity but give a working model that completely obscures the physical source of the thermoelectric potential. The model is both unhelpful and misleading for anyone analyzing thermocouples or trying to avoid the common errors in thermocouple practice. Periodically the basic principles are rediscovered, most often when large errors result from the use of thermocouple thermometers in new and unusual applications, or when some large industry loses millions of dollars because of the misunderstanding.

In this chapter, we will cover the construction of thermocouple thermometer, the errors that occurring use, and calibration methods for thermocouple thermometers that do work in practice. Before we do, however, we shall spend some time developing a clear description (hopefully) of the operating principles of thermocouple thermometers.

7.2  Construction of Thermocouple Thermometer

There is no standard way to construct a thermocouple thermometer, as they have been adapted to a wide variety of situations. Where possible a thermocouple assembly should be obtained from a well-known supplier because specialized materials and techniques can be involved for some applications. The main steps involved in construction are covered here primarily to help the user specify the thermocouple thermometer when purchasing. They will also provide general guidance for the construction of thermocouple thermometers.

7.2.1  Junctions

The sole purpose of a thermocouple thermometer junction is to provide electrical continuity. Whereas twisting and soft solder may well be suitable at low temperatures, for reliable high temperature exposure the junction should be welded.

7.2.2  Sheaths and Thermo Wells

While completely bare wire is sometimes used, especially in applications requiring the heavier gauges, it is more common to cover the wire to provide electrical insulation and environmental protection. A wide variety of insulating materials are available to suit many purposes and the user is advised to consult a catalogue to select an appropriate covering material. For higher temperatures, thermocouple thermometers are commonly hand assembled from bare wire and ceramic beads. Cleanliness is essential for this operation. Avoid work hardening the wire during handling. Bare junctions can be used to achieve a low mass or small size. If there is a risk of contamination the wire may need to be replaced frequently or if an increase in size and mass can be tolerated, a sheath can be used to provide protection.

7.2.3  Mineral-Insulated Metal Sheaths

MIMS (Mineral-Insulated, Metal-Sheathed) thermocouple thermometers are a very convenient form of thermocouple thermometer cable (See Figure 7.4). They offer the same protection as a metal

sheath while retaining a reasonable amount of flexibility. Various sizes from 0.25mm to 6mm diameter are often available from stock with special diameters up to 24mm made to order. The smaller diameters make it possible to preserve the size and mass advantage of thermocouples in a protective sheath.

7.2.4  The Paltrier Effect

Figure: 7.5 shows a simple thermocouple thermometer circuit made from two dissimilar wires, all at the same temperature. Now consider what happens when an electric current flows around the circuit. At one junction (in this case, the left-hand junction), the electrons move from a metal where they carry a lot of chemical potential energy to one where they carry less (at the same temperature). So the electrons carry that energy across the junction and then must come to thermal equilibrium with the different metal. In doing so they give some of their spare energy to the lattice and we see this as heat.

Figure 7.5 : The Pettier effect. Electrons moving from one conductor to another change state and may take in heat or release heat. The effect is reversible; changing the direction of the current moves heat in the other direction

7.2.5  The See beck Effect

Figure: 7.6 shows a single conductor exposed to a temperature gradient, but with no current flowing. The electrons within the conductor behave much like a gas. At the hot end of the conductor, the electrons have a high kinetic energy so move around violently and diffuse towards the cold end of the conductor. Similarly cold electrons

Figure 7.6 : The Thomson effect. Electrons moving from cold parts of a conductor into hotter parts take up heat and cool the conductor. The effect is reversible; heat is released as electrons move from hotter parts to cooler parts

Figure 7.7 : The See beck effect. The electrons in a conductor behave like a gas and expand under the influence of temperature. We observe the redistribution of electrons as a change in voltage along the length of the conductor. This occurs only where there is heat flowing; that is, only where there is a temperature gradient diffuse towards the hot end, but not so vigorously. The hot electrons carry heat to the cool parts of the conductor, while cool electrons take up heat from the hot parts of the conductor. The diffusion of free electrons is the main reason that metals have a high thermal conductivity.

In the same way, the change in See beck voltage, Es, occurs only where there is a temperature gradient and in proportion to the temperature gradient:

dEs = s(T )dT,

Figure 7.8 : The siphon analogy of the See beck effect. The change in pressure in the fluid-filled hose occurs only where there is a gradient.

Figure 7.9 : A defect (bubble) that occurs at a gradient has an effect on the pressure in the siphon, whereas a defect in an area where there is no gradient has no effect.

where s(T ) is called the See beck coefficient of the conductor, and dEs and dT are small changes in See beck voltage and temperature respectively. The See beck coefficient depends on the electronic properties of the conductor so is different for every metal and alloy, and varies with defect or contaminant concentration.

