Clinical Thermometer

What is called a Clinical Thermometer?

Clinical thermometer is used for medical treatment purposes to measure the body temperature at different locations of the patient’s body.

A clinical thermometer may be of following types –

  1. Liquid thermometers – It may be a Mercury thermometer or Alcohol thermometer. It may be used as oral or rectal thermometer.
  2. Infrared thermometer – It is a contactless thermometer for measuring temperature from a distance. It is either a forehead thermometer or tympanic membrane thermometer.
  3. Digital thermometer – It is a resistance thermometer. It can be used as oral or rectal thermometer.

Human Body Temperature

Human body normal temperature for an adult person is found around 98.6 \degree F \left ( 37 \degree C \right ) .

The body temperature varies according to the following factors –

  1. Age of the person. It is found that an older adult person has low body temperature.
  2. Physical activeness of the person. A physically active person has high body temperature.
  3. Health and medical conditions. People having hypothyroidism have lower body temperatures and people suffering with cancer have higher body temperatures.
  4. Body temperature also changes throughout the day. In the evening, body temperature increases.

Children Body Temperature

Children have higher body temperatures in comparison to the adults. The average body temperature for children is found 97.2 \degree F \left ( 36.4 \degree C \right ) .

Infants have higher body temperatures than older children and adults. The average body temperature for newborn babies are around 99.5 \degree F \left ( 37.5 \degree C \right ) .

Infants body temperature is higher because they have higher ratio of body surface area to body weight. Also, their body are more metabolically active which generates heat.

Cause of change in Body Temperature

Human body temperature is controlled and regulated by brain. Hypothalamus is a part of human brain which regulates the body temperature. If the body temperature rises or falls below the normal range of 98.6 \degree F \left ( 37 \degree C \right ) , the hypothalamus comes to action to regulate this change in temperature.

  • If the body temperature is lower than normal range, the hypothalamus sends signals to make the body shiver, which warms up the body to bring the temperature normal.
  • If the body temperature goes high to normal range, it sends messages to begin sweating to relieve from body heat to bring the temperature normal.

Poor medical conditions and infections causes most fevers. A fever develops as the body immunization system fight against germs causing infections.

Human body temperature is said to be abnormal or it is called a condition of fever, when the body temperature is found

\ge 100.4 \degree F \left ( 38.0 \degree C \right ) – it is termed as “fever”.

\ge 103.1 \degree F \left ( 39.5 \degree C \right ) – it is termed as “high fever”.

\ge 105.8 \degree F \left ( 41.0 \degree C \right ) – it is termed as “very high fever” called hyperpyrexia. This can lead to permanent organ damage and death. Hyperpyrexia requires urgent medical assistance.

Types of Clinical Thermometer

Clinical Thermometer can be classified into following categories depending upon the place of use for medical purposes. These are –

  1. Oral Thermometer.
  2. Forehead Thermometer.
  3. Infrared Thermometer.
  4. Tympanic Membrane Thermometer.
  5. Rectal Thermometer.
  6. Digital Thermometer.

The above thermometers are explained as under –

1. Oral Thermometer

Oral thermometers are most common clinical thermometer used in hospitals to take the temperature inside of mouth. The bulb of the thermometer is placed under the tongue to take the body temperature. Oral thermometers may be of type –

  1. Mercury thermometer.
  2. Hicks mercury free thermometer.
  3. Alcohol thermometer.
  4. Digital thermometer.
  5. Infrared thermometer.

Oral thermometers has following features –

  • Most common oral clinical thermometer is the Hick’s mercury thermometer.
  • Modern oral thermometers are digital thermometers. They usually gives sound beep when the reading is stable which makes them user’s friendly for most domestic purposes.
  • Oral thermometers are easier to use in adults. This should be placed under the tongue and close the mouth and keep the thermometer in place for about 20 seconds to get an accurate reading.

A typical Hicks clinical thermometer is shown in figure.


2. Forehead Thermometer

Forehead thermometer is a kind of infrared thermometers. It is a good option for use in children, as it gives quick reading and children do not require them to sit still for a long time.

  • Forehead thermometers are becoming more popular for home use.
  • These devices are moderately accurate, but not as accurate as rectal thermometers.

There are two different types of Forehead thermometers –

  1. One type of forehead thermometer is placed on the temporal artery in the forehead. It uses infrared light to measure a person’s temperature.
  2. Second type of forehead thermometer is a plastic strip thermometer. It is placed on the forehead. These strips gives the body temperature within a range. These strips can detect a fever, but they do not give an exact reading.

3. Infrared Thermometer

An infrared thermometer is a temperature measuring device which measures the infrared radiation energy emitting from a body to get its temperature. Infrared thermometers are sometimes called as laser thermometers as they use laser beam to help in aiming the thermometer at pin point for non-contact thermometers or temperature guns.

Infrared thermometers can take quick temperature readings from a distance. However, these are not as accurate as other methods.

4. Tympanic Membrane Thermometer


It is basically an infrared thermometer. In research, it has found that an accurate reflection of body core temperature is ensured using ear thermometers called Tympanic Membrane Thermometers.

Tympanic Membrane Thermometer is a hand-held device that measures the temperature of the tympanic membrane or eardrum using an infrared sensor.

  • Tympanic membrane thermometers, or digital ear thermometers, use an infrared sensor to measure the temperature inside the ear canal and can give results within seconds.
  • This gives accurate results. However, ear thermometers may not be as accurate as contact thermometers like Oral or Rectal Thermometers.
  • Ear thermometers are noninvasive, hygienic, quick, and easy to use.
  • It is also more comfortable for use by children. However, it is not advisable to use an ear thermometer on infants who are below the age of 6 months.
  • Ear thermometers can give results within a few seconds. This makes them a good option when dealing with very young children, who may find it difficult to sit still for a prolonged period of time.

A typical Tympanic Membrane thermometer is shown in figure.

5. Rectal Thermometer

Rectal thermometers measure the temperature of rectum inserted through the anus. It provides the most accurate readings for infants and toddlers up to 3 years of age. A rectal thermometer may be of type –

  1. Mercury thermometer.
  2. Hicks mercury free thermometer.
  3. Alcohol thermometer.
  4. Digital thermometer.

A rectal thermometer may be a Mercury clinical thermometer or Alcohol Thermometer or Hicks Mercury Free Thermometer.

  • Specialized digital rectal thermometers are common for taking the body temperature of young children. It is required to clean and lubricate the end of these devices before inserting them into the anus. The device will alert the user when it has made a reading and is safe to remove.
  • A rectal thermometer takes the temperature of the rectum. It represents the most reliable and highly accurate readings of body temperature in comparison to Oral or Ear thermometers.
  • Rectal thermometers are the best option when caring for infants.

6. Digital Thermometer or Resistance Thermometer

Digital thermometers uses the property of a thermistor. Thermistors are resistors whose resistance changes considerably with small change in temperature. It uses linear variations of electrical resistance or resistivity of conductors with rise in temperature.

The governing thermometric relation is expressed as –

T = \left [ \left ( \frac { R_t - R_0 }{ R_{100} - R_0 } \right ) \times 100 \degree C \right ]

Where ( R ) is the electrical resistance of a conductor.

A digital clinical thermometer is shown in figure.


This thermometer has a tiny battery that sends a current through a wire towards the tip where the thermistor is present. According to the temperature of the thermistor, the current is impeded and received on the other end of the processing unit.