A Study on Standard Measurement System In Bangladesh
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.
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)
Figure 2.1 : Organogram of BSTI
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
Fig. 2.2: Hierarchy of Standards
2.7 Important Definitions Related to Measurement System
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.
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.
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.
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.
18.104.22.168 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 <href=”#associates”>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).
22.214.171.124 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:
- Member bodies.
- Correspondent members.
- 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.
- 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
- 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
Figure 3.1 : 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
Figure 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 dayampereAelectric 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%.KelvinKthermodynamic 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).molemolamount of substanceThe 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.candelacdluminous 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
Figure 4.2 : SI Base Units
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|
|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|
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|
in terms of
other SI unitsExpression
in terms of
SI base unitsplane angleradian (a)rad-m·m-1 = 1 (b)solid anglesteradian (a)sr (c)-m2·m-2 = 1 (b)frequencyhertzHz-s-1forcenewtonN-m·kg·s-2pressure, stresspascalPaN/m2m-1·kg·s-2energy, work, quantity of heatjouleJN·mm2·kg·s-2power, radiant fluxwattWJ/sm2·kg·s-3electric charge, quantity of electricitycoulombC-s·Aelectric potential difference,
electromotive forcevoltVW/Am2·kg·s-3·A-1capacitancefaradFC/Vm-2·kg-1·s4·A2electric resistanceohm V/Am2·kg·s-3·A-2electric conductancesiemensSA/Vm-2·kg-1·s3·A2magnetic fluxweberWbV·sm2·kg·s-2·A-1magnetic flux densityteslaTWb/m2kg·s-2·A-1inductancehenryHWb/Am2·kg·s-2·A-2Celsius temperaturedegree Celsius°C-Kluminous fluxlumenlmcd·sr (c)m2·m-2·cd = cdilluminanceluxlxlm/m2m2·m-4·cd = m-2·cdactivity (of a radionuclide)becquerelBq-s-1absorbed dose, specific energy (imparted), kermagrayGyJ/kgm2·s-2dose equivalent (d)sievertSvJ/kgm2·s-2catalytic activitykatalkat 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|
|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|
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.
Figure 5.1 : Schematic Diagram of Electronic Balance
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
|± ?m 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|
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:
- Functionality Check.
- Leveling Check.
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.
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 indications
The 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.
Maximum capacity: 600 g.
Minimum capacity: 0.01 g.
Readability: 0.01 g.
Drift: 0.01 g.
Environmental condition :
|Air pressure||1012 mbar|
|Indication||599.95 g||599.94 g||599.95 g|
Indication Test :
|U=||±(0.00931 + 1.39 x 10-04 x M)||g|
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.
Figure 6.1 : Different Temperature Scale
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 ? 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.
Figure 6.2 : Standards Platinum Resistance Thermometer
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 Standar