How Thermometers Work: Principles & Types Explained
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).
Cryometer
Correctly, thermometers are used for low temperatures.
Medical
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.
FAQs on Temperature Measurement Using Thermometers in Physics
1. What is a thermometer and what is the fundamental principle behind its operation?
A thermometer is a scientific instrument used to measure temperature or a temperature gradient. The operation of any thermometer is based on a fundamental principle: it uses a thermometric property. This is a physical property of a substance that changes consistently and measurably with temperature. Examples include the expansion of a liquid like mercury or alcohol, the change in electrical resistance of a metal, or the pressure of a gas kept at a constant volume.
2. What are the main types of thermometers and what are their specific uses?
There are several types of thermometers, each designed for different applications based on its range, sensitivity, and the physical principle it uses. The main types include:
- Liquid-in-Glass Thermometers: These use the expansion of a liquid (like mercury or alcohol) in a capillary tube. Laboratory thermometers use this principle for general-purpose experiments.
- Clinical Thermometers: A specific type of mercury or digital thermometer designed to measure human body temperature. It has a narrow range (e.g., 35°C to 42°C) for higher precision.
- Digital Thermometers: These use electronic sensors like a thermistor or a thermocouple. A thermistor changes its resistance with temperature. They provide fast, accurