Medical Thermometers

Medical Thermometers

Medical thermometers are utilized to measure human body temperature, with the thermometer's tip inserted into various locations such as the mouth under the tongue (oral or sub-lingual temperature), under the armpit (axillary temperature), or into the rectum via the anus (rectal temperature).

Liquid-filled

The classic thermometer consists of a glass tube with a bulb at one end containing a liquid that expands uniformly with temperature changes. The tube, typically capillary in nature, features calibration markings along its length.

Mercury is often used in traditional thermometers, although some use colored alcohol. In medical settings, a maximum thermometer is commonly employed, indicating the highest temperature reached even after removal from the body. To take a reading, the bulb is positioned in the area where the temperature is to be measured and left until thermal equilibrium is reached, usually about three minutes. Maximum readings are obtained through a constriction near the bulb. As the bulb's temperature rises, the liquid expands up the tube through the constriction. Upon cooling, the liquid column breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube. After reading, the thermometer must be reset by briskly swinging it to shake the liquid back through the constriction.

Mercury

Mercury-in-glass thermometers have long been regarded as the most accurate liquid-filled thermometers. However, due to mercury's toxicity, it has only been used in clinical thermometers when the tube is protected against breakage.

The tube must be narrow to minimize the amount of mercury—the tube's temperature is uncontrolled, so it must contain much less mercury than the bulb to minimize the tube's temperature effect. This narrow mercury column makes reading somewhat difficult as it is not very visible, though this is less of an issue with colored liquids.

In the 1990s, concerns about the risks associated with mercury-based thermometers led to their replacement by electronic digital thermometers or, less commonly, thermometers using non-mercury liquids (such as galinstan, colored alcohols, and heat-sensitive liquid crystals).

Electronic

With the availability of compact and affordable temperature measurement methods, electronic thermometers (often called digital due to their numeric displays) have become prevalent. Many provide readings with high precision, but accuracy must be checked and maintained through periodic recalibration. The first electronic clinical thermometer, employing a flexible probe containing a Carboloy thermistor, was invented in 1954.

Contact

Electronic thermometers may operate by contact, with the electronic sensor placed in the measurement location until equilibrium is reached. They typically reach equilibrium faster than mercury thermometers, with some models emitting a beep or specifying the required time in the manufacturer's documentation.

Remote

Other electronic thermometers use remote sensing, with an infrared sensor responding to the radiation spectrum emitted from the measurement location. While they do not directly touch the area being measured, they may contact part of the body (e.g., an ear thermometer inserted into the ear canal to measure the eardrum's temperature). To prevent cross-infection, disposable probe covers and single-use clinical thermometers are commonly used in clinics and hospitals.