Thermometry Concepts: Thermometer Accuracy

Any thermometer is only as good as the temperatures it takes, so thermometer accuracy is of the utmost importance.


The United Kingdom Accreditation Service  (UKAS) provides a way for calibrated thermometers and their temperatures to be traceable to a national standard, thereby giving the user a guarantee of accuracy.


Traceability is characterized by several essential elements, including:

1. An unbroken chain of comparisons going back to stated references, usually a national or international standard (i.e. UKAS)
2. Measurement uncertainty. The uncertainty of measurement for each step in the traceability chain must be calculated or estimated according to agreed methods and must be stated so that an overall uncertainty for the whole chain may be calculated or estimated. (This is represented as a plus sign over a minus sign in front of a number (e.g. ± 0.5° C).
3. Documentation. Each step in the chain must be performed according to documented and generally acknowledged procedures, and the results must be recorded.
4. Competence. The laboratories or bodies performing one or more steps in the chain must supply evidence for their technical competence (e.g. by demonstrating that they are accredited). The calibration facilities at ETI are UKAS accredited calibration labs.



The level to which accuracy could be measured took a major leap forward with the invention of electronic thermometers and digital displays. No more was it necessary to peer at the liquid in a glass tube thermometer, or a dial to try and discern which mark the measurement was closest to.


Now we depend upon very accurate temperature measurements. Slight increases or decreases in temperature can have profound effects upon the growth of bacteria, the pliability of plastics, the interaction of chemicals, the health of a patient, etc, and electronic thermometers with digital displays make it easy to measure temperature within a tenth of degree or less.


What is Drift?

Drift is the  potential for instruments to lose accuracy over time. Drift in thermometers necessitates periodic calibration against standards. In the United Kingdom, UKAS is responsible for setting the standards by which the accuracy of instruments are checked and reset.


Electronic thermometers with computer circuitry can sometimes perform very complex calculations, factoring in such things as the effect of ambient temperature on the thermometer's own circuitry to determine a measurement with greater accuracy and reproducibility. But by separating the temperature sensor or probe from the temperature calculator and display (or meter) into distinct devices, they also introduce the possibility of additional error.


With mechanical thermometers like liquid thermometers and dial thermometers, the display is directly manipulated by the physical properties of the temperature sensor itself, the expansion of the liquid or bimetal coil. Dial thermometers need frequent weekly, if not daily recalibration, but they only need one calibration at a time.


Electronic thermometers, particularly those that take interchangeable probes, may only need to be calibrated once a year, depending upon use, but both the probes and the meter should be calibrated for accuracy. Electronic thermometers and probes that are calibrated together can often mitigate against the potential for composite errors. Such probe/meter calibrations are said to be system calibrated Accuracy can also vary over the full range of temperatures measured by a given thermometer. 




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