Thermometer And Its Types

Edited By Vishal kumar | Updated on Jul 02, 2025 06:36 PM IST

Thermometers are essential instruments that measure temperature, a fundamental aspect of our daily lives and various scientific applications. From checking a fever to ensuring precise temperature control in industrial processes, thermometers play a crucial role in both personal health and technological advancements. They come in various types, each suited to different needs and environments. Understanding the different types of thermometers—ranging from traditional mercury thermometers to modern digital and infrared versions—can help us make informed choices for accurate temperature readings. In real life, whether you're cooking a meal, monitoring weather conditions, or conducting scientific experiments, the type of thermometer you use can significantly impact the accuracy and efficiency of your measurements. This article explores the various types of thermometers and their practical applications, highlighting their importance in everyday situations and specialized fields.

This Story also Contains

Thermometer and its Types
Solved Examples Based on Thermometer and its Types
Summary

Thermometer And Its Types

Thermometer and its Types

Thermometers are indispensable tools in our everyday lives, used to measure temperature accurately in various settings. From checking if a child has a fever to calibrating machinery, these devices help us monitor and control temperature effectively. There are several types of thermometers, each designed for specific applications and environments. Traditional mercury thermometers, with their precision and reliability, have been widely used but are gradually being replaced by safer alternatives. Digital thermometers offer quick and easy readings, while infrared thermometers provide non-contact measurements, ideal for a range of practical uses.

Thermometry

A branch of science that deals with the measurement of the temperature of a substance is known as thermometry.

An instrument used to measure the temperature of a body is called a Thermometer. The principle on which it works is by absorbing heat from the body.

There are various kinds of thermometers which are briefly classified into three types

Liquid Thermometers

In liquid thermometers, mercury is usually preferred over other liquids. The reason behind this is its expansion is large and uniform. The main reason behind all these is that it has high thermal conductivity and low specific heat.

Range of temperature :(freezing point of mercury) - ( boiling point of mercury) which is $-50^{\circ} \mathrm{C}$ to $350^{\circ} \mathrm{C}$

The formula for the calculation of temperature at any length of l

$t=\frac{l-l_0}{l_{100}-l_0} \times 100^0 \mathrm{C}$

Here $-l=$ length of the mercury column at the given temperature $t$.
$l_0=$ length of the mercury column at the $0^{\circ} \mathrm{C}$ temperature
$l_{100}=$ length of the mercury column at the $100^{\circ} \mathrm{C}$ temperature

Gas Thermometers

In this gases are used as thermometric material. Gas thermometers are more sensitive and accurate than liquid thermometers as the expansion of gases is more than that of liquids. In this gas are used as a thermoelectric substances are called ideal gas thermometers. These are basically two types

Constant Pressure Gas Thermometers

If pressure is constant, then for an ideal gas, volume is directly proportional to temperature. So,

$V \propto T$

The formula for the calculation of temperature at any volume V: $t=\frac{V-V_0}{V_{100}-V_0} \times 100^{\circ} \mathrm{C}$

Here, $V=$ volume of the gas column at the given temperature $t$.
$V_0=$ volume of the gas column at the $0^{\circ} \mathrm{C}$ temperature.
$V_{100}=$ volume of the gas column at the $100^{\circ} \mathrm{C}$ temperature.

Constant Volume Gas Thermometers

If the volume is constant, then for an ideal gas, pressure is directly proportional to temperature. So,

$P \propto T$

The formula for calculation of temperature at any pressure P: $t=\frac{P-P_0}{P_{100}-P_0} \times 100^{\circ} \mathrm{C}$

Here, $P=$ pressure of the gas column at the given temperature $t$.
$P_0=$ pressure of the gas column at the $0^{\circ} \mathrm{C}$ temperature.
$P_{100}=$ pressure of the gas column at the $100^{\circ} \mathrm{C}$ temperature.

Resistance thermometers: Usually Platinum and Germanium are used in resistance thermometers due to their high melting point and large value of temperature coefficient of resistance. This type of thermometer can be used for high temperatures.

