Calibration Services
KSP SOLUTIONS & ASSESSMENT PVT.LTD calibration services.
Calibration service is a service aimed at detecting the inaccuracy and uncertainty of a measuring instrument or piece of equipment. In calibration, the device under test (DUT) is compared to a reference of known value to determine the deviation of the measurement from the true value. Deviation of the actual value from the measured value is called an error. After the error has been detected, the next move is to adjust the DUT to obtain more accurate measurements, but this is a separate process called adjusting or trimming.
Calibration is a documented activity performed in a designated and controlled calibration laboratory by an accredited calibration service provider. Some calibration service providers perform this service in the site where the measuring equipment is situated to prevent operational downtime.
The calibration certificate is issued by the provider to the requesting organization; the certificate is specific to the calibrated instrument for traceability and documentation purposes. It reports the information and results of the calibration performed on the DUT. A calibration sticker may be seen on some measuring equipment to readily distinguish a calibrated piece of equipment.
Any Query/ Get A Quote
Measurement Traceability
International System of Units
The International System of Units (SI system) is a standardized system of measurement. SI is an abbreviation the organizations French name, “Système International d unités”, which is known as the metric system. The goal of the SI system is to communicate measurements precisely through a coherent and consistent expression of units describing the magnitudes of physical quantities. The SI system serves as the universal language for measurement systems and is adopted in organizations, businesses, and research facilities worldwide.
The SI system was established in 1960 by the 11th General Conference on Weights and Measures (CGPM, Conférence Générale des Poids et Mesures). Under the authority of the CGPM is the Bureau International des Poids et Mesures (BIPM), an intergovernmental agency based in Paris, France that ensures the worldwide unification of physical measurements.
The SI system is made up of seven base units, which are used to define the 22 derived units. The seven base units are used to describe the seven fundamental quantities, which originated from the most stable reference—one that does not deteriorate over time: the distance traveled by light in a vacuum in 1/299792458 second is designated as one meter. It is an immutable definition of the most basic unit of length; all measurements of distance are based on it. It is only amended on the basis of the latest international definitions and scientific advances that lead to a clearer and more acceptable definition of the base unit. When the base units are multiplied by each other, the 22 derived units are obtained. The table below presents the seven fundamental quantities:
Prefixes are attached to a base unit to indicate its magnitude. The magnitude of the scale is increasing by multiples of ten. This makes the SI system a convenient way of conveying and comparing quantities because we can easily convert from one unit to another.
Metrological Traceability
Metrological traceability, or measurement traceability, as defined by the International Vocabulary of Metrology, is the “property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.” It is one of the most important aspects of measurement and calibration as it authenticates the conformity to the international standards. To visualize the chain of calibrations, take a look at the measurement traceability pyramid:
The calibration chain starts with the working standards or, alternatively, process calibrators of higher accuracy are used to calibrate the DUTs. These working standards and process calibrators are assumed to have the highest accuracy in a plant or site. Before working with DUTs, they are sent to an accredited calibration laboratory for calibration by a standard or calibrator with higher accuracy. The standards and calibrators in the accredited calibration laboratory are sent to the national metrology institutes (NMI‘s) of the governing state. Finally, the NMIs coordinate with international metrological agencies to assist in the fulfillment of the SI units and the conformance based on international definitions and comparisons of the calibrations and measurements being performed in their home countries. All uncertainties must be declared in each level of calibration.
The SI units are the foundation of all measurement standards and sit at the top of the measurement traceability pyramid. The SI units serve as the measurement standard with perfect accuracy and “true value” for all measurements.
The BIPM and NMIs of the participating countries help in preserving the accuracy of SI units as the calibration level moves down the lineage—from the primary and secondary standards to the process DUTs, which sit at the base of the traceability pyramid. From NMI-level standards, “true value” is communicated down the lineage. As we move up the pyramid, the accuracy becomes closer to the true value dictated by the SI system, but the cost of the standards also becomes higher. As we go down the pyramid, the measurement error and level of uncertainty is magnified.
To summarize, measurements must be related to the next reference of higher accuracy in the hierarchy. The measurement is traceable if it satisfies these three conditions:
- The calibration of the measuring equipment is performed at regular intervals. The calibration of measuring equipment is only valid for a certain period. Remember that traceability expires once the calibration expires.
- The calibration in each level of the chain is documented. A calibration certificate is issued by the calibration service provider, which is accredited by the higher governing body.
