Basic Thermometry Concepts: Accuracy
One of the most basic attributes of any thermometer is its accuracy, and because a thermometer is only as good as the temperatures it takes, accuracy is of the utmost importance.
The National Institute of Standards and Technology (NIST) 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:
- An unbroken chain of comparisons going back to stated references acceptable to the parties, usually a national or international standard (i.e. NIST)
- 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. ± 0.7°F.)
- Documentation. Each step in the chain must be performed according to documented and generally acknowledged procedures, and the results must be recorded.
- Competence. The laboratories or bodies performing one or more steps in the chain must supply evidence for their technical competence.
The level to which accuracy could be measured took a major leap forward with the invention of electronic thermometers and digital displays. Anyone older than Generation X can likely remember peering at the liquid in a glass tube thermometer, or even the dial on a bimetal thermometer, and trying to determine which hashmark the measurement was closest to.
Sophisticated industrial and scientific processes have come to 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 can make it easy to determine temperature within a tenth of degree or less.
The potential for instruments to lose accuracy over time is sometimes called "drift." Drift in thermometers necessitates periodic calibration against standards. In the United States, NIST 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 affect of ambient temperature on the thermometer's own circuitry to determine a measurement with greater accuracy and reproducibility. But by separating the temperature sensor (the probe) from the temperature calculator and display (the 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" (see Basic Thermometry Concepts: Regulations & Calibration). Accuracy can also be variable over the full range of temperatures measured by a given thermometer (see Basic Thermometry Concepts: Range).