In message <[email protected]>, "ce-test, qualified testing bv - Gert Gremmen" <[email protected]> writes
>That said; without GUM, no decent traceable internal calibration is >possible. So I suggest that you take a look at GUM first: >http://en.wikipedia.org/wiki/Measurement_uncertainty I feel very sceptical about this subject. Granted that it is necessary to consider how accurate a result is, but there seems to be a lot of disingenuous language designed to cover up that estimates of accuracy cannot be other than largely arbitrary, because the quantities involved are those which are inherently unknown - if they were known they would not be 'uncertainty'. Consider for example: Type A uncertainties are those that are evaluated by statistical methods, meaning 'guessed using numbers' and Type B uncertainties are evaluated by other means. meaning 'guessed without any numbers being available.' Because of the close association of 'uncertainty' with assessment and certification of laboratories, it is becoming ritualized, with implied presumptions that uncertainty is some sort of 'fall from perfection' or even 'evil'. It isn't; it's an inherent property of this Universe. When threatened with increasing demands for more and more complicated evaluations of uncertainty and/or pressure to reduce it, I suggest to bear in mind that the required accuracy of a result depends entirely on what its to be used for, We don't measure the dimensions of (agricultural) fields to the nearest millimetre, even though with a laser tellurometer, we could measure to even smaller units. I have coined the phrase 'measuring jelly (Jello) with a micrometer' about some EMC tests. This is particularly relevant to EMC, or any other measurements relating to limits. The permissible uncertainty isn't constant: close to the limit it's very small (one hopes), but far from the limit in can indeed be 'agricultural'. If I measure an emission as -20 dB referred to the limit, an uncertainty of +/- 19.5 dB could be tolerated. In practice, +/- 6 dB should be acceptable without question. Another example is sound pressure level. A change of 1 dB is just perceptible under instant comparison. With a 4-hour gap between presentations, some people cannot detect a 3 dB change. But an uncertainty of +/-1 dB is regarded as 'poor' in some circles. End of rant. -- This is my travelling signature, adding no superfluous mass. John M Woodgate - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
