How Do Temperature Measurement Devices Work?
In our daily life, we note that a utensil placed on fire appears hot when we touch it. A piece of ice taken in hands appears cold. If steam is passed in water then it becomes warm. When one end of the metal rod is kept in fire the other end of it becomes hot after some time. Have you ever wondered about the causes of the above facts? The answer lies in knowing the thermal properties of matter which involves the study of heat and temperature. In this article, we are going to learn about what heat and temperature are and how are they measured.
Daniel Gabriel Fahrenheit
Daniel Gabriel Fahrenheit was a physicist, inventor, and builder of scientific instruments who lived from May 24, 1686, until September 16, 1736. Daniel Gabriel Fahrenheit, a Dutch scientist, and inventor produced the first dependable thermometer in 1714, utilizing mercury instead of alcohol and water combinations.
He proposed a temperature scale in 1724, which is currently (slightly adjusted) named after him. He was able to do so because he was the first to make thermometers using mercury (which has a high coefficient of expansion), and the quality of his production allowed for a finer scale and more reproducibility, resulting in widespread acceptance.
What is Heat?
We know that everybody is made up of a large number of tiny particles called molecules. Depending on the nature of the substance (solid, liquid, and gas) and its temperature, the molecule may possess translation motion i.e., motion along a straight line, vibrational motion i.e., to and fro motion about the mean position of the molecules, and rotational motion i.e., rotation of the molecules about their axes.
Every type of motion provides some kinetic energy to the molecules and hence to the body. In fact, heat possessed by a body is the total thermal energy of the body which is the sum of kinetic energies of all the individual molecules of the body due to the translational, vibrational, and rotational motion of the molecules. We know that a hot cup of tea if left on the table then it cools down and ice-cold water if left on the table it warms up. In both cases, heat is transferred from one body at a higher temperature to another body at a lower temperature. Thus heat is a form of energy that is transferred from one body at a higher temperature to another body at a lower temperature when they are placed in contact with each other. The SI unit of heat is the joule.
What is the Temperature?
Often, we confuse temperature with heat but the concept of temperature is different from the concept of heat. Generally, we define the temperature of a body as the degree of hotness or coldness of the body. When two bodies at different temperatures are kept in contact with each other then the heat flows from a body at a higher temperature to a body at a lower temperature, till their temperatures become equal. Therefore, redefining the temperature of a body as the thermal state or condition of the body, which would determine the direction of flow of heat when this body is placed in contact with another body.
Different Types of Temperature Scales
For quantitative measurement of temperature we use thermometers. A thermometer calibrated for a temperature scale is used to measure the value of a given temperature on that scale. In order to define any standard temperature scale, for the measurement of temperature, two reference points are needed, for which the physical phenomena that always occur at the same temperature. The two convenient fixed reference points are the ice point and the steam point of water at standard pressure which are known as the freezing point and boiling point of water at standard pressure. The following temperature scales are;
Celsius Temperature Scale:
It is formally known as the centigrade temperature scale and it is designed by Andre Celsius in the year 1741. Here the freezing point of ice at standard pressure is regarded as 0 oC and the boiling point of water at standard pressure as 100 oC. The space between these two fixed points is divided into 100 equal parts and each part represents 1 oC.
Fahrenheit Temperature Scale:
It was designed by Gabriel Fahrenheit in the year 1717. Here the freezing point of ice at standard pressure is regarded as 32 oF and the boiling point of water at standard pressure is 212 oF. The space between these two fixed points is divided into 180 equal parts and each part represents 1 oF. If tC and tF are temperature values of a body on the Celsius temperature scale and Fahrenheit temperature scale respectively then we can write,
$(t_C-0)/100=(t_F-32)/212$
Kelvin Temperature Scale:
In science, the Kelvin scale is the most widely used temperature scale. It's an absolute temperature scale with 0 K at absolute zero, or the coldest temperature imaginable. Water's freezing and boiling points are 273.15 K and 373.15 K on this scale, respectively. Unlike other temperature measures, the Kelvin scale is absolute.
