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Correctly understanding and comparing instrument specifications

Fluke (UK) : 12 November, 2012  (Technical Article)
Since specifications are based on the performance statistics of a sufficiently large sample of instruments, they describe group behaviour rather than that for a single, specific instrument. Fluke has provided an application note to explain the implications.
Correctly understanding and comparing instrument specifications

The initial selection of prospective equipment is usually based on its written specifications. They provide a means for determining the equipment’s suitability for a particular application. Specifications are a written description of the instrument’s performance in quantifiable terms and apply to the population of instruments having the same model number.

Good specifications have the following characteristics:

  • complete for anticipated applications easy to interpret and use
  • define the effects of environment and loading

Completeness requires that enough information be provided to permit the user to determine the bounds of performance for all anticipated outputs (or inputs), for all possible environmental conditions within the listed bounds, and for all permissible loads. It is not a trivial task even for a simple standard. Designing complete specifications for a complex instrument like a multifunction or multi-product calibrator is a big challenge.

Other considerations

Uncertainty specifications are an important part of determining whether or not a particular calibrator will satisfy the need. There are, however, many other factors that determine which instrument is best suited for an application. Some of these are listed as follows.
The workload: Remember that the instrument’s specification must match the workload requirements. There is a tendency for manufacturers to engage in a numbers race, with each new instrument having more and more impressive specifications, although often this has little bearing on true workload coverage.

  • Support standards: The support standards will typically be three to ten times more accurate than the instruments supported. This is known as the test uncertainty ratio (TUR). Specialized instruments or those that require exotic support equipment on an infrequent basis may best be served by an outside service bureau.
  • Manufacturer support: One should also consider the level of manufacturer support. Can the manufacturer provide support as calibration needs grow and vary? Do they have in-house experts who can assist with technical issues? Are training programs available? Are service facilities conveniently located? Do they have an adequate line of support products and accessories?
  • Reliability: Reliability is another important consideration in how useful an instrument will be. Precision electronic instruments can have a seemingly high failure rate. Any condition that causes the instrument to fall outside its extremely tight tolerance constitutes a failure. One should ask for a Mean Time To Fail rate (MTTF) to determine when the first failure might occur. Failures upon delivery usually make this interval shorter than the Mean Time Between Failure rate (MTBF). Whichever is quoted, consider whether the number is based on actual field experience or just calculated projections.
  • Service philosophy: When an instrument does fail, the manufacturer’s approach to service becomes very important. A responsive service organisation is essential to getting equipment back in action fast. Issues to consider include service center locations and one’s proximity to them, stocking levels for spare parts and subassemblies, availability of service manuals, and service training for one’s own technicians, all of which go into determining how soon equipment can be returned to service.
  • Reputation: Finally, the manufacturer’s reputation should be assessed. Overall, how credible are its claims with respect to performance, reliability, and service? Will the company still be around five years from now? All of these issues define the true cost of owning and using a calibrator.

This application note has been adapted from “Chapter 31: Instrument Specifications,” in Calibration: Philosophy in Practice, Second Edition.

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