7.3  Errors in Thermocouple Thermometer

Reconsider Figure 7.10, which shows a measurement model for a thermocouple thermometer. Unlike other temperature sensors, the active part of the thermocouple thermometer is distributed over a long length and is thus exposed to a wide variation in environmental conditions, making an error assessment very difficult. Figure 20 summarizes the various error effects that must be considered.

Figure 7.10 : A summary of the sources of error in a thermocouple thermometer temperature measurement.

There are 8 (eight) types of errors in Thermocouple Thermometer

  • Thermal Effects.
  • Inhomogeneity Errors.
  • Heat Treatment.
  • Isothermal Errors.
  • Reference Junction Errors.
  • Interference Errors.
  • Wire Resistance Errors.
  • Linearization Errors.

7.4  Experimental Data Sheet for Thermocouple Thermometer Calibration



Scale interval reference 0.001 0C Scale interval 0.01 0C
δtimmersion 0.001 0C Resolution of item 0.01 0C
δbath homogenity (uniformity) 0.01 0C Range of measurements 0-1000 0C
δbath stability 0.01 0C


0 500 1000
-0.001 0.02 -0.001 500.001 500.01 500.001 1000.002 1000.01 1000.002
-0.002 0.02 -0.002 500.003 500.01 500.001 1000.003 1000.01 1000.002
-0.002 0.02 -0.002 500.002 500.02 500.002 1000.003 1000.02 1000.003
-0.003 0.03 -0.001 500.001 500.02 500.002 1000.004 1000.01 1000.003
-0.001 0.02 -0.001 500.004 500.02 500.002 1000.004 1000.02 1000.003
-0.001 0.02 -0.002 500.004 500.03 500.001 1000.004 1000.03 1000.004
-0.002 0.02 -0.002 500.006 500.03 500.004 1000.005 1000.03 1000.005
-0.001 0.03 -0.001 500.007 500.04 500.002 1000.006 1000.02 1000.005
-0.003 0.02 -0.003 500.008 500.05 500.001 1000.006 1000.03 1000.006
-0.003 0.02 -0.003 500.008 500.01 500.002 1000.007 1000.02 1000.006

7.5  Results of Thermocouple Thermometer  Calibration



Standard (0C) Indication (UUT) (0C) Uexp  (0C)
0.100 0.02 0.068
499.755 500.02 0.070
999.756 1000.02 0.070

Calibration of electronic balance has been performed with reference weight F1. At the time of calibration temperature was 260C, relative humidity (RH) was 70%, air pressure was 1012 mbar. The capacity of the balance was 600g. Off-center loading/eccentricity test, indication test/linearity test, repeatability test were done very carefully. Calculation of uncertainty was satisfactory, because it was within the limit.

Standard platinum resistance thermometer (SPRT) and thermocouple thermometer calibration were done in Temperature Measurement Laboratory of National Metrology Laboratory (NML), BSTI. The calibration were performed in ambient conditions under normal atmospheric pressure, temperature of 23°C ± 5°C and a relative humidity (RH) of 50% ± 20%. At the time of calibration of SPRT, it was seen that the resistance ideally varies linearly with temperature. SPRT can be used up to 6600C, but the measurement was taken from 00C to 500C. The results of SPRT calibration was good, because it was within the standard limit.

Thermocouple Thermometer calibration was done using an ice bath. The thermocouple thermometer calibration can be done in the temperature range from 2000C to 26000C. But this calibration was done in the temperature range from 00C to 10000C. The results of calibration were found within the standard limit.

The reported calibration work involves a very little money and it is very accurate, reliable and precise. Electronic balance is suitable for accurate weighing, time saving and also easy to use.

Bangladesh Standards and Testing Institution (BSTI) spends huge foreign exchange for calibration of its procured National Metrology Laboratory (NML) equipment from abroad which is not desirable to us. So, BSTI should purchase National and Primary  Standards used in the measurement system for calibration of NML equipment.

BSTI should continue calibration services of weights, measures, weighing and measuring instruments used in laboratory, academia, industries, wholesale and retail traders etc. to establish standard measurement system in Bangladesh.

9.1  Conclusion

Calibration of Electronic Balance, Standard Platinum Resistance Thermometer (SPRT) and Thermocouple Thermometer ensure accurate, reliable and precession measurement in the Research Laboratories, Academic Institutions and  Industrial Sectors.

National Metrology Laboratory (NML) of Bangladesh Standards and Testing Institution (BSTI) has established chain of traceability in the measurement system in Bangladesh as well as other NML’s of the world.

9.2  Recommendations

Bangladesh Standards and Institution (BSTI) should continue Standard Calibration for all the equipment used in National etrology Laboratory (NML) by procuring Primary and National Standards used in the Measurement System.

Bangladesh Accreditation Board (BAB) should be strengthened within shortest possible time, so that it can start accreditation activities for all organizations, bodies, systems, individuals of our country as per ISO 17025.