Different Types of Temperature Measurement Devices
In today’s world, the temperature is measured by various types of thermometers with different types of thermometer units of measurement such as Celsius, Fahrenheit, and Kelvin. Some of these are explained below;
Mercury Thermometer:
The mercury thermometer was created by Gabriel Fahrenheit in 1974.The stem of a mercury thermometer is a long, thin, and uniform glass tube. On the stem are indicated the scales that are used to gauge temperature. At the end of the stem, there is a little bulb that holds mercury. When a thermometer comes into touch with a heated body, mercury expands in a capillary tube inside the glass stem.
Clinical Thermometer:
Thermometers like this may be found in homes, clinics, and hospitals. These thermometers have a kink in them that prevents the mercury from returning to the patient's mouth when it is removed, making it easier to record the temperature. On either side of the mercury thread are two temperature scales, one in Celsius and the other in Fahrenheit.It can produce temperatures ranging from 35oC(94oF) to 42oC(108oF). The Celcius scale is more sensitive than the Fahrenheit scale. As a result, the temperature is expressed in degrees Fahrenheit (°F).
Laboratory Thermometer:
Laboratory thermometers can be used in school laboratories or other labs for scientific study to keep track of the temperature. In industries, these are also used to measure the temperature of solutions or instruments. When compared to a clinical thermometer, the stem and bulb of a laboratory thermometer are longer. There is no kink in a scientific thermometer. It only has a temperature scale in Celsius. It can withstand temperatures ranging from -10oC to 110oC.
Thermocouples
Thermocouples are voltage devices that use a change in voltage to indicate temperature measurement. The output voltage of the thermocouple rises as the temperature rises, but not always linearly.
The thermocouple is frequently enclosed in a metal or ceramic shield to protect it from a range of conditions. Many types of outer coatings, such as Teflon, are available for trouble-free usage in acids and strong caustic solutions on metal-sheathed thermocouples.
Silicon Diode Sensors
A silicon diode sensor is a gadget that was created specifically for use at cryogenic temperatures. In essence, they are linear devices in which the diode's conductivity grows linearly in low cryogenic regions.
Interesting Facts
Water was used instead of mercury in the first thermometers invented. Because water freezes at 32 degrees Fahrenheit (0 degrees Celsius), the inventors turned to mercury and alcohol.
Thermometers that used liquids worked on a straightforward principle. When the liquid was heated, it expanded, indicating a temperature increase.
Conclusion
In this article, we studied the concept of heat and learned the difference between heat and temperature. We discussed different temperature-measuring scales along with different temperature-measuring instruments. Different thermometers, thermocouples, and digital thermometers are used to measure temperature. Thermometers can have mercury or any other liquid in them. The expansion of mercury is exactly proportional to temperature. The expansion of fluid occurs as the temperature rises. As a result, the volume of fluid can be used to determine the temperature.
FAQs on Temperature Measurement Devices: Definitions, Types & Uses
1. What are the main types of temperature measurement devices?
There are several types of devices used to measure temperature, each based on a different physical principle. The most common types include:
- Liquid-in-glass thermometers: These use the uniform expansion of a liquid like mercury or alcohol in a narrow tube.
- Bimetallic strip thermometers: These use two different metals bonded together, which bend when heated due to different rates of thermal expansion.
- Resistance thermometers: These measure temperature based on the change in electrical resistance of a material, such as a platinum wire (e.g., Platinum Resistance Thermometer).
- Thermocouples: These generate a small voltage at the junction of two different metals, which varies with temperature.
- Pyrometers: These are non-contact devices that measure the temperature of very hot objects by detecting the thermal radiation they emit.
2. What is the fundamental principle behind a thermometer?
The fundamental principle behind any thermometer is the use of a thermometric property. This is a physical property of a substance that changes consistently and measurably with temperature. For example, in a mercury thermometer, the thermometric property is the volume of the mercury, which expands or contracts predictably as temperature changes. In a resistance thermometer, the property is the electrical resistance of a metal.