- The estimate of the measurement uncertainty is reported. Measurement uncertainty is critical information in calculating the true value.
Note: The NMI of the United States of America is the National Institute of Standards and Technology (NIST), which is part of the US Department of Commerce.
Calibration Importance
There is a phrase: “If you can‘t measure it, you can‘t improve it.” Measurement is the foundation of quality, safety, efficiency, and overall development. Most industries and sectors rely heavily on the accuracy of measuring instruments to improve the quality of life. The goal of a calibration service is to minimize the measurement error and increase the assurance of making precise measurements.
The accuracy of all measuring instruments deteriorates over time
Factors such as environment, usage frequency, and handling can increase measurement uncertainty and error. Hence, measuring devices must be subjected to timely calibration. Calibration lends to the improvement of repeatability and reproducibility of the produced data.
The accuracy of measuring instruments does not deteriorate over time. Instruments that are very stable and used for many years are the best standards. Their uncertainty and stability can be well documented due to their long history proving the best standards, which may not be the true value but documentation makes them valuable.
Calibration is necessary if your measuring instrument significantly affects the accuracy and validity of your testing procedure, which is crucial to the credibility of the data produced by laboratories and testing facilities. This aspect is immediately apparent in medical and legal decisions.
The ISO/IEC 17025:2017, or the General Requirements for the Competence of Testing and Calibration Laboratories, provides the scope of competencies of the facilities seeking accreditation. Measurement uncertainty analysis and measurement traceability are parts of the ISO/IEC 17025 scope, and many laboratories are cited for these deficiencies. Besides, compliance to this standard assures many intangible benefits and ease of operation for the testing facilities that have been granted accreditation.
Another reason to calibrate your measuring instrument is if it is vital in the detection of variations in your process and such variations can have detrimental effects on product quality, health, and safety. Reliable measurements help engineers to minimize the assignable causes of variation by prevention and early detection.
Minimizing variation is critical in meeting the specification of manufactured products or parts. There are instances where large variations can even put others‘ lives at risk. When process parameters and operating conditions are left uncontrolled in a factory or site, it can endanger the people and the environment in the vicinity. This aspect is also seen in the manufacturing of aircraft and automobile parts and medications where any deviation can adversely affect the health and safety of the users.
Lastly, calibration upholds the geographical consistency of all measurements performed and keeps all measuring instruments up to date with international agreements and standards. Parties in different regions must agree if they were presented with a specific measured quantity. This is essential in international and domestic trading as quantities of goods equate to revenues. For example, one cubic meter of gasoline exported by a country must be exactly one cubic meter of gasoline when it is measured in the importing country.
Calibration Service Process
BIPM formally defines calibrators as the “measurement standard used in calibration”. Calibrators may be an instrument with higher accuracy used to calibrate a DUT, a source, or a Certified Reference Material (CRM).
A source is an instrument that produces a known and precise output. The output is measured by the DUT. The setting of the source equipment is considered the exact value.
A Certified Reference Material (CRM) is a form of a standard that is characterized by a metrologically valid procedure. Its exact measurement value is known. Individual CRM samples must be stable and must be authenticated by a certificate. CRMs are typically used in analytical and clinical chemistry.
The general procedure in calibrating any process parameter is as follows:
Using an instrument of higher accuracy
- Use the DUT and the calibrating equipment to measure the set/s of sample/s using the same test conditions.
- Calculate the difference between the measurement made by the DUT and the calibrating equipment. The resulting value is called the measurement error.
- Repeat the steps above until the required number of data points by the calibration procedure is satisfied.
- Interpret the calibration data.
Using a source or a Certified Reference Material (CRM)
- Use the DUT to measure the output of the source or the CRM.
- Calculate the difference between the measurement made by the DUT and the exact value. The resulting value is called the measurement error.
- Repeat the steps above until the required number of data points by the calibration procedure are satisfied.
- Interpret the calibration data.
The comparison of the measurements obtained from the DUT and the calibrator can have one of the two possible outcomes:
The measurement error is within the acceptable level. This means that the DUT has passed the calibration. Therefore, adjusting for the error made by the measuring instrument is not necessary.
The measurement error is outside the acceptable level. This means that the DUT has failed the calibration. Therefore, there is a need to adjust the measuring instrument to correct the measurement error or to apply a calibration correction.
The commonly used metrics in determining the calibration result are the Test Accuracy Ratio (TAR) and Test Uncertainty Ratio (TUR). Most calibration laboratories today consider a TAR or TUR of 4:1. This means that the DUT has at least 25% accuracy of the reference standard, or the reference standard is four times more accurate than the DUT.
