Understanding Different Types of Thermometers

Thermometers all answer the same question, but they do it in different ways and at different places on the body. That is why two thermometers can give two different numbers for the same person at the same moment. Understanding how each type works makes the readings much easier to interpret.

What a thermometer measures

The body keeps its internal temperature within a narrow range. Thermometers estimate that temperature from a convenient site: the mouth, armpit, rectum, ear or forehead. Each site sits at a slightly different temperature, and each is affected by different things.

Electronic (digital probe) thermometers

The most common type. A heat sensor in the tip measures temperature, and the result appears on a digital display. MedlinePlus notes that the probe can be placed in the mouth, rectum or armpit.

Oral

The tip goes under the tongue with the mouth closed. Suitable for adults and older children. Hot or cold drinks, food and smoking affect the reading; wait 20 to 30 minutes.

Armpit (axillary)

The tip is held in the armpit with the arm pressed against the body. Simple and safe at any age, but generally the least precise method.

Rectal

Used mainly for infants, on a pediatrician’s advice. MedlinePlus states that rectal temperatures are generally considered more accurate when checking for fever in a young child. A thermometer used this way should be kept for that purpose only.

Example: our Digital Thermometer with Flexible Tip.

Infrared forehead thermometers

These measure infrared energy given off by the skin of the forehead. There are two designs: contact models that are swept across the forehead, and non-contact models held a short distance away.

The FDA notes that non-contact infrared thermometers may reduce the risk of spreading disease between people and are easy to clean, but that improper use may lead to inaccurate measurements. For a reliable reading:

• use the thermometer in a draft-free space, out of direct sun and away from radiant heat;
• let it adjust to the room for 10 to 30 minutes before use;
• make sure the forehead is clean, dry and not covered by hair, a hat or a headband;
• hold the sensor perpendicular to the forehead at the distance given in the instructions;
• keep the person still.

Example: our Non-Contact Infrared Forehead Thermometer.

Infrared ear (tympanic) thermometers

A probe placed in the ear canal measures infrared energy from the eardrum. These are quick, but the probe must be aimed correctly, and earwax or a small, curved ear canal can affect the result. MedlinePlus states that electronic forehead thermometers are more accurate than ear thermometers and that their accuracy is similar to that of probe thermometers.

Plastic strip thermometers

Strips placed on the forehead change color to indicate temperature. They show whether a temperature is raised but do not give a precise measurement.

Mercury-in-glass thermometers

The traditional type. MedlinePlus reports that the American Academy of Pediatrics recommends against using glass thermometers with mercury, because the glass can break and mercury is a poison. If you still have one, do not put it in the household trash; ask your local waste authority about hazardous waste disposal.

Other types you may see

• Pacifier thermometers for babies: convenient but less reliable.
• Wearable and smart thermometers: track temperature continuously and send it to a phone.
• Basal thermometers: read to two decimal places for fertility tracking.

How readings compare by site

MedlinePlus gives these general relationships, taking an average normal oral temperature of 98.6°F (37°C):

• Rectal: 0.5°F (0.3°C) to 1°F (0.6°C) higher than oral.
• Ear: 0.5°F to 1°F higher than oral.
• Armpit: most often 0.5°F to 1°F lower than oral.
• Forehead scanner: most often 0.5°F to 1°F lower than oral.

These are approximations. The practical lesson is to use the same thermometer and the same site each time, and to tell your doctor which method you used.

What is normal?

There is no single normal temperature. The FDA notes that although 98.6°F (37.0°C) is typically considered normal, studies have shown that normal body temperature can range from 97°F (36.1°C) to 99°F (37.2°C). Temperature also varies with:

• time of day, being lower in the morning and higher in the late afternoon;
• age;
• physical activity;
• the menstrual cycle;
• clothing and surroundings.

What is a fever?

A temperature of 100.4°F (38°C) or above is usually considered a high temperature. MedlinePlus adds that a reading more than 1 to 1.5°F above your own normal indicates a fever. Fever is usually a symptom of infection and is part of the body’s defense.

Comparing the types at a glance

• Digital probe: inexpensive and versatile; slower; needs cleaning.
• Non-contact forehead: fastest and most hygienic; sensitive to technique and surroundings.
• Ear: fast; technique-dependent; not ideal for very young babies.
• Strip: rough guide only.
• Mercury: not recommended.

Getting the best from any thermometer

1. Read the instructions.
2. Wait at least an hour after heavy exercise or a hot bath.
3. Clean the device as directed.
4. Take a second reading if the first seems wrong.
5. Replace the battery when the low-battery symbol appears.
6. Note the time, the reading and the method.

When to seek help

• A baby under 3 months with a temperature of 100.4°F (38°C) or higher
• A fever with a rash that does not fade when pressed, a stiff neck, trouble breathing or confusion
• A fever that lasts more than a few days or keeps returning
• Anyone who seems seriously unwell

A normal reading does not rule out illness. Trust how the person looks and behaves.

The bottom line

Different thermometers measure at different sites and give slightly different numbers. Choose one type, learn its technique and use it consistently. See the options in our medical supplies range, and read our guide on how to choose a digital thermometer.

Sources

1. MedlinePlus. Temperature measurement.
2. U.S. Food and Drug Administration. Non-contact Infrared Thermometers.
3. NHS. High temperature (fever) in adults.
4. MedlinePlus. Fever.