Temperature Measurement - Thermometer

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What is Temperature Measurement?

Temperature measurement is the process of obtaining the current temperature locally either for immediate requirements or for later. Temperature measurement is needed in various fields, from our kitchen to multi-billion dollar industries, from metallurgy to baking, and more. In the process control industry, there is a wide array of needs, and there has been a development of a vast number of devices and sensors to meet these needs. The most common instrument used for the measurement of temperature is known as a thermometer.

Temperature measurement is further known as thermometry, describes the measuring a current local temperature process for immediate or later evaluation, as discussed. Datasets consisting of repeated standardized measurements are used to assess temperature trends.

Temperature Sensors: Thermometer

Usually, the thermometer measures temperature in the units of centigrade.

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The thermometer is the earliest form (measuring temperature using thermometer) of a temperature sensor, although the first working model entered into the market very late. 16th century, the first thermometers were designed, and it was filled with gas, unlike how we remember the thermometer. The present design of a hollow cylindrical tube filled with mercury was conceived and perfected by Daniel Gabriel Fahrenheit, who chose mercury metal for two primary reasons. Irrespective of the temperature, mercury exhibits an equal rate of thermal expansion, and it does not stick to the glass tube.

Thermal expansion is a phenomenon that has happened. The expansion of mercury because of heat is what represents the temperature. If we calculate the mercury expansion per degree change in temperature, we can also calibrate the thermometers with a scale depending on the expansion. This is the working nature of a thermometer. We have named the temperature in honor of Fahrenheit's discovery scale after him.

Temperature sensors are calibrated in such a way that the numbers available on the thermometers represent the body temperature. Now we are aware of that for any scale, we require two reference points. The most sufficient and distinct stable reference points are the boiling point and the melting point of water; being 100° C and 0° C. At 100 degrees, the water boils over into the vapour state, and at 0 degrees, the water melts from solid to the liquid state.

This would turn out to be 212° F for the boiling point, whereas 32° F for the melting point on the Fahrenheit scale. The Fahrenheit scale is used in North America and a few other countries and whereas the Celsius scale is widely used by the other parts of the world. The formulae for its conversion can be given as,

T\[_{c}\] = (T\[_{f}\] - 32) . \[\frac{5}{9}\]

Primary and Secondary Thermometers

A thermometer is referred to as primary or secondary depending on how the raw physical quantity it measures is mapped to a temperature. As reviewed by Kauppinen et al., "For primary thermometers, the measured property of matter is so well known that the temperature can be calculated without any unknown quantities. These thermometer examples are based on the equation of state of a gas, and on the thermal noise voltage or electrical resistor current, on the angular anisotropy of gamma-ray emission of a few radioactive nuclei exists in a magnetic field, and the velocity of sound in a gas."

By comparison, it is stated that "the secondary thermometers are utilized typically in most of the situations because of their flexibility. Also, they are often much more sensitive compared to the primary ones. Considering the secondary thermometers, knowledge of the measured property is insufficient to allow for the direct temperature calculation. They should be calibrated against a primary thermometer at one temperature or several fixed temperatures, at least. At fixed stages, for example, superconducting transitions and triple points exist reproducibly at identical temperatures."

Applications of a Thermometer

Thermometers use a wide range of physical effects to measure the temperature. These sensors are used broadly in scientific and engineering applications, especially measurement systems. Also, thermometers are used in roadways in cold weather climate conditions to determine the existence of icing conditions. In indoor, thermistors are used in climate control systems like freezers, air conditioners, heaters, water heaters, and refrigerators. Galileo thermometers can be used to measure indoor air temperature because of their limited measurement range.

Some of the applications of thermometers are given below.

Nano Thermometry

Nano thermometry is an emergent research field that deals with the temperature knowledge in the sub-micrometric scale. Conventional thermometers cannot measure the object's temperature, smaller than a micrometre, and new materials and methods have to be used, and in such cases, Nano thermometry is used. Nanothermometers are divided as luminescent thermometers (if they use light for temperature measurement) and non-luminescent thermometers (systems where thermometric properties are not related to luminescence directly).


  • Correctly, thermometers are used for low temperatures.


  • Typically, oral and Rectal thermometers have been mercury, but with a digital readout, these have since largely been superseded by NTC thermistors

  • Ear thermometers tend to be infrared ones

  • The forehead thermometer is the best example of a liquid crystal thermometer

Different thermometric techniques have been used throughout history, like the Galileo thermometer, to thermal imaging. Medical thermometers such as infrared thermometers, mercury-in-glass thermometers, liquid crystal thermometers, and pill thermometers are used in the health care settings to determine if the individuals have a fever or they are hypothermic.

FAQ (Frequently Asked Questions)

1. Explain the Environmental Applications of Thermometers?

  • The heat meter uses a thermometer to measure the rate of heat flow

  • Indoor-outdoor thermometer

  • Thermostats have used the bi-metallic strips, but digital thermistors have since become popular

Infrared thermometers, Alcohol thermometers, mercury-in-glass thermometers, thermistors, Six's thermometers, and recording thermometers are used in climatology and meteorology in various levels of the oceans and atmosphere. Aircraft use hygrometers and thermometers to determine if any atmospheric icing conditions exist along their flight path. These measurements are used to initialize the weather forecast models. Thermometers are also used in cold weather climates in roadways to help determine if icing conditions exist and indoors in climate control systems.

2. Explain the Indirect Measures of Temperature Measurement?

A few of the indirect measurements of temperature are given below.

Thermal Expansion

  • Some liquids possess high expansion coefficients relatively over useful temperature ranges, thereby forming the basis for a mercury or alcohol thermometer. The alternative designs using this principle are the Beckmann differential thermometer and the reversing thermometer

  • Utilizing the thermal expansion property of different phases of matter

  • Pairs of solid metals having various expansion coefficients can be used for bi-metal mechanical thermometers. The other design using the same principle is Breguet's thermometer

  • Gases can be used to form a gas thermometer also as with liquids


  • Galileo thermometer


  • Vapour pressure thermometer


  • A few compounds exhibit thermochromism at distinct temperature changes. Therefore, by tuning the phase transition temperatures for a series of substances, the temperature can be quantified in discrete increments, which is a form of digitization. This is the basis for a thermometer of liquid crystal type.