Questions and Answers on Calibration

What is calibration?

Formally, calibration is a documented comparison of the measurement device to be calibrated against a traceable reference standard/device.

The reference standard may be also referred to as a “calibrator.” Logically, the reference standard should be more accurate than the device to be calibrated. The reference standard should also be calibrated traceably.

Formally the calibration does not include adjustment or trimming, although in everyday language it is often included.

What is traceability in calibration?

Formally, traceability is a property of the result of a measurement, through an unbroken chain of comparisons each having stated uncertainties.

In practice, traceability means that the reference standard has also been calibrated using an even higher-level standard. The traceability should be an unbroken chain of calibrations so that the highest-level calibration has been done in a National calibration center or equivalent.

So, for example, you may calibrate your process measurement instrument with a portable process calibrator. The portable process calibrator you used, should be calibrated using a more accurate reference calibrator. The reference calibrator should be calibrated with an even higher-level standard or sent out to an accredited or national calibration center for calibration.

What is calibration uncertainty?

Calibration uncertainty is a property of a measurement result that defines the range of probable values of the measurand.

Uncertainty means the amount of “doubt” in the calibration process, so it tells how “good” the calibration process was. Uncertainty can be caused by various sources, such as the device under test, the reference standard, calibration method or environmental conditions.

In the worst case, if the uncertainty of the calibration process is larger than the accuracy or tolerance level of the device under calibration, then calibration does not make much sense.

What is TAR and TUR in calibration?

In a calibration procedure, the test accuracy ratio (TAR) is the ratio of the accuracy tolerance of the unit under calibration to the accuracy tolerance of the calibration standard used.

In a calibration procedure, the test uncertainty ratio (TUR) is the ratio of the accuracy tolerance of the unit under calibration to the uncertainty of the calibration standard used.

We commonly hear about using a TAR ratio of 4 to 1, which means that the reference standard is 4 times more accurate than the device under test (DUT). I.e. the accuracy specification of the reference standard should be 4 times better (or smaller) than the one of DUT.

Why should you calibrate?

In industrial process conditions, there is various reason for calibration. Examples of the most common reasons are:

  • Accuracy of all measurements deteriorates over time
  • Regulatory compliance stipulates regular calibration
  • Quality System requires calibration
  • Money – money transfer depends on the measurement result
  • Quality of the products produced
  • Safety – of customers and employees
  • Environmental reasons
  • Various other reasons

How often you should calibrate?

A common question is how often should instruments be calibrated?

There is no one correct answer to this question, as it depends on many factors. Some of the things you should consider when setting the calibration interval are, but not limited to:

  • The criticality of the measurement in question
  • Manufacturer’s recommendation
  • Stability history of the instrument
  • Regulatory requirements and quality systems
  • Consequences and costs of a failed calibration
  • Other considerations

What is As found and As Left calibration?

The term “As Found” is used for the first calibration you make–the way you found the instrument. If there are errors found and you make an adjustment, then after the adjustment you make another calibration which is called the “As left” calibration– the way you left the instrument.

To summarize the process: Make “As Found” calibration – Adjust if necessary – Make “As Left” calibration.

What is a calibration certificate?

The definition of calibration includes the word “documented.” This means that the calibration comparison must be recorded. This document is typically called a Calibration Certificate.

A calibration certificate includes the result of the comparison and all other relevant information of the calibration, such as equipment used, environmental conditions, signatories, date of calibration, certificate number, the uncertainty of the calibration, etc.

What is Pass and Fail calibration?

Most often when you calibrate an instrument, there is a tolerance limit (acceptance limit) set in advance for the calibration. This is the maximum permitted error for the calibration.

If the error (the difference between DUT and reference) at any calibrated point is larger than the tolerance limit, the calibration will be considered as “failed.”

In the case of a failed calibration, you should take corrective actions to make the calibration pass. Typically, you will adjust the DUT until it is accurate enough.

Source: Beamex