Kinds of Medical Thermometers
MEDICAL THERMOMETER Medical thermometers are used for measuring human body temperature, with the tip of the thermometer being inserted either into the mouth under the tongue (oral or sub-lingual temperature), under the armpit (axillary temperature), or into the rectum via the anus (rectal temperature).
CLASSIFICATION BY TECHNOLOGY
Liquid-filled
The traditional thermometer is a glass tube with a bulb at one end containing a liquid which expands in a uniform manner with temperature. The tube itself is narrow (capillary) and has calibration markings along it. The liquid is often mercury, but alcohol thermometers use a colored alcohol. Medically, a maximum thermometer is often used, which indicates the maximum temperature reached even after it is removed from the body. To use the thermometer, the bulb is placed in the location where the temperature is to be measured and left long enough to be certain to reach thermal equilibrium—typically three minutes. Maximum-reading is achieved by means of a constriction in the neck close to the bulb. As the temperature of the bulb rises, the liquid expands up the tube through the constriction. When the temperature falls, the column of liquid breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube. After reading the value, the thermometer must be reset by repeatedly swinging it sharply to shake the liquid back through the constriction.
Mercury
Mercury-in-glass thermometers have been considered the most accurate liquid-filled types. However, mercury is a toxic heavy metal, and mercury has only been used in clinical thermometers if protected from breakage of the tube. The tube must be very narrow to minimize the amount of mercury in it—the temperature of the tube is not controlled, so it must contain very much less mercury than the bulb to minimize the effect of the temperature of the tube—and this makes the reading rather difficult as the narrow mercury column is not very visible.
CLASSIFICATION BY TECHNOLOGY
Liquid-filled
The traditional thermometer is a glass tube with a bulb at one end containing a liquid which expands in a uniform manner with temperature. The tube itself is narrow (capillary) and has calibration markings along it. The liquid is often mercury, but alcohol thermometers use a colored alcohol. Medically, a maximum thermometer is often used, which indicates the maximum temperature reached even after it is removed from the body. To use the thermometer, the bulb is placed in the location where the temperature is to be measured and left long enough to be certain to reach thermal equilibrium—typically three minutes. Maximum-reading is achieved by means of a constriction in the neck close to the bulb. As the temperature of the bulb rises, the liquid expands up the tube through the constriction. When the temperature falls, the column of liquid breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube. After reading the value, the thermometer must be reset by repeatedly swinging it sharply to shake the liquid back through the constriction.
Mercury
Mercury-in-glass thermometers have been considered the most accurate liquid-filled types. However, mercury is a toxic heavy metal, and mercury has only been used in clinical thermometers if protected from breakage of the tube. The tube must be very narrow to minimize the amount of mercury in it—the temperature of the tube is not controlled, so it must contain very much less mercury than the bulb to minimize the effect of the temperature of the tube—and this makes the reading rather difficult as the narrow mercury column is not very visible.