A datasheet may state a very fine resolution without every measurement being accurate to that level. When selecting a displacement sensor, read the specifications together and check the conditions under which they apply. Terminology matters as much as the figures.
The distinctions below draw on the International Vocabulary of Metrology, or VIM. They help you ask the right questions before choosing a wireless crack monitor.
01 Resolution describes a distinguishable change
Resolution concerns the smallest change in the measured quantity that produces a perceptible change in the indication. It depends on the system and the measurement conditions. The number of decimal places displayed does not, by itself, establish the resolution that can usefully be achieved on site. S4
Consider a purely illustrative example: a display might change in steps of 0.001 mm while other error contributions are several hundredths of a millimetre. Three decimal places do not remove those contributions or the effects of an unstable mounting arrangement. This example does not describe an ADVARNA product.
02 Distinguish accuracy from precision
French commercial datasheets may use “précision” for a stated performance value or interval. Ask what that figure covers: a specified maximum error, linearity, the performance of an assembly or another characteristic. It should not automatically be translated as the metrological term “precision”.
The VIM distinguishes accuracy, concerning agreement with a true value, from precision, concerning agreement between repeated measurements under specified conditions. Accuracy is a qualitative concept. The English term “measurement precision” corresponds to the French “fidélité” and must not be confused with accuracy. S1, S2
03 Repeatability concerns scatter
Repeatability is precision under repeatability conditions, including the same procedure and measuring system, the same location and a short time period. Closely grouped results indicate low scatter under those conditions. S3
However, they may still be offset from a reference value. Conversely, taking readings over several months, at changing temperatures or after altering the installation, does not automatically constitute a repeatability test.
04 Uncertainty accompanies a result
Measurement uncertainty characterises the spread of values that could be attributed to the measured quantity, based on the information available. Its evaluation may consider several contributions: calibration, resolution, thermal behaviour, method and installation. It cannot be inferred from resolution alone. S5
A stated uncertainty needs the information required to interpret it, particularly when it is an expanded uncertainty. Ask which measurement and conditions it applies to. A commercial specification labelled “accuracy” is not automatically a complete uncertainty budget.
05 Compare like with like
First establish whether the figure covers the sensor, the acquisition module or their combination. Then check the units and the basis of any percentage: full scale or reading. These have different implications when the measurement range changes.
For another illustrative example, 0.1% of a 100 mm full-scale range is 0.1 mm. That calculation does not turn a linearity specification into total measurement uncertainty. It simply expresses the percentage in a comparable unit. No product-performance claim is derived from this example.
06 Consider the installed measurement chain
A well-characterised sensor cannot compensate for a support that moves independently of the phenomenon being monitored. Mounting, alignment and thermal conditions must suit the measurement objective. Our guide to preparing a crack sensor installation explains these points.
Before approving a configuration, bring together the sensor reference, usable range, environmental conditions, available verification documents and the characteristics of the associated module. For OBSESSIO X1 and X2, consult the ADVARNA configuration comparison and the corresponding technical datasheets.
07 Ask questions that prevent misleading comparisons
What exactly does the stated figure mean? Does it include the displacement probe? Under which conditions was it established? Which contributions remain installation-dependent? Does a certificate cover the supplied equipment or a different configuration?
The answers connect specifications to your requirements. They provide a more useful basis for selection than ranking sensors solely by the smallest number in a table.
Clarify my measurement configuration