Specifications are used to quantify an instrument's performance and are often the main determinant in the selection of temperature equipment. Therefore marketers sometimes use a tactic termed 'specmanship' to artificially enhance their instrument's performance.
Specmanship is defined as presenting instrument specifications in a misleading fashion, and can come in many forms. The following are some of the most common seen in the temperature world.
Incomplete specifications
Completeness means providing all the information necessary for a user to form proper expectations of an instrument's performance over its intended range and for its intended purpose. It is usually not practical to define an instrument's specifications in every detail at every increment over its range (or for every imaginable application). However, this is no excuse for intentionally omitting or relocating critical specifications in an effort to give a false impression of instrument performance.
For example, temperature baths are generally used as a stable medium for comparison calibration of thermometers. For the user to make an educated decision, the manufacturer should, at a minimum, publish stability and uniformity over the full range of the bath. Some manufacturers only specify stability. Furthermore, the stability specification may only be given at one temperature point, which gives little indication of the instrument's performance over its full range. Some specifications of stability rely on (without explicit say-so) the use of additional equipment installed in the bath - such as an equilibration block - giving the impression that the bath, by itself, is more stable than it really is.
Make sure specifications provide all the information needed to adequately predict an instrument's performance over your intended range of use.
Specifications derived from single-unit evaluations
Specifications that are based on statistics from a significant sample of an instrument group may be usefully applied to the entire instrument group. Any individual piece of equipment may significantly outperform its peers in various aspects of performance, but this does not represent the entire instrument group. Some manufacturers specify their instruments based on single-unit evaluations. But how are these evaluations applicable to the unit any one customer will purchase?
All components have performance variation from unit to unit. A specification based upon a single unit ignores the performance variation of components as well as potential variations in manufacturing.
Be wary of specifications written around single-unit evaluations.
Misuse of the terms 'typical' and 'best performance'
Unfortunately, manufacturers are not bound by a common standard defining how to derive or present specifications. Ideally, manufacturers are adequately conservative when publishing specifications so that actual performance is better than the stated specifications. This should be done not only to provide a high level of confidence to the user when they validate the instrument's performance, but also to account for applications that may not be 'typical' or that do not fit the 'best performance' criteria.
Beware when critical specifications read 'typical' or 'best performance'. Unless 'typical' or 'best performance' are clearly defined or accompanied with a guaranteed spec, the user can have no reasonable level of confidence that their use of the equipment will yield said performance. Often these terms are stated so the manufacturer may publish the most attractive specification possible while trying to build a defensible position for when the instrument does not perform to the stated specification. Do not be caught by these terms, which exaggerate your instrument's performance.
This is not to say that such terms may never be used. The term typical may be used in literature to give users a general indication of performance on non-critical specifications that have several variables. For example, the length of time that a bath will maintain an ice mantle within a triple point of water cell will depend largely on how well the ice mantle is formed in the cell. In this case, using the term 'typical' proves useful in that it provides the user with an indication of performance when that performance is more within the user's control than the manufacturer's.
'Relative' temperature specifications
Another way manufacturers overstate instrument performance is by using the term 'relative'. Often this term is found in the footnotes, asterisks, or subscripts of specifications. For example, a dry-well manufacturer may specify the temperature range of a dry-well to be from '-40°C* to 125°C'. The asterisk explains that the temperature range is relative to ambient, meaning the temperature range is -17°C to 125°C when used at an ambient of 23°C.
Furthermore, some dry-well manufacturers craft the accuracy specification of built-in reference thermometers to appear as though it includes the errors of the electronics and reference standard. Yet, the fine print states that accuracy is relative to the reference standard. Users will find the combined error of the electronics and the reference standard to be much larger than what is stated in the specification.
Declaring specifications that are difficult to interpret or apply
Not enough can be said for clearly stated specifications that are easy to interpret and apply. Some specmanship artisans ambiguously represent specifications so customers are left to their own interpretation of the specification and how to apply it. Once a customer has clarified the 'true' meaning, he or she often finds the specification is not based on the general application - thus rendering it useless.
Properly written specifications should apply to the anticipated application. For example, when a manufacturer specifies the expanded uncertainty of fixed-point cells, the user would find it most useful to know the uncertainty of the cell when realised by traditional methods and when using traditional maintenance devices. In an effort to have their specs appear more favourable, however, a manufacturer might calculate the uncertainty of their fixed-point cells based only on cell impurity and state uncertainty in terms of a depression from the ITS-90.
Though a technically correct calculation, the uncertainty statement is not useful in any practical application as it is based upon a theoretical calculation and not on a verifiable measurement.
Conclusion
Specmanship exists in the temperature calibration markets today. We suggest a thorough examination of instrument specifications on completeness and application use.
Hart Scientific is a leading supplier of temperature calibration equipment to the world and services a wide variety of industries. It is represented locally by the sole authorised southern African distributor, Comtest.
For more information contact Comtest, +27 (0)11 695 9162, [email protected]
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