Basic Thermometry Concepts: Accuracy

One of the most basic attributes of any thermometer is its accuracy. Accuracy is often expressed with an uncertainty specification—usually a plus sign over a minus sign in front of a number (e.g. ± 0.7°F). This number represents the outer limit of potential error for a given thermometer, probe or system (e.g. displaying a reading that is actually 0.7°F above or below the actual temperature of the target being measured). It is frequently a composite number, representing the total of all possible errors in a system.

Accuracy 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, as very 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.

Drift
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, a government agency called the National Institute for Standards and Technology (or NIST) is responsible for setting the standards by which the accuracy of instruments is 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).