Temperature is used to express how hot or cold something is with reference to a standard value. Measuring temperature is especially important in laboratories. For example, to determine the purity of different substances by establishing their boiling points and melting points, to determine the flash points of flammable oils or to have control over distillation processes.
Attempts were made to standardize temperature measurement as early as 170 AD when physician Claudius Galenus mixed equal parts boiling water and ice to create a neutral standard. However, it wasn’t until the 1600s that the modern scientific system for temperature measurement began.
Because temperature can fluctuate to such extremes, it cannot be measured using a single device. In this article, we’ll explore the different types of instruments that are used to measure temperature in various situations.
Temperature is measured using a range of devices, including thermometers — which are the most common. Thermometers can be used to measure how hot or cold solids, liquids and gases are and will usually give readings using the Kelvin, Fahrenheit or Celsius scales.
In most situations, thermometers measure temperature. However, there are some other instruments that are used, such as Langmuir probes, pyrometers and resistance temperature detectors (RTDs).
To find out more about the type of equipment that’s used to measure temperature, keep reading.
What are the different types of thermometers?
The most common temperature-measurement devices are thermometers, which take many different forms:
Also known as “glass thermometers” or “liquid in glass thermometers”, these thermometers are narrow glass tubes which contain liquid (usually mercury or red-coloured alcohol) that rises up the tube from a glass bulb as it expands with heat. When the temperature falls, the liquid contracts and goes back down into the bulb. The tube is known as the “stem” or “capillary tube”, and it has standardized readings printed on the outside, which are often both the Celsius and Fahrenheit temperature scales.
A maximum thermometer is a type of liquid thermometer that’s commonly used to measure body temperature. The liquid doesn’t fall as soon as the temperature drops, which means the reading can be observed even after it is removed from the environment in which it is taken.
Liquid thermometers can’t take readings above or below certain temperatures due to the liquid used inside them. Mercury, for example, becomes a solid at minus 37.89 degrees Fahrenheit (38.83 degrees Celsius), so it cannot measure temperatures below this point, while alcohol boils at around 172 degrees Fahrenheit (78 degrees Celsius), so it cannot measure temperatures above this point.
Here are some of the reasons why these liquids are used in glass thermometers:
- They have low freezing points
- They have high boiling points
- They expand linearly
- They give accurate readings
- They give a fast response time
Some of the other liquids used in glass thermometers are creosote, pentene and toluene.
Liquid thermometers cannot measure temperature using the Kelvin scale. To measure temperature in Kelvins, an electronic thermometer is usually used.
These types of thermometers detect changes in temperature using a thermosensitive device, which is incorporated into the tip of a probe. This device will either be a thermistor or a thermocouple, as they are very small and can respond to changes in temperature rapidly.
Probe thermometers are used to take the temperature of semi-solid porous substances, like food, oil and grease.
Portable and easy to use, they are immersed into the substance and give a temperature reading on a simple digital display.
Probe thermometers vary in size, material and manoeuvrability and come in two forms:
- Fixed probes — These have a pen-like structure and are cheap and easy to use.
- Wired probes — These have at least a one-meter cable and are a little more complicated to use, due to their extra features.
Aside from in laboratories, probe thermometers are most commonly used in restaurants and cafes to check that the food is hot enough and safe to eat, as well as in retail outlets and manufacturing industries.
Bimetallic strip thermometers
These types of thermometers are used in various household appliances, for example, ovens, grills, wall thermometers and thermostat switches.
They are made up of bimetallic strips which expand and contract according to the temperature — a process which is one of the oldest techniques to measure temperature.
They are capable of measuring high temperatures of up to around 500 degrees Fahrenheit (260 degrees Cenitgrade).
There are various specialized thermometers too. Some of these are:
- Cryometer thermometers — These are used to measure very low temperatures, such as temperatures in space.
- Infrared thermometers — Also known as a “laser thermometer”, an infrared thermometer is one of the most complicated thermometers to use. They are used by astronomers to measure temperatures at great distances and in manufacturing processes because they can measure extremely high or low surface temperatures. They are also used in air conditioning systems and in the automotive trade.
- K-type thermometers — K-type thermometers are extremely precise, which means they are used in laboratories and industrial processes that deal with intense temperatures. These thermometers fall into different categories (liquid, air, surface or penetration), and each has a specific purpose.
