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How does a digital thermometer work?

Updated April 17, 2017

Caregivers---from the home to the hospital and everywhere in between---use fever thermometers to measure internal body temperature. Many of them have come to rely on digital thermometers to give them a fast, accurate reading of a patient's body temperature.

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Older, glass tube thermometers use the tendency of matter to expand as temperature rises (thermal expansion) to determine temperature; their space age digital decedents rely instead on a "thermistor" to determine temperature by measuring electrical resistance.

The thermistor in a digital thermometer acts as a as a temperature-sensitive electric resistor. At low temperatures, a thermistor will not conduct electricity, but as its temperature rises, the thermistor's state changes and it becomes more and more conductive. Thermistors used in digital fever thermometers undergo this change in conductivity (resistance) at temperatures near 98° F.

In a glass thermometer, mercury or alcohol contained in a narrow glass tube expands as the temperature increases. The caregiver determines the temperature by matching the height of the fluid to a scale etched into the outside of the glass.

A simple and inexpensive computer (called a microcontroller) inside the thermometer determines the thermistor's temperature by measuring its electric resistance. The microcontroller then displays the temperature on a liquid crystal-based screen.

Digital thermometers offer users significant benefits compared to the older glass tubes. They provide an easy-to-read, LCD display, while glass tube thermometers often require the caregiver to estimate a final temperature.

A digital thermometer generates a temperature within a minute, making taking a temperature easier for both the patient and the caregiver.

Unlike glass, digital thermometers won't shatter if dropped; a safety measure for equipment often used around barefooted patients.

Almost any store (both brick-and-mortar and on the internet) that sells home health care products will offer inexpensive digital thermometers for sale.

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About the Author

Steve Gross has been writing professionally for 35 years. Gross holds a Bachelor of Arts in English and a Bachelor of journalism and Master of Arts in journalism from the University of Missouri. He edited and published "DuPont’s Journal of Teflon®" and "Engineering Design Magazine" for 23 years. He founded "Seaside Properties" real estate magazine in 1991 and now works as a full-time freelancer.

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