Becky Crossley, CBET, is a BMET specialist in the Biomedical Engineering Department at the University of Pittsburgh Medical Center Susquehanna-Williamsport in Williamsport, PA. Email:crossleyrk@upmc.edu

Becky Crossley, CBET, is a BMET specialist in the Biomedical Engineering Department at the University of Pittsburgh Medical Center Susquehanna-Williamsport in Williamsport, PA. Email:crossleyrk@upmc.edu

Temperature taking has been a key focal point during the COVID-19 pandemic. In fact, our biomedical engineering department has been asked to give guidance to nursing regarding the most accurate type of thermometer. This article will discuss the different types of thermometers and their accuracy.

Several different types of digital thermometers are available, including resistance temperature detectors (RTDs), thermocouple, thermistor, contact, and remote. Thermometers can also be classified by the location of the body from which temperature is taken, such as oral, armpit, rectal, ear, and forehead. The forehead type of thermometer can be temporal artery or plastic strip.

Although other types of thermometers exist (e.g., liquid filled, mercury, phase change, liquid crystal), we do not think of or use most of these today because of the pervasiveness of electronic thermometers.

One of the newest types of thermometers is the smart thermometer. In the future, we will likely see increased use of smart thermometers in healthcare facilities. However, given the scope of this article, I will cover the most common types of thermometers found in the hospital environment.

Normal temperature varies to a small degree based on the location of the body at which it is taken. Numerous studies have been conducted on variation in temperature at different body locations, and as one can imagine, much discussion has occurred in the medical field on this topic. I acknowledge the debate regarding variations in temperature readings at different body locations; however, rehashing these discussions is not relevant to this article. With that said, it's worth noting that the “gold standard” for temperatures readings is rectal (considered core temperature).Differences among body location temperature readings typically are about ±0.5°F/±0.3°C, and acceptable ranges in temperature exist for various locations. Also of note, various locations may involve factors, such as physical barriers, that impede the correct reading of temperature; this is not the fault of the thermometer. An example of this would be the taking of an unusually low temperature in the ear canal. Factors such as incorrect placement or ear wax build-up could account for this low reading.

Electronic thermometers help improve the accuracy of body temperature readings because, as most electronic thermometers have digital readings, they remove reader error from the equation. Electronic thermometers also increase the precision of guaranteed accuracy, which means that if the thermometer has been calibrated, the accuracy should be to a precision of ±0.1°F/±0.56°C. Electronic thermometers are smaller and less costly than other types of thermometer.

Different types of digital thermometers vary in terms of accuracy. Resistance temperature detectors (RTDs), which typically are built with thin platinum wires and have a serpentine shape, are devices that use the principle that the resistance of a metal changes with temperature. Platinum is the usual choice for the thin film used in RTDs because it is almost linear over a wide range of temperatures, making it very accurate. RTDs can also be made of copper and nickel. RTD thermometers are considered to provide stable results over long periods of time. They also are easier to calibrate. Some disadvantages of RTD thermometers are that they are more expensive and not very rugged. Unfortunately, these types of thermometers are not particularly common in the modern medical field. The closest example to this type of thermometer is the type used for cardiac outputs. However, this older, bulkier technology is not often preferred by medical personnel.

The most important considerations for obtaining accurate human temperature readings are the way in which a thermometer will be used, removal of nondevice factors such as physical barriers, and periodic device calibration.

The most important considerations for obtaining accurate human temperature readings are the way in which a thermometer will be used, removal of nondevice factors such as physical barriers, and periodic device calibration.

Given their high sensitivity, thermocouple thermometers respond quickly to small changes in temperature. These thermometers have two wires that are made of different metals and are joined together. A thermoelectric voltage is generated between the wires, and the size of the opening correlates to the difference in temperature. These materials tend to corrode over time, thereby affecting their accuracy. These types of thermometers are used in sterilizers and in certain laboratory applications.

Thermistor thermometers are the most sensitive type. These are semiconductor devices with a resistance that corresponds to a temperature. The temperature inversely relates with the resistance, meaning that when the temperature increases, the resistance decreases. Thermistors are small and fairly stable. These devices are not a good choice for use in temperature extremes. Examples of thermistor thermometers in the medical setting include esophageal, foley catheter, and skin surface temperature thermometers.

Contact thermometers work exactly as their name implies. A sensor comes in contact with the area where a temperature reading is needed. They are left in place until temperature becomes stable. These thermometers are much faster than the mercury type of years past. The majority of these thermometers will emit a beep sound when stability has been reached. Contact thermometers are the most common type used in the medical field, as well as the type most often found in people's homes.

Remote thermometers are also known as infrared thermometers. Infrared thermometers acknowledge the radiation spectrum emitted from the body. This is also known as the fraction of a body's energy that is emitted as ultraviolet radiation. Medical examples of these types of thermometers are the ear drum type or forehead type.

As one can see, a great many types of thermometers exist. Thermometer technology has evolved to enable ease of use and to be less intrusive. A number of factors can affect the accuracy of these devices, including the manner in which they are used, physical barriers, and calibration. Rather than choosing a thermometer based on its brand and/or technology, the most important considerations for obtaining accurate human temperature readings are the way in which the thermometer will be used, removal of nondevice factors such as physical barriers, and periodic device calibration.