- Nanothermometers — Nanothermometers were developed by researchers at Havard University. They are injected into a single living cell to measure temperature changes inside the cell. It is hoped that in the future, they will be able to kill cancer at the cellular level.
- Pill thermometers — These thermometers are used in athletics to prevent and treat heat-related medical conditions such as heatstroke. Once swallowed, the thermometer can read the body’s core temperature for 18 to 30 hours.
- Pyrometer thermometers — This type of thermometer is used to measure very high temperatures. The steel industry, for example, uses them to measure the temperature of metals like iron.
- Temperature data loggers — These can either take temperatures continuously or at set intervals and are often used to evaluate the shelf-life of products. The data is shown in either digital or graphic form and can be shared online or stored for future review.
How is air temperature measured?
Air temperature near the surface of the earth is measured at weather stations and meteorological observatories, usually using thermometers in shelters. These thermometers are positioned 1.25 to 2 meters above the ground, per directions from the World Meteorological Organization (WMO).
Average daily temperatures can be determined by a thermograph that gives frequent readings, for example, every 24 hours or every 6 hours. They can also be determined by the mean of the minimum and maximum readings given each day however, this method is not as accurate as, depending on what time the reading is taken, it could be hotter or colder than the true mean.
Are there any other devices that are used to measure temperature?
Aside from thermometers, the following instruments are also used to measure temperature:
- Langmuir probes
- Resistance temperature detectors (RTDs)
What unit is temperature measured in?
There are three main scales that are used to measure temperature: Kelvin (K), Fahrenheit (°F) and Celsius (°C). The Kelvin scale uses the Kelvin as a unit of measurement and is used in science and engineering, while Celsius and Fahrenheit use degrees to measure temperature in everyday situations.
The Kelvin scale is used in the fields of science and engineering to record precise temperatures. It is essential for calculating the temperature of objects in outer space.
Everyday temperatures are not read in the Kelvin scale because such large numbers are used. A 75-degree Fahrenheit day, for example, is the equivalent of 297 Kelvins.
Note that Kelvins are not read as “degrees Kelvin” or “Kelvin degrees”, like Fahrenheit and Celsius often are.
The Fahrenheit scale is part of the imperial system. The United States, Burma and Liberia are the only three countries in the world that use the Fahrenheit Scale to measure temperature.
The Fahrenheit scale is named after the Polish physicist, Daniel Gabriel Fahrenheit, who developed it and who also invented the alcohol and mercury liquid thermometers.
The Celsius scale is part of the metric system. This is the official measurement system for every country in the world, apart from the three that use the Fahrenheit scale. However, in these countries, scientists use the Celsius or Kelvin scale to measure temperature. This is so they can quickly and easily communicate with one another, no matter where they are from.
Other temperature scales
Temperature is also measured using the following scales:
- Delisle (°D/°De)
- Newton (°N)
- Rankine (°Ra/°R)
- Rømer (°Rø/°R)
- Réaumur (°R/°Ré/°Re)
The table below gives an idea of how the different units of temperature compare to one another.
|Average human body temperature||310.0 ±0.7||36.8 ±0.7||98.2 ±1.3||94.8 ±1.1||12.1 ±0.2||557.9 ±1.3||26.8 ±0.4||29.4 ±0.6|
|Average surface temperature on earth||287||14||57||129||4.6||517||15.4||12|
|Highest recorded surface temperature on earth||329.8||56.7||134||65.0||18.7||593.7||37.3||45.3|
|Lowest recorded natural temperature on earth||184||−89||−128||284||−29||331||−39||−71|
|Temperature of the sun’s surface||5800||5526||9980||−8140||1823||10440||2909||4421|
|Water’s boiling point||373.15||100||212||0||33||672||60||80|
|Water’s freezing point||273.15||0||32||150||0||491.67||7.5||0|
The most common temperature-measuring device is the liquid thermometer, which is also known as a "glass thermometer" or "liquid in glass thermometer". Some other types of thermometers are electric thermometers, probe thermometers and Bimetallic strip thermometers. There are also various specialized thermometers, such as cryometer thermometers, pyrometer thermometers, infrared thermometers, pill thermometers, nanothermometers, temperature data loggers and K-type thermometers. Some of the other instruments for measuring temperature are Langmuir probes, pyrometers and resistance temperature detectors (RTDs).
The three main scales that are used to measure temperature are the Kelvin scale, the Fahrenheit scale and the Celsius scale. Temperature is also measured using the Delisle, Newton, Rankine, Rømer and Réaumur scales.