The formula for calculation of temperature at any resistance R: $t=\frac{R-R_0}{R_{100}-R_0} \times 100^{\circ} \mathrm{C}$

Here, $R=$ Resistance of the material at the given temperature $t$.
$R_0=$ Resistance of the material column at the $0^{\circ} \mathrm{C}$ temperature.
$R_{100}=$ Resistance of the material column at the $100^{\circ} \mathrm{C}$ temperature.

Solved Examples Based on Thermometer and its Types

Example 1: An instrument used to measure the temperature of the body is known as

1) Hydrometer

2) Manometer

3) Barometer

4) Thermometer

Solution:

Thermometers

An instrument used to measure the temperature of a body is called a thermometer.

Hence, the answer is the option (4).

Example 2: The temperature range of the liquid thermometer is

1) $0-100^{\circ} \mathrm{C}$
2) $50^{\circ} \mathrm{C}-150^{\circ} \mathrm{C}$
3) $50^{\circ} \mathrm{C}-350^{\circ} \mathrm{C}$
4) $-50^{\circ} \mathrm{C}-350^{\circ} \mathrm{C}$

Solution:

Liquid Thermometers
$
t=\frac{l-l_0}{l_{100}-l_0} \times 100^{\circ} \mathrm{C}
$

wherein
Range of temperature $-50^{\circ} \mathrm{C}$ to $350^{\circ} \mathrm{C}$

Hence, the answer is the option (3).

Example 3: A branch of science that deals with the measurement of temperature is known as

1) Hydrometry

2) hygrometry

3) Thermometry

4) Barometry

Solution:

A branch of science that deals with the measurement of the temperature of a substance is known as thermometry.

Hence, the answer is the option (3).

Example 4: The temperature range of the resistance thermometer is

$\begin{aligned} & \text { 1) }-200^{\circ} \mathrm{C}-200^{\circ} \mathrm{C} \\ & \text { 2) }-200^{\circ} \mathrm{C}-1000^{\circ} \mathrm{C} \\ & \text { 3) } 200^{\circ} \mathrm{C}-1200^{\circ} \mathrm{C} \\ & \text { 4- } -200^{\circ} \mathrm{C}-1200^{\circ} \mathrm{C}\end{aligned}$

Solution:

Resistance Thermometers

$
T=\frac{R_t-R_0}{R_{100}-R_0} \times 100^{\circ} \mathrm{C}
$
Temperature Range $-200^{\circ} \mathrm{C}$ to $1200^{\circ} \mathrm{C}$

Hence, the answer is the option (3).

Example 5: For a constant volume gas thermometer, which of the following expressions is correct?

1) $t=\frac{P-P_{100}}{P-P_0} \times 100^{\circ} \mathrm{C}$
2) $t=\frac{P-P_0}{P_{100}-P_0} \times 100^{\circ} \mathrm{C}$
3) $t=\frac{P_{100}-P}{P_{100}-P_0} \times 100^{\circ} \mathrm{C}$
4) $t=\frac{P-P_0}{P_{100}-P} \times 100^{\circ} \mathrm{C}$

Solution:

Constant Volume gas Temperature
$P \propto T$ (V = constant)

$
t=\frac{P-P_0}{P_{100}-P_0} \times 100^{\circ} \mathrm{C}
$

Hence, the answer is the option (2).

Summary

Thermometers are crucial instruments for measuring temperature in various applications, from everyday health checks to industrial processes. They come in several types, including liquid thermometers using mercury, gas thermometers with constant pressure or volume, and resistance thermometers made from materials like platinum and germanium. Each type has unique principles and applications, such as the precise temperature range of liquid thermometers or the sensitivity of gas thermometers. Understanding these differences helps in selecting the appropriate thermometer for accurate temperature measurement in diverse situations.

Frequently Asked Questions (FAQs)

1. How do liquid crystal thermometers work?

Liquid crystal thermometers use heat-sensitive liquid crystals that change color at different temperatures. These crystals are typically arranged in a series of thin strips, each designed to change color at a specific temperature. As the temperature changes, different strips change color, allowing for a visual temperature reading. These are often used in aquariums, mood rings, and forehead thermometers.

2. What is the difference between precision and accuracy in thermometers?

Precision refers to the consistency of measurements, while accuracy refers to how close the measurements are to the true value. A precise thermometer will give consistent readings when measuring the same temperature multiple times, even if those readings are not necessarily accurate. An accurate thermometer gives readings that are close to the actual temperature. Ideally, a good thermometer should be both precise and accurate.

3. How do gas thermometers differ from liquid-in-glass thermometers?

Gas thermometers use the expansion and contraction of a gas (usually hydrogen or helium) to measure temperature. Unlike liquid-in-glass thermometers, gas thermometers can measure a wider range of temperatures and are more accurate, especially at very low temperatures. They work based on the principle that the pressure of a fixed volume of gas is directly proportional to its temperature (Gay-Lussac's law).

4. What is the triple point of water, and why is it important in thermometry?

The triple point of water is the temperature and pressure at which water coexists in all three phases: solid, liquid, and gas. It occurs at exactly 273.16 K (0.01°C) and a pressure of 611.657 Pa. This point is crucial in thermometry because it provides a fixed reference point that can be precisely reproduced, making it valuable for calibrating thermometers and defining temperature scales.

5. How do alcohol thermometers compare to mercury thermometers?

Alcohol thermometers use colored alcohol (usually ethanol) instead of mercury. They are safer and more environmentally friendly than mercury thermometers. Alcohol expands more than mercury for the same temperature change, making alcohol thermometers more sensitive. However, they have a smaller temperature range and are less accurate at higher temperatures compared to mercury thermometers. Alcohol thermometers are commonly used in outdoor thermometers because alcohol doesn't freeze at temperatures where mercury would.

6. What is a thermometer and how does it work?

A thermometer is a device used to measure temperature. It works based on the principle that most substances expand when heated and contract when cooled. Common thermometers use liquids like mercury or alcohol that expand and rise in a narrow glass tube as temperature increases, allowing us to read the temperature on a calibrated scale.

7. Why is mercury often used in thermometers?

Mercury is commonly used in thermometers because it has several advantageous properties: it expands uniformly with temperature changes, has a wide liquid range (-38.83°C to 356.73°C), doesn't stick to glass, and has high visibility due to its silvery color. However, due to its toxicity, many countries have phased out mercury thermometers for safer alternatives.

8. How does a digital thermometer differ from a traditional liquid-in-glass thermometer?

Digital thermometers use electronic sensors to measure temperature, typically employing thermistors or thermocouples. Unlike liquid-in-glass thermometers, they don't rely on the expansion of liquids. Digital thermometers provide quick, accurate readings, are easy to read, and often have additional features like memory storage and fever alerts.

9. What is the principle behind a bimetallic strip thermometer?

A bimetallic strip thermometer uses two different metals bonded together. As temperature changes, the metals expand or contract at different rates, causing the strip to bend. This bending is mechanically linked to a pointer that moves across a temperature scale. This principle is used in many household thermostats.

10. How does an infrared thermometer measure temperature without contact?

Infrared thermometers measure the thermal radiation emitted by an object. All objects above absolute zero emit infrared radiation, and the amount increases with temperature. The thermometer detects this radiation and converts it into a temperature reading. This allows for quick, non-contact temperature measurements, useful in many industrial and medical applications.

11. What is the principle behind a thermostat, and how does it relate to thermometers?

A thermostat is a device that maintains a system at a constant temperature. It uses a temperature sensor (often a bimetallic strip or electronic sensor) to detect temperature changes, much like a thermometer. However, unlike a simple thermometer, a thermostat uses this information to control a heating or cooling system. When the temperature deviates from the set point, the thermostat triggers the system to heat or cool until the desired temperature is reached.

12. What is the difference between Celsius and Fahrenheit scales?

Celsius and Fahrenheit are two different temperature scales. The Celsius scale is based on the freezing (0°C) and boiling (100°C) points of water at standard atmospheric pressure. The Fahrenheit scale sets the freezing point of water at 32°F and the boiling point at 212°F. To convert between them: °F = (°C × 9/5) + 32 or °C = (°F - 32) × 5/9.

