Many tests require the data to be validated by a certificate of calibration, this can easily and quickly become an expensive overhead. So, the first question, is it necessary? The answer is subjective and probably not. We will explore this answer later in the blog but to do this we have to understand calibration, so what is it?
Calibration is the comparison of two measurements obtained in a similar way under similar conditions. One measurement of known magnitude or correctness and the other “the unit under test” or item requiring calibration.
Traditionally, if you wanted to ascertain if an instrument was accurate within its specification, say +/-1%, you would compare the measurement with that taken by a superior instrument previously calibrated to +/0.25%, giving an uncertainty ratio of 4:1 These days with more sophisticated electronic systems it is normally 10:1
So, if I we need to calibrate a 50N force gauge in tension at two relevant fixed points, we could acquire a 1Kg and 4Kg weight certified accurate to 0.1%, 1gm and 4gms respectively, suspend them individually from the gauge and take readings. If the indicated readings were within 990gm and 1010gm for the 1Kg weight, then between 3960gm and 4040gm for the 4Kg weight, then we could realistically say thet the force gauge was calibrated, of course quoting the test procedure and identity and certification (traceability) of the weight used.
One can see, that this actually is only an indication, as we have only examined 2 fixed points and a nonlinear measurement function could have significant error mid scale at 3Kg while not being made visible by the calibration check. Now comes the debate, how many calibration points required, and increasing cost of equipment (calibrated weights) and length/ difficulty of the procedure. Costs escalate as the measurement range increases, as well as logistics and safety of using larger weights. hence the gravitation to comparison of test instrument with a “standard” instrument and automating the process. In the next blog we will examine the use of the DFTS5000i and LabView application to self calibrate.