Understanding a calibration certificate
When a measuring instrument is sent to be calibrated, it is returned to the user with a calibration certificate. But what exactly is a calibration certificate? What information does it tell us? And how is this information used?
What is calibration?
In order to understand a calibration certificate, we must first be familiar with what the term ‘calibration’ means. The BIPM defines ‘calibration’ as the
operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication
This is a slightly cumbersome definition, but in simple terms ‘calibration’ can be defined as
the process which establishes, under specific conditions, the relationship between the values of quantities indicated by a measurement instrument and the corresponding certified values of standards – accompanied by an associated statement of uncertainty.
A ‘calibration’ takes a measuring instrument and uses it to measure a known quantity (be this a mass, temperature, length, density, pressure etc.) whose value is traceable to international standards (and certified as such).
To use an example from our own field, to calibrate a density meter one would take a certified density standard, put it into the density meter, and note the result. If the value of the standard was, say 998.200kg/m³, and the value indicated by the density meter was 998.305kg/m³, we would know that the meter was +0.095kg/m³ adrift from reality.
Once a calibration has been performed, the results are summarised in a calibration certificate.