13. What is an absolute temperature scale, and why is it important?

An absolute temperature scale is one that starts at absolute zero, the lowest theoretically possible temperature. The Kelvin scale is the most common absolute scale used in science. It's important because many physical laws and equations work only with absolute temperatures. The Kelvin scale starts at 0 K (-273.15°C), where all molecular motion theoretically stops.

14. How does a maximum-minimum thermometer work?

A maximum-minimum thermometer records the highest and lowest temperatures over a period. It typically uses a U-shaped tube with mercury and two small iron indicators. As temperature changes, the mercury pushes the indicators along the scale. One indicator shows the highest temperature reached, while the other shows the lowest. The indicators remain in place until reset, usually with a magnet.

15. What is a thermocouple, and how does it measure temperature?

A thermocouple is a temperature sensor consisting of two different metal wires joined at one end. When the junction experiences a temperature different from the other ends of the wires, it produces a small voltage due to the thermoelectric effect. This voltage is proportional to the temperature difference, allowing for accurate temperature measurement. Thermocouples are widely used in industrial settings due to their wide temperature range and durability.

16. How does altitude affect the boiling point of water, and why is this important for thermometer calibration?

As altitude increases, atmospheric pressure decreases, which lowers the boiling point of water. At sea level, water boils at 100°C (212°F), but at higher altitudes, it boils at lower temperatures. This is important for thermometer calibration because many thermometers use the boiling point of water as a reference point. Calibration must account for altitude to ensure accuracy.

17. What is a resistance temperature detector (RTD), and how does it work?

An RTD is a temperature sensor that operates on the principle that electrical resistance of metals changes with temperature. Typically made of platinum, RTDs provide very accurate and stable temperature measurements. As temperature increases, the resistance of the metal increases in a predictable way, allowing for precise temperature determination. RTDs are commonly used in industrial and scientific applications where high accuracy is required.

18. What is the principle behind a Galileo thermometer?

A Galileo thermometer, also known as a Galilean thermometer, consists of a sealed glass cylinder containing a clear liquid and several floating glass bulbs of varying densities. Each bulb is tagged with a temperature. As the ambient temperature changes, the density of the liquid changes, causing the bulbs to rise or sink. The temperature is read from the lowest floating bulb. This device is based on the principle of buoyancy and relative density.

19. How do thermistors differ from traditional thermometers?

Thermistors are electronic devices that change their electrical resistance with temperature. Unlike traditional liquid-in-glass thermometers, thermistors don't use expanding liquids. They're typically made of ceramic or polymer materials and can be very small, allowing for rapid response to temperature changes. Thermistors are widely used in electronic devices and can be more sensitive than many other types of temperature sensors.

20. What is a pyrometer, and in what situations is it used?

A pyrometer is a non-contact device used to measure high temperatures, typically above 600°C. It works by measuring the amount of thermal radiation emitted by an object. Pyrometers are used in situations where it's impractical or dangerous to use contact thermometers, such as in metallurgy, glass and ceramic production, or measuring the temperature of moving objects. They can measure temperatures far beyond the range of traditional thermometers.

21. How does a clinical thermometer differ from a laboratory thermometer?

Clinical thermometers are designed specifically for measuring human body temperature. They have a narrow temperature range (usually 35°C to 42°C) but high precision within this range. They often have features like a constriction in the capillary tube to prevent the mercury from falling back into the bulb after reading. Laboratory thermometers, on the other hand, have a wider temperature range but may be less precise. They are designed for general-purpose temperature measurements in scientific experiments.

22. How do fiber optic temperature sensors work?

Fiber optic temperature sensors use changes in the optical properties of a fiber to measure temperature. One common type uses the fact that the refractive index of the fiber changes with temperature, affecting how light travels through it. Another type uses special coatings on the fiber that fluoresce differently at different temperatures. These sensors can provide distributed temperature sensing over long distances and are immune to electromagnetic interference, making them useful in harsh environments.

