Measuring Equipment Calibration or Verification

    In 7.1.5.2 of ISO9001:2015, it requires that measuring tools shall be calibrated or verified or both:

So in which cases shall the measuring equipment be verified, in which cases shall they be calibrated, and in which cases shall they be done both?

    To answer this, one first needs to understand the difference between verification and calibration. The measuring process is a process to assign a value to the measured characteristic. There are two values which should be noted here: one is the true value of the characteristic ut, and the other is the assigned value of the characteristic ua, i.e. the result obtained from the measurement. Verification is the serial of activities done to prove that the difference between ut and ua is within the acceptable range (which can be defined internally, according to customer requirements, according to national standard, etc.). So the result from verification is a judgement of “Acceptable” or “Not Acceptable”. Calibration, however, is the serial of activities which are done to establish the proper relationship between ut and ua, so that acceptable ua can be obtained from the measuring process. Below are two examples of calibration:
Example 1: a digital caliper gives the reading of a dimension. It first transfers the measured dimension ut into an electrical signal, and then transfers the electrical signal into a digital reading ua. The manufacturer of the digital caliper must study how the signal transferring from the measured dimension into the digital reading is done and establish their relationship properly. Otherwise, the reading ua may depart significantly from the actual dimension ut.
Example 2: an electronic scale is used to weigh the products. An operator measured 5 samples with known value and obtained following results:

From the above measurements, the operator found that the reading is always 0.1kg greater than the true value. So for future measurements, when he records the weight of products (please be noted that here ua is the recorded value, not the reading from the scale), he’ll subtract 0.1kg from the reading.

    Now let’s go back to the question: when shall the measuring equipment be calibrated, when shall they be verified, and when shall they be done both? The answer is given as below:

• If the measuring equipment is purchased, the suppliers have already done calibration, so the equipment shall be first verified before use. If it does not pass the verification (i.e. the judgement is “Not Acceptable), then it should be further calibrated.
• If the measuring equipment is homemade, the equipment shall be calibrated first. Once the calibration is done, for future use, it should be further verified, and calibrated if necessary, just as the purchased equipment.