Test Accuracy Ratio (TAR). TAR is the ratio of the DUT tolerance to the reference standard tolerance. It is a simplified pass-or-fail indicator in calibration practice, but it does not consider the measurement uncertainties associated with the process.
Test Uncertainty Ratio (TUR). TUR is the ratio of the DUT tolerance to the estimated calibration uncertainty. It considers the influences in the calibration process, which can affect the accuracy of the measurements such as environmental factors, process variations, technician errors, and instruments used in the procedure. The estimated calibration uncertainty is typically expressed at a 95% or a 99% confidence level..
Interpreting Calibration Certificates
A calibration certificate is issued by an accredited service provider after the calibration has been successfully performed under their supervision. It summarizes the details, procedure, and results of the calibration performed. The certificate must contain the following information:
- Title of the certificate (i.e., “Certificate of Calibration”)
- Name and address of the accredited calibration laboratory
- Issuance details such as the date and certificate number
- Name and address of the requesting organization
- Details of the measuring device such as device type, model, serial number, and specifications
- The calibration procedure
- Environmental conditions (e.g., ambient temperature, humidity) at the place and time the calibration is performed
- Calibration results
- Validity
- Evidence of measurement traceability
- Name and signature of the calibration technician/s and approvers.
A calibration sticker is attached to the equipment to verify the calibration validity and status. It usually indicates the equipment serial number and the date the next calibration is due. However, it does not equate to the legitimacy and traceability of a calibration certificate. It is only used as a visual reference to inspect the calibration of the equipment.
Calibration Correction
Calibration correction is the difference between the measurements obtained by the DUT during calibration and the exact value of the reference standard. This data is presented on the calibration certificate. The calibration correction will be added to the future measurement readings obtained by the DUT. This will help the DUT get closer to the true value, thereby improving its accuracy.
Expanded Uncertainty
Expanded uncertainty is the defined interval in the calibration report wherein the true values can be confidently asserted to lie. It is determined statistically and accounts for all uncertainty sources. The lower the expanded uncertainty of a DUT is, the higher precision of the measurements.
Coverage factor
The coverage factor, or the K-factor, indicates the confidence level the expanded uncertainty is derived from. K-factors of 2 or 3 are recommended for most industries.
A K-factor of 2 corresponds to a 95.45% confidence level. It means that 95.45% of the time, the measurements made lie within the expanded uncertainty. The same concept is also applied to other K-factors; a K-factor of 3 corresponds to a 99.73% confidence interval. Higher K-factor is applied in DUTs performing critical measurements, wherein measurement failure can be costly and dangerous.
False Accept and False Reject
A key factor in the acceptance of calibrations is measurement decision risk that are expressed as false accept risk or false reject risk, which are the metrics used to determine the quality of a calibration.
False Accept
There are two definitions of false accept risk, which are unconditional and conditional. Unconditional false accept refers to the possibility that equipment parameters may be out of tolerance but viewed as being in tolerance. With conditional false accept, it is possible that the equipment is out of tolerance. With high false accept, there is the possibility of severe negative outcomes regarding the performance of an item.
False Reject
False reject refers to in tolerance readings being rejected, which can lead to increased costs due to unnecessary adjustments, repairs, re-calibrations, and less time between calibrations.
Conclusion
- Calibration service is a service used to verify the accuracy of measuring equipment by comparing the measurement obtained by the DUT to the standard value. The deviation from the standard value is called error.
- The SI definition of the base units serves as the highest-level standard.
- Traceability is an important aspect of measurement. Measurements must be related to a reference of a higher accuracy through an unbroken chain of calibrations.
- Calibration is important in laboratories, testing facilities, and industrial processes. It is also important in the geographic consistency of measurements.
- TAR and TUR are metrics that determine the calibration results. TAR or TUR of 4:1, or 25% accuracy, must be met by the DUT to pass the comparison.
- A calibrator may be another instrument with higher accuracy, a source, or a CRM.
- The calibration certificate reports the details, procedure, and results obtained during the calibration service. Calibration service is performed by an accredited service provider.
- Calibration correction is used to adjust the measurement value, bringing it closer to the true value.
- Expanded uncertainty indicates the interval where the true value is confidently asserted to lie. It is statistically determined under the recommended K-factor. This value is one of the most overlooked details in calibration reports