3. Why is mercury used in thermometers despite being a relatively poor conductor of heat?
While mercury is not the best heat conductor, its other properties make it exceptionally suitable for thermometers. The key reasons for its use are:
- Uniform Expansion: Mercury expands at a very consistent rate for each degree rise in temperature, ensuring an accurate and linear scale.
- High Boiling Point: It has a high boiling point (357°C), allowing it to measure a wide range of high temperatures.
- Low Freezing Point: Its freezing point is low (-39°C), making it useful for most common climates.
- Visibility: It is a shiny, opaque liquid that is easily visible inside a thin glass tube.
- Non-Adhesive Property: Mercury does not stick to the glass, which allows for precise readings as it rises and falls.
4. How do you convert temperatures between the Celsius, Fahrenheit, and Kelvin scales?
Converting between the three main temperature scales involves using specific formulas. As per the CBSE/NCERT curriculum for 2025-26, the key conversion formulas are:
- Celsius to Fahrenheit: °F = (°C × 9/5) + 32
- Fahrenheit to Celsius: °C = (°F - 32) × 5/9
- Celsius to Kelvin: K = °C + 273.15
The Kelvin scale is the SI unit of temperature and is based on absolute zero, the point where all thermal motion ceases.
5. What are the key differences between a clinical thermometer and a laboratory thermometer?
A clinical thermometer and a laboratory thermometer are both liquid-in-glass devices but are designed for different purposes. The main differences are:
- Temperature Range: A clinical thermometer has a very narrow range (approx. 35°C to 42°C) to measure human body temperature. A laboratory thermometer has a much wider range (e.g., -10°C to 110°C) for scientific experiments.
- Constriction (Kink): A clinical thermometer has a constriction or kink in the tube just above the bulb. This prevents the mercury from falling back immediately, allowing time to take a reading after removing it from the body. Laboratory thermometers do not have this feature.
- Use Case: Clinical thermometers are strictly for medical purposes, while laboratory thermometers are used for general-purpose experiments in science labs.
6. How does a platinum resistance thermometer work, and what are its advantages over a mercury thermometer?
A platinum resistance thermometer (PRT) operates on the principle that the electrical resistance of a platinum wire changes in a precise and linear way with temperature. By measuring the wire's resistance, an accurate temperature can be determined.
Its main advantages over a mercury thermometer include:
- Higher Accuracy: PRTs are extremely accurate and are used as a standard for calibrating other thermometers.
- Wider Temperature Range: They can measure a much broader range of temperatures, from very low (around -200°C) to very high (around 650°C).
- Greater Sensitivity: They can detect very small changes in temperature more effectively than a mercury thermometer.
7. What is the importance of measuring temperature in various real-world applications?
The measurement of temperature is critical across many fields for safety, quality control, and scientific understanding. For example:
- In Medicine: Monitoring body temperature is vital for diagnosing illnesses like fever and tracking a patient's response to treatment.
- In Meteorology: Temperature readings are essential for weather forecasting and climate change studies.
- In Industry: Processes like chemical manufacturing, food processing, and metalwork require precise temperature control to ensure product quality and safety.
- In Daily Life: We use temperature measurement for cooking (ovens), home heating (thermostats), and ensuring food is stored safely (refrigerators).
8. For what kind of measurements would you use a pyrometer instead of a standard thermometer?
A pyrometer is used when direct contact with an object is impossible or dangerous. You would choose a pyrometer for measuring extremely high temperatures where a contact-based device like a mercury or resistance thermometer would be destroyed. Its applications include measuring the temperature of:
- Molten metals in a foundry or furnace.
- The surface of a star or distant celestial body.
- The interior of an industrial kiln or engine.
A pyrometer works on the principle of measuring the intensity of thermal radiation emitted by the hot object, making it a non-contact measurement device.