23. What is the difference between a thermometer and a calorimeter?

While both devices involve temperature measurement, they serve different purposes. A thermometer is used to measure the temperature of an object or substance directly. A calorimeter, on the other hand, is used to measure the amount of heat released or absorbed during a chemical reaction or physical change. Calorimeters often use thermometers as part of their setup to track temperature changes, but they also include insulated containers and other components to measure heat flow.

24. How do thermocouples compare to RTDs in terms of accuracy and application?

Thermocouples and RTDs (Resistance Temperature Detectors) are both widely used temperature sensors, but they have different characteristics. Thermocouples have a wider temperature range and faster response time, making them suitable for extreme temperatures and rapidly changing conditions. However, RTDs are generally more accurate and stable over time. RTDs are often used where high precision is required, such as in laboratory settings, while thermocouples are common in industrial processes and high-temperature applications.

25. What is the working principle of a constant volume gas thermometer?

A constant volume gas thermometer works based on the principle that the pressure of a fixed amount of gas at constant volume is directly proportional to its temperature (Charles's Law). It consists of a bulb filled with a gas (usually hydrogen or helium) connected to a manometer. As the temperature of the gas changes, its pressure changes proportionally. By measuring the pressure change, the temperature can be determined. This type of thermometer is highly accurate and is often used as a standard for calibrating other thermometers.

26. How do thermometers designed for extremely low temperatures (cryogenic thermometers) differ from standard thermometers?

Cryogenic thermometers are specialized devices designed to measure very low temperatures, typically below -150°C. They often use different principles than standard thermometers. Some common types include:

27. What is the principle behind a quartz thermometer, and why is it highly accurate?

A quartz thermometer uses the temperature-dependent resonant frequency of a quartz crystal to measure temperature. As temperature changes, the crystal's frequency of vibration changes in a very predictable manner. This change is measured electronically and converted to a temperature reading. Quartz thermometers are highly accurate because the frequency of a quartz crystal can be measured with extreme precision, and the relationship between frequency and temperature is very stable and reproducible.

28. How do thermometers used in food safety differ from other types of thermometers?

Food safety thermometers are designed to quickly and accurately measure the internal temperature of food. They often have features like:

29. What is a Beckmann thermometer, and why is it used in certain scientific experiments?

A Beckmann thermometer is a highly sensitive differential thermometer used to measure small temperature differences rather than absolute temperatures. It has a large bulb and a very fine capillary tube, allowing it to measure temperature changes as small as 0.01°C. The unique feature of a Beckmann thermometer is that its temperature range can be adjusted by adding or removing mercury from the bulb. It's commonly used in experiments where precise measurement of small temperature changes is crucial, such as in determining freezing point depression or boiling point elevation in solutions.

30. How do infrared ear thermometers work, and why are they popular for measuring body temperature?

Infrared ear thermometers measure the thermal radiation emitted by the eardrum and surrounding tissues. The eardrum shares blood supply with the temperature control center in the brain, making it a good reflection of core body temperature. These thermometers work by:

31. What is the principle behind a Galileo thermoscope, and how does it differ from modern thermometers?

A Galileo thermoscope, one of the earliest thermometers, consists of a series of glass bulbs filled with colored water in a larger container of clear liquid. As temperature changes, the density of the liquid changes, causing the bulbs to rise or sink. The principle is based on the fact that the density of liquids changes with temperature. Unlike modern thermometers, thermoscopes don't have a numerical scale and only show relative temperature changes. They were more of a qualitative instrument, showing if temperature was increasing or decreasing, rather than providing precise measurements.

32. How do thermometers used in weather balloons operate in extreme conditions?

Thermometers used in weather balloons, known as radiosondes, are designed to operate in extreme conditions of the upper atmosphere. They typically use:

33. What is the working principle of a reversing thermometer used in oceanography?

A reversing thermometer is a specialized mercury-in-glass thermometer used to measure water temperature at specific depths in the ocean. It consists of two parts:

34. How do thermometers in smartphones work, and what are their limitations?

Smartphone thermometers typically use temperature sensors integrated into the device's circuitry. These are often thermistors or other electronic temperature sensors. They work by measuring the temperature of the device's components, such as the battery or processor. However, they have several limitations: