How to understand the traceability and calibration of laboratory measurement

The correctness and reliability of standards/tests carried out in the laboratory depend on many factors, among which "traceability and calibration of measurement" is one of the important factors. The laboratory accreditation criteria (CNCL 201-1999) and the measurement traceability policy (CNCL 206-1999) formulated by the National Accreditation Committee of Chinese Laboratories put forward requirements for measurement traceability to prove whether the laboratory has the specified calibration/testing ability technically. 1. It is generally required that all equipment used for calibration/testing, including auxiliary measuring equipment (such as measuring equipment used for environmental conditions) that has a significant impact on the accuracy or effectiveness of calibration results, test results or sampling results, must be calibrated or verified before being put into use in laboratories that have applied for accreditation. For the calibration or verification of the measuring equipment, the laboratory must make implementation plans and procedures in advance. The plan shall include measurement standards, reference standards as measurement standards, selection, use, calibration/verification, verification, control and maintenance of measurement equipment and test equipment for calibration/detection, and form a system. This is article 9. 1 of the laboratory accreditation guide and article 5.6.65438 of the draft ISO 17025 (ISO/DIS/17025:1998), which will soon replace ISO/IEC guide 25 (/). In this system, the measuring equipment that needs calibration/verification should be listed and identified first, and some equipment (such as DC power supply, sine template, etc.). Only the initial calibration and the function check are required) shall also be listed and identified. Secondly, we should distinguish which belong to our laboratory and which can be calibrated by ourselves, that is, the measurement standard or reference standard used for calibration is owned by the laboratory itself; Which belongs to external calibration, that is, external institutions (such as metrology institutes, metrology centers and calibration laboratories) with corresponding levels in the traceability system diagram must be selected, and these measuring devices are always in the control state of calibration or inspection through the system of sending them to school or inspection. For different types of laboratories, the contents and requirements of this external calibration and self-calibration will be different, but in general, it must include the following seven points: (1) Identify the measuring equipment that needs calibration/verification; (2) the date of the last and next delivery or inspection of measuring equipment; (3) When necessary, provide proof of the calibration/verification ability of external institutions, and record how the measuring equipment is delivered to external institutions and sent back to our room for calibration or inspection. (4) Explain how the laboratory saves the calibration/verification certificate from external institutions, and pay special attention to whether the information in the certificate is consistent with the corresponding calibration/detection range and uncertainty or accuracy level of the laboratory; (5) Provide the proof of measurement traceability, which proves that the laboratory has the ability to calibrate some measuring equipment by itself; (6) Explain the procedures and methods used for self-calibration, how to keep its records and certificates, and provide the qualification certificates of self-calibration personnel; (7) Record the maintenance, verification, change trend and control of calibration/verification status of measuring equipment. In fact, based on these points, the laboratory can establish historical files and databases for calibration/verification of measuring equipment. Using this database, the laboratory can often get a lot of useful information, such as: (1) If the same parameter or index of the same measuring equipment is out of tolerance repeatedly in calibration, verification or verification, the laboratory should pay close attention to it at this time, study its possible causes and follow-up problems, and take appropriate measures if necessary, such as adjustment, repair, degradation or even scrapping (that is, deleting it from the list of measuring equipment in use) (. (3) For the measuring equipment showing that some parameters exceed the predetermined control limit on the state trend and control chart, it should also be suspended and marked until the cause is found out, corrected and calibrated/verified correctly. The laboratory should be equipped with special personnel or specialized agencies to identify and identify the calibration/verification status of measuring equipment. The above-mentioned "measuring equipment" refers to the general name of measuring instruments (measuring instruments), measuring standards, reference materials, auxiliary equipment and materials necessary for measurement. "Measurement standard" refers to a physical measuring tool, measuring instrument, reference substance or measuring system used as a reference in order to define, realize, save or reproduce the unit or one or more values of quantity. Such as 1kg quality standard,100Ω standard resistance, cesium frequency standard, standard hydrogen electrode, reference solution with certified cortisone concentration in plasma, etc. "Reference standard" means the measurement standard that usually has the highest metrological characteristics in a given area or a given organization, and all measurements made there are derived from this standard. For example, China's provincial, municipal and county-level social public measurement standards, national professional measurement station social public measurement standards, the highest measurement standards of departments, the highest measurement standards of enterprises and institutions. Second, the requirements for calibration laboratories For calibration laboratories, the formulation and implementation of calibration/verification plans for such measuring equipment must ensure that the measurements carried out by the laboratory can be traced back to SI measurement units, including related natural constants. The traceability of measurement must be guaranteed by the calibration service that the laboratory is engaged in, that is, the calibration service shows the laboratory's ability, measurement ability and measurement traceability. In the calibration certificate issued by the laboratory, it must be indicated that there is an uninterrupted traceability chain, which can link calibration with measurement benchmark or realize the natural constant of SI unit. The measurement results on the calibration certificate must include the statement or statement of measurement uncertainty and/or conformity with the specified measurement specifications. This is the main content of article 9.2 of CNCAL 201-1999 and article 5.6.2. 1 of ISO 17025. The above-mentioned "SI" (International System of Units) refers to the consistent system of units adopted and recommended by the International Metrology Conference (CGPM). At present, SI is based on the following seven basic units: length unit meter (m), mass unit kilogram or kilogram (kg), time unit second (s), current unit ampere (a) and thermodynamic degree-kelvin (k). The meaning of "measurement standard" is that it has the highest measurement characteristics, and its value does not need to refer to other standards of the same quantity, designated or recognized measurement standards. The concept of benchmark applies equally to basic quantity and derived quantity. "Traceability" refers to the characteristic that the measurement results or measurement standards can be related to the specified reference standards (usually national measurement standards or international measurement standards) through an uninterrupted comparison chain with specified uncertainty. This uninterrupted comparison chain is also called "traceability chain". Traceability is called traceability or traceability in ISO9000 family standards, and its meaning is very wide. In the field of laboratory accreditation and measurement, traceability reflects a characteristic of measurement results or measurement standards, that is, the values of any measurement results and measurement standards must ultimately be linked with national or international measurement standards to ensure the unity of measurement units and the accuracy, reliability, comparability, repeatability and reproducibility of the values. And its way is to trace back to the source of measurement (measurement benchmark) according to this comparison chain. Traceability is a term widely used in the field of international metrology. As far as its technical content is concerned, it is similar to the commonly used term "value transfer" in China, and the traceability system formed by uninterrupted comparison chain is similar to "verification system" However, there are great differences in management methods. The traceability of measured values is from bottom to top. According to the requirements of measurement accuracy, enterprises can independently find reference standards with good uncertainty to calibrate measuring equipment, and even compare or calibrate them with national or international measurement standards across regions and borders, so as to reasonably meet the use requirements. The value transfer is from top to bottom, especially for the compulsory verification of measuring instruments, which will inevitably lead to the repeated setting of measuring and verification institutions and the increase of transfer links, thus losing part of the measurement accuracy. In order to be in line with international practice and meet the needs of the development of socialist market economy, we should advocate the traceability of quantity. China's quality traceability system is based on China's metrology. Article 10 of China's Metrology Law stipulates: "Metrological verification must be carried out in accordance with the national metrological verification system table. The national metrological verification system table shall be formulated by the metrological administrative department of the State Council ". The legal status of the metrological verification system table depends on this, and its contents generally include text description and system block diagram, such as the description of measurement benchmark, measurement standard and the name of working measuring instruments, measurement range, uncertainty or allowable error, verification method and value transfer relationship, etc. Every national measurement standard should have a corresponding measurement verification system. So far, China has formulated 93 tables of metrological verification systems. In Article 5.2 of CNCAL 201-1999, it is also stipulated that the quality manual and its corresponding quality documents should include "5.2(g) Procedures for realizing the traceability of laboratory values". When making this plan, we should pay attention to the block diagram of laboratory-related working procedures and traceability system, especially how to determine the recalibration or reinspection period. In principle, the recalibration period depends on the measurement risk and economic factors, that is, the risk of measuring equipment exceeding the allowable error in use should be as small as possible, and the annual calibration cost should be kept at the lowest level, that is, how to optimize the balance between risk and cost. When determining or selecting the verification period of measuring equipment, the following six points should be considered: (1) the provisions of the relevant metrological verification regulations on the verification period; (2) Requirements or suggestions of relevant departments when type approval is carried out; (3) the requirements or suggestions of the manufacturer; (4) Frequency or times of use; (5) the severity of the use environment and its influence; (6) Requirements for allowable error or accuracy level. The above-mentioned "international standards of measurement" refers to the standards of measurement recognized by international agreements, which serve as the basis for formulating the values of other standards of measurement related to quantity internationally. "National measurement standard" refers to the measurement standard recognized by the state, which is the basis for formulating the values of other measurement standards related to quantity within a country. "Specified measurement specification" means that the calibration certificate must clearly indicate which specification the measurement is based on or explicitly refer to. "Natural constant" refers to the basic physical constant involved in the definition of quantum measurement standards with higher accuracy, stability, reliability and universality. For example, the superconducting Josephson effect is used to define DC voltage according to 2e/h, and the quantized Hall effect is used to define DC resistance according to h/e2, where h is Planck constant and e is electron charge. These are natural constants and will not change with time, place, materials that make up Josephson junction and other factors. Three. Requirements for testing laboratories For testing laboratories, Article 5.6.2.2 of ISO 17025 stipulates: "The requirements for calibration laboratories in Article 5.6.2. 