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Thermometers are the first medical instruments that we encounter as children. It is important to measure temperature as it indicates underlying infections. Our understanding of what a normal temperature is has now changed; the time-tested 98.6°F has been challenged and normal temperature is now thought of in terms of a range between 97-99°F. A temperature of over 100.4°F is considered a fever, and the range between 99-100°F is considered a low grade fever.

Read about normal body temperature.

A thermometer is a device that measures the temperature of an object. There are many types of thermometers that you can use to check your temperature, and a big distinction is between mercury and digital thermometers. The former has now fallen out of favour because of the dangers of mercury and the convenience and speed of digital thermometers. 

It is important to know how to accurately measure your temperature, as incorrect readings can lead you to take the wrong steps. 

Read about home remedies for fevers.

Here is a list of different types of thermometers and how to use them.

  1. Digital thermometers and how to use them
  2. Non-contact Infrared Thermometers (NCIT)
  3. Tympanic thermometers
  4. Temporal Artery Thermometers
  5. Mercury thermometers
  6. Other types of thermometers
Digital thermometers and how to use them

Digital thermometers are the most commonly used ones in homes. This is because they are cheap, easy to use and store. You can take temperature either orally (the mouth), through the rectum, or the armpit (axillary). 

Axillary temperature readings are considered the least accurate but are handy if the oral or rectal routes can’t be used. The rectal method is most commonly used in infants under 3 months of age, and often used in children under 3 years of age. This is because infants may not be comfortable with the probe in their mouth for too long - the mouth needs to be closed as the temperature gets recorded. 

Here are the steps to accurately take temperature using a digital thermometer. 

Oral method

  • Wash your hands before handling the thermometer.
  • Clean the thermometer with cool water, sterilize with rubbing alcohol, and wait for it to dry.
  • Make sure you haven’t eaten or had a beverage in the last 5-10 minutes as this can alter the readings.
  • Turn on the thermometer, and wait for it to calibrate. Usually the display will read ‘Low’ to signal readiness.
  • Place the tip of the thermometer under the tongue, close your mouth and do not speak.
  • Wait for the thermometer to beep - this will indicate that the temperature reading is complete.
  • You can retake the temperature reading in some minutes to make sure it is accurate.
  • Repeat the cleansing procedure and return the thermometer to its case. 

Rectal method

Never use the same thermometer for oral and rectal use. Buy separate gadgets and clearly mark their purpose. Here are the steps to take rectal temperature.

  • As always, make sure the thermometer is clean. Rinse with water and use rubbing alcohol.
  • Place a towel under the person if the probe triggers a fecal reaction.
  • Put some vaseline or lubricant on the tip of the probe to ensure that it doesn’t hurt on insertion.
  • Gently insert the thermometer into the anus, and do not force it at all. Stop if there is resistance.
  • Hold the device in place until there is a beeping sound.
  • Thoroughly wash the thermometer and return it to its case.

Axillary method

If the rectum or mouth is not available or a convenient option, you can use the armpit as well. Note that this method is the least accurate, but is still useful in giving an approximation of the temperature. Here is how to use it. 

  • Again, clean the thermometer and rinse it before taking a reading.
  • It is not necessary to take off your shirt, but you can.
  • Place the tip of the thermometer in your armpit.
  • Put your arm across your chest so that the thermometer is held in place. 
  • Wait for the beep and then check the temperature.
  • Clean and put the thermometer back in its case.

Remember to keep your armpit dry when taking a reading. Also remember that you should add 0.5°F to 1°F to readings taken from the oral or axillary route.

It is also important to keep your digital thermometer stored safely. Make sure to store the device in its packaging, and keep it away from direct sunlight. Further, make sure the batteries are working properly - low battery levels will muddle readings and alter the functioning of the device.

Non-contact Infrared Thermometers (NCIT) have entered the limelight since the global pandemic caused by the novel coronavirus infection took over the world. Since they don’t require physical contact, the obvious advantage is the device’s role in reducing transmission of an infectious disease. Further, they are easy to use, clean, disinfect and are very fast in displaying readings. 

