A Tool for Data Validation
For any testing in which the environmental operating conditions of a transducer vary with time and/or location, several requirements must be fulfilled before measurement uncertainty analysis is justified. Included among the requirements are good measurement system design practices, such as adequate low- and high-frequency response and data-sampling rates, appropriate anti-aliasing filter selection, proper grounding and shielding and much more.
In addition to these requirements, data validation must be performed to establish that each transducer responds only to the environmental stimulus for which it is intended. For piezoelectric and piezoresistive transducers, "placebo" (IEST-RP-DTE011.1) transducers enable data validation to be accomplished. The referenced IEST standard defines a placebo transducer as 'identical to a "live" unit in every parameter except for mechanical sensitivities.' The placebo transducer should respond only to extraneous "environmental factors." Ideally, its output would be zero. Any signal output from it would indicate that signals from the "live" transducers could be corrupted.
Every transducer responds to its environment in every way it can. For example, accelerometer specifications include their response to thermal, acoustic, strain and radiation stimuli, to name a few. While accelerometers must have their response to acoustic pressure specified, pressure transducers must have their response to acceleration specified. Thus, one transducer's desired response becomes another's undesired response.
These undesired responses can cause a change in transducer sensitivity or can result in additive, spurious signals at the transducer's output attributable to thermoelectric, electromagnetic, triboelectric and other self-generating noise phenomena. Since the test or instrumentation engineer has the best understanding of the test environment, he/she becomes responsible for data validation. The transducer manufacturer can assist by supplying "placebo" transducers to support this validation process.
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