Calibration is a word that is often used in many different contexts and the word has a different meaning depending on the industry that is defining it. In most cases, the definition of calibration references the comparison of a device or test specimen with a standard device to determine that the item being calibrated is functioning within its manufacturer’s specifications or will produce accurate results.
In a calibration laboratory, calibration is defined as the “comparison of specific measurement values measured or generated by a piece of test equipment that is compared with a traceable reference standard”. The traceable reference standard is required to produce values more accurate than the device being calibrated. The reference standard could be another measurement device, a device generating the value to be measured or a physical reference, such as a gage block or reference gas.
The purpose of calibration is to ensure that the device being calibrated is capable of either measuring or generating values that are accurate, repeatable and reliable.
Some measuring devices may exhibit a measurement ‘drift’ when measurements are taken over time. This can be caused by normal wear of the device, circuit component changes for electronic equipment, or changes based on the environment in which it is being used, such as environments with drastic temperature changes or where vibration is present. Therefore, it is important that calibrations are performed at regular intervals to be confident in the accuracy and reliability of the measurement results of the equipment throughout its lifetime
Measurement values are often used to make decisions on production processes or quality requirements, so it is important that the values obtained are accurate and that the values remain repeatable when they are collected over a period of time or by different operators. Decisions made based on flawed data could have negative consequences which could result in reworked parts, safety issues and damage to the reputation of the business.
There are several functions within a calibration process where there are misunderstandings or outright ‘myths’ of how the process works. Below are some of the most common of those myths.
The term ‘Calibration’ only relates to the verification of the measurement values in comparison with an appropriate laboratory standard. Calibration, per definition, does not involve the adjustment of the equipment being calibrated. Adjustments to the equipment are a completely different step after the equipment has been calibrated.
If adjustments are required, they are usually performed as a part of the calibration process depending on the adjustments that are needed. Many times, the customer needs to be contacted before any adjustments are performed if the adjustments require a lengthy process or to verify the decision rules for the measured values. The calibration laboratory will issue the calibration certificate with both the ‘As Received’ and ‘As Left’ measurement values to assist the customer in determining the reliability of their equipment.
A number of equipment types such as Oscilloscopes and Analytical Balances contain ‘Self-Test’ or ‘Self Calibration’ routines as part of their menu functions. Many people think that because an item has a self-test function, the item does not need to be calibrated. Most self-tests will only check the proper operation of general equipment functions or internal modules or they will set a reference zero or reference value for their measurements. They cannot verify the actual measured values or compare them with an external standard of higher accuracy, as in a proper calibration. While performing the ‘self-test’ may help to maintain the functioning of the equipment, it does not replace the need for regular calibrations.
Some customers believe that as long as they receive a calibration certificate back with their piece of equipment, that the equipment met its specification and ‘passed’ its calibration. In some cases, calibration laboratories do not provide specific statements of whether an item was received within its stated specifications, such as ‘In Tolerance’ or ‘Passed’. These ‘Statements of Conformity’ are only to be included on the calibration certificate if requested by the customer or if there is an agreement with the customer on the decision rule that defines the statement of conformity. If the statement is not included, the customer will need to review the measurement data themselves to determine if the measurements were within the stated specifications.
In other instances, there may be measurements that were out-of-specification and could not be adjusted back within the specifications. In this case, a ‘Limited Calibration’ may need to be applied to the equipment to document the out-of-specification condition. The customer will typically be contacted to approve the limitation and the limited calibration information will be listed on the calibration certificate for reference.
Many customers also believe that the calibration laboratory sets the calibration intervals for the equipment and they are required to use that interval. Actually, it is the responsibility of the customer to define the specific calibration intervals for their equipment as a part of their quality program and to provide the calibration laboratory with that information. It is generally considered a ‘conflict of interest’ for the calibration laboratory to assign the calibration intervals. If the customer does not provide a specific calibration interval as part of the contract review, the calibration laboratory cannot provide any recommendation on a calibration interval or include the interval and appropriate calibration due dates on the certificate or calibration label.
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