Read some tips on maintaining social distancing, and safe ways to engage socially during COVID-19.

NCITs work by recording the heat signatures of the body they are pointed at. Here is how to use an NCIT:

  • Point the device directly at the person’s forehead, and make sure it is not obstructed by clothing.
  • The instruction manual of the instrument will tell you the right distance to take the reading from - just make sure to not touch the person. 
  • The reading will be displayed on the screen of the thermometer. 

You need to take care of some conditions before using NCITs: 

  • Ideally, the forehead should not be exposed to direct sunlight when the temperature is being measured.
  • The thermometer works best in calm conditions - windy conditions and places with a lot of draft can throw off readings, as can places with other sources of heat.
  • The device should also be in the environment for around 30 minutes before being used so that it gets calibrated. This will vary by product so make you check beforehand.
  • Certain cosmetic products, or excessive head coverings such as bandanas can also alter readings.
  • The thermometer may be less accurate in extremely hot conditions - usually hotter than 40°C. Again, check device information before using it.

NCITs are very useful when a large number of people need to be checked in quick succession, and in reducing transmission risks. They are likely to become a staple in our lives as we live with the pandemic. Make sure you read the instructions carefully and correctly calibrate the device before use. 

Tympanic thermometers

These are digital ear thermometers that use infrared rays to measure the temperature of the ear canal. Tympanic thermometers are useful if the oral route is a problem - they are fast and particularly popular with families with young children. Their use is not recommended for infants under three months of age however. Here is how to use them:

  • Make sure the probe has been cleansed and rinsed before use.
  • Pull the ear and push it back slightly to open up the ear canal.
  • Gently insert the probe.
  • A beeping sound will alert you when the temperature is recorded.

Tympanic thermometers are less effective if there is a high amount of earwax, so make sure your ears are clean before using the device.

Here are some tips to remove earwax safely

Temporal Artery Thermometers

Temporal artery, or forehead thermometers, use infrared heat to take a temperature measurement from the forehead. Unlike NCITs, they need to touch the forehead, so they are contact thermometers. 

While they deliver rapid results and are easy to clean, they may not be as accurate as classical digital thermometers. Further, they are much more expensive than regular thermometers which means they are not used as often.

Again, for those who resist or are unable to follow the oral or rectal approaches, this could be a suitable replacement.

Mercury thermometers

Glass mercury thermometers were commonly used before and were a part of every household. Mercury rises in contact with warm temperatures and this basic principle was used in calibrated thermometers. However, mercury thermometers are no longer recommended because mercury is poisonous - if the glass breaks it can be ingested. The glass also makes them more vulnerable and a constant safety hazard. They can also be a little challenging to read. 

Since digital thermometers are now easily available, they have made the mercury ancestors obsolete. 

Using mercury thermometers is largely the same as using digital ones vis a vis the cleaning and inserting procedures. Keep mercury thermometers away from children and try to minimize fall hazards as much as you can. 

To reset mercury thermometers, shake the thermometer vigorously so that the mercury can go back down by overpowering the kink holding it in place.

Other types of thermometers

For babies, there is another type of thermometer known as pacifier thermometer. Pacifier thermometers are pacifiers with sensors in them that record temperature. This is a good approach with babies since they are fond of pacifiers and therefore not averse to the device. However, the pacifier needs to be in for 5 minutes, and studies have shown that they are not as accurate as alternatives. They are generally not recommended for this reason. 

There are also plastic strip thermometers - but again, these are not recommended since they are not considered accurate enough. They also just measure the temperature of the surface of the skin.


  1. FDA [Internet]. US Food and Drug Administration Non-contact Infrared Thermometers
  2. NHS [Internet]. National Health Services; How do I take a temperature?
  3. EPA [Internet]. United States Environmental Protection Agency; Mercury Thermometers
  4. Mayo Clinic [Internet]. Mayo Foundation for Medical Education and Research (MFMER). Rochester, MN; Thermometers: Understand the options
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