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| How To Interpret Instrument Specifications | |
This section presents information which should help you in interpreting the specifications of our instruments as well as those of others. All companies do not specify performance in the same way. And some companies actually are incorrect in the way they do specify their instruments. For example the resolution and accuracy of an instrument are generally much different than each other, yet some companies will present the resolution of the instrument as the accuracy. Or they may specify the basic resolution of the A/D converter as the resolution of the instrument. The true resolution of the measurement must take into account noise in the signal and other factors. For example, measurements with 24 bit A/D converters may typically be only 21 or 22 bits at best. This is 4 (22 bits) or 8 (21 bits) times worse resolution in the measurement than could be attained with a true resolution of 24 bits. We suggest that you really do not need to understand everything or anything about bits, counts, % of full scale, or ppm in order to determine if a particular instrument is suitable for your needs. If you are looking for accuracy in temperature measurements and are having trouble interpreting a company's specifications, then simply ask them what is the accuracy (in units of temperature, °C or °F) of the measurement obtained by their instrument and their connected probes or sensors in the temperature range you are interested in. The following definitions and commentary are intended to assist you in understanding the details of instrument specifications: Accuracy : Measurement accuracy is a measure of the measured value relative to the true value. At GEC Instruments we specify temperature measurement accuracy in direct units of temperature, for example ±0.1 °C. Actually the term accuracy implies inaccuracy. That is, an accuracy specification of ±0.1 °C is really saying that the inaccuracy, or the error, of the measurement is less than ±0.1 °C. Some companies specify accuracy of temperature instruments as % of reading, or as % of reading plus % of full scale. These are generally specifications of the voltage measuring device and do not include other factors which affect the temperature measurement accuracy. Such specifications can be confusing or misleading when applied to temperature instruments. For example an accuracy specification in terms of % of reading applied to a measurement at 0 °C would imply no error at this temperature. We know this is not possible. The same specification applied to 32 °F (which is the same as 0 °C) would result in an error number. Every thermocouple circuit has a reference junction temperature which must be accounted for in the measurement. Our specifications for thermocouple instrument accuracy include all factors which affect accuracy. These include the accuracy of the thermocouple voltage measurement, the accuracy of the voltage to temperature conversion and the accuracy of the reference junction. Resolution : The smallest detectable change that a system can measure. Our thermocouple instruments have a solid resolution of ±0.01 °C or better. We have thermistor based instruments with resolutions of 0.0001 °C at 0 °C and 0.0002 °C at 60 °C. Sensitivity : The smallest quantity that can be measured and displayed. Same as resolution. Repeatability : The relative ability to display the same reading when the input is not changing. Stability : The amount that the accuracy of the reading may be expected to change. Stability may be specified as short term or long term. Stability over a very short period would be the same as repeatability. Drift : Generally refers to long term stability. Uncertainty : An estimate of the possible deviation of a reading from its true value. Related to accuracy but includes considerations of statistical probability. GEC Instruments is located in Gainesville, Florida, USA and can be reached |
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