A key aspect of implementing an accelerometer calibration system is understanding ISO 16063-21 (methods for calibration of vibration and shock transducers – vibration calibration by comparison to a reference transducer). This standard presents methods for measurements by comparison calibration for both laboratory and field conditions. Successfully selecting measurement instrumentation based upon this standard requires an understanding the resultant measurement uncertainties as influenced by the reader’s choices.
A common (mis) interpretation of ISO16063-21 is to construe the recommendations as a comprehensive list of requirements rather than the real world purpose: to provide requirements and helpful guidelines for good measurement equipment selection. The standard also includes best calibration practices that combine to provide a platform that can be used to achieve adequate measurement uncertainties. Due to the overlapping nature of optional configurations (Scenario A vs Scenario B) and generations of measurement hardware technology (RMS on DVM vs. FFT on DSA), as well as excitation (air bearing exciter vs flexure-based exciter), the evolved standard written from a perspective that does not inadvertently serve to constrain a practice to an aging measurement structure that can be reshaped quickly by changing technology.
This is why we see stated in Clause 3.3 of the standard:
“In practice, these limits may be exceeded depending on the uncertainty with which the reference transducer has been calibrated; the response characteristics of the reference transducer and the transducer to be calibrated; and the vibratory characteristics of the exciter and the instrumentation used in the measurement apparatus. It is the responsibility of the laboratory or end user to make sure that the reported values of expanded uncertainty are credible.”
The interpretation is straightforward: the uncertainty “requirements” are not rigid, but simply are an example as to what can be obtained with available vibration measurement equipment, environment and practices.
Sometimes there can be confusion with the standard stemming from the “…limits may be exceeded…” statement in Clause 3.3 and the "requirements" in Clause 4. Clarity can be achieved when a reader analyzes the statement in 3.3. This clause mentions the reference transducer's uncertainty contribution, the test and/or reference transducer's response characteristics, along with "...the vibratory characteristics of the exciter and the instrumentation used in the measurement apparatus." These items represent possible sources for the exceeding of the limits -- the limits which are mentioned in the examples given throughout Clause 4.
Clause 4 includes a list of required parameters for apparatus and environmental conditions and their respective impact on measurement uncertainty. These parameters are given with recommended specifications that should lead to the obtainable uncertainties given in Clause 3. These are, however, not required performance specifications.
As stated in clause 4.1:
“If the recommended specifications listed below are met for each item, the uncertainties given in Clause 3 should be obtainable over the applicable frequency range depending on the uncertainty with which the reference transducer has been calibrated, and the response characteristics of the reference transducer and transducer to be calibrated. Other combinations of requirements can, however, lead to the same uncertainty.”
This clause specifically connects Clause 3.3 with Clause 4, as: “It is the responsibility of the laboratory or end user to make sure that the reported values of expanded uncertainty are credible.”
Next month, we’ll examine how we might think about this consideration in more detail….