1 also apply to measuring equipment and testing equipment with measuring function, unless the influence of relevant calibration uncertainty on the expanded uncertainty of measurement results has been confirmed. If this confirmation occurs, the laboratory must ensure that the measuring equipment used has the necessary measurement accuracy. " In other words, the compliance of the testing laboratory with the requirements of Article 5.6.2. 1 (that is, the previous section) depends on the relative influence (contribution) of the calibration uncertainty on the expanded uncertainty of the measurement results. If calibration is the main factor, the requirements in the previous section should be strictly observed. However, if calibration is not one of the main influence (contribution) components of the expanded uncertainty, other methods given in the following article 5.6.2.2 can be used to provide the confidence of the measurement results. Article 5.6.2.2.2 requires that when the traceability of SI measurement units in the laboratory is impossible and/or irrelevant, the following three other methods must be used to provide measurement confidence: (1) Use appropriate certified reference materials to give the reliable characteristics of materials; (2) Using the protocol standard or protocol method clearly defined and agreed by all parties concerned; (3) Participate in the appropriate inter-laboratory comparison plan or capability verification plan. In fact, this article contains the requirements of Article 9.3 of CNACL 20 1- 1999, that is, in the case that it cannot be traced back to the national measurement standards, the laboratory should provide satisfactory evidence for the correlation of the measurement results, such as participating in appropriate inter-laboratory comparison or capability verification. For testing laboratories and calibration laboratories, measurement uncertainty has been mentioned many times. In fact, it is a parameter that represents the dispersion of the measured value, which is related to the measured result. Uncertainty should be included in the complete expression of measurement results. It can be the standard deviation or its multiple, or it can be the half width of the interval indicating the confidence level. The uncertainty expressed by standard deviation is called standard uncertainty, expressed by u, and the uncertainty expressed by multiples of standard deviation is called extended uncertainty, expressed by u, and the extended uncertainty represents the half width of the interval with higher confidence probability. Uncertainty consists of multiple components, and the standard uncertainty of each component should be evaluated. The evaluation methods are divided into two categories: A and B. The evaluation of category A is characterized by experimental standard deviation through statistical analysis of observation columns. Class B evaluation is characterized by the standard deviation estimated by other methods different from Class A. The composition of each standard uncertainty component is called composite standard uncertainty, which is expressed by uc and is the estimated value of the standard deviation of measurement results. In order to get the expanded uncertainty, the digital factor multiplied by the combined standard uncertainty is called the inclusion factor, which is generally expressed by K, and the inclusion factor when the confidence probability is p is expressed by kp. In order to determine the influence or contribution of calibration uncertainty to the expanded uncertainty of measurement results, that is, to determine whether it is dominant or dominant, partial judgment or partial calibration can often be adopted. China's quantitative traceability policy cncal 206- 1999 stipulates in "Handling when it is difficult to establish traceability relationship" that when traceability to national/international measurement standards is impossible or inapplicable, division calibration can be adopted, or relevant certificates can be provided by participating in appropriate capacity verification. Iv. Other requirements for quoting standards (1). Laboratories must have plans and procedures for calibrating their reference standards, and the calibration of reference standards must be carried out by institutions that can provide measurement traceability. For such a reference standard, the laboratory must only be used for calibration and not for other purposes, unless it can be proved that its performance as a reference standard will not fail. The above provisions of article 5.6.3. 1 of ISO 17025 actually include the requirements of articles 9.5 and 9.4 of cncal 20 1- 1999. (2) About reference materials. If possible, reference materials must be traced back to SI measurement units or certified reference materials. According to the technical and economic feasibility, the internal reference materials must be verified. The above provisions of 5.6.3.2 clause of ISO17025 actually include the requirements of Article 9.7 of cncal 20 1- 1999. (3) About the mid-term verification. In order to maintain confidence in the calibration status of reference standards, benchmarks, transfer standards, working standards and reference materials, inspections must be carried out in accordance with the prescribed procedures and progress. The above provisions of ISO17025 5.6.3.3 actually include the requirements in Article 9.6 of cncal 20 1- 1999, that is, calibration (or verification) must go through "operation inspection". (4) About transportation and storage. In order to prevent reference standards and reference substances from being polluted or damaged and ensure their integrity, the laboratory must have procedures for safe disposal, transportation, storage and use. When reference standards and reference substances are used for field detection, calibration or sampling outside the fixed laboratory, some additional procedures may need to be formulated. The above is the 5.6.3.4 clause of ISO17025. In cncal 206- 1999, the traceability policy "About Traceability to Foreign Metrology Standards" stipulates that calibration/testing laboratories must provide effective traceability certificates when imported measuring equipment cannot be traced back to China's national metrology standards. It also stipulates that overseas laboratories accredited by the National Laboratory Accreditation Committee of China must provide valid certificates that can be traced back to domestic/international measurement standards.