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Thermometer: Clinical Laboratory Thermometer


Thermometers, in general, are devices used to measure the temperature of an object or the temperature gradient. The temperature gradient is a dimensionless quantity that measures the rate of change of temperature in any direction. Any thermometer has two very essential components. The first is a means by which the temperature of an object can be sensed. The other component is a scale that can help identify the numerical value of temperature. An Italian physician named Santorio is said to have invented the first working temperature in the 17 th century. Depending on how the thermometer measures the crude physical quantity and relates it with temperature, thermometers are classified into primary and secondary thermometers.

  • Primary thermometers: These kinds of thermometers can measure temperature without any unknown quantities. Some examples of primary thermometers are thermometers based on the equation of state of a gas, the velocity of sound produced in a gaseous medium, the thermal noise voltage or current of an electrical resistor etc.
  • Secondary thermometers: In comparison with primary thermometers, these kinds of thermometers are used more often owing to their convenience and also because secondary thermometers are relatively more sensitive.

Clinical Laboratory Thermometer

A clinical laboratory thermometer, also called a medical thermometer, is a device often used to observe the temperature readings of a human or an animal. The clinical laboratory thermometer was eventually derived from a device known as a thermoscope which was invented by Galileo Galilei. The thermoscope was also a device used to measure temperature, but it did not have an accurate temperature scale and was not precise in nature because external factors like changes in atmospheric pressure affected it. Daniel Gabriel Fahrenheit, a scientist from the Netherlands in 1709, made the alcohol-based thermometer and then eventually, in 1714, invented the mercury thermometer, which is still in use today. He established that using mercury instead of the originally used material was better because mercury had a lower boiling point than water. Hence, it was much more sensitive to temperature changes. He also created the Fahrenheit scale in 1724.

Classification by Location

The temperature reading of a person can be observed by placing the tip of a medical thermometer in several locations on the body like the mouth (under the tongue), under the armpits, inside the rectum, inside the ear and on the forehead. The measured temperatures at these different locations will not be the same but are often within a stable range. And because of this reason, when a temperature is noted, along with the value, the location on the body where the temperature was measured should also be furnished.

  • Oral: Temperature readings are taken this way only if the patient is able to hold the tip of the thermometer firmly under the tongue. If a hot or cold substance was consumed by a patient, sometime must be allowed to pass before a temperature measurement is taken.
  • Armpit: A thermometer must be held tightly under the armpit, otherwise known as the axilla, for a few minutes to get an accurate temperature reading. Axilla readings are generally less reliable compared with rectal measurements.
  • Rectum: Rectal temperature measurements are used predominantly only in the case of very young children. To avoid pain or uneasiness, it is suggested to use a water-based lubricant before taking temperature readings inside the rectum. Rectal temperature is the most accurate among the readings taken at other places.
  • Ear: The hypothalamus is the part of the brain that regulates the core temperature of the body. Owing to this, Dr Theodor H. Benzinger wanted to note down temperature values as close to the brain as possible and invented the ear thermometer in 1964. These thermometers are placed at the blood vessels in the ear canal. Although being extremely accurate in their reading, these thermometers can be unreliable at times because failure to place the probe of the thermometer exactly at the desired position in the ear canal will produce faulty readings.

Classification by Technology

Thermometers can further be classified into another set of categories based on the working principle involved.

  • Liquid filled: These consist of a glass tube with a bulb at one end containing a liquid that expands in accordance with the rise in temperature. Traditional thermometers used to use colored alcohol, but now mercury is predominantly used owing to its accuracy. However, if the bulb is broken accidentally, its mercury can pose dangerous hazards because mercury in nature is very toxic.
  • Digital: Electronic thermometers, also called digital thermometers, are much more accurate than conventional thermometers and display the temperature readings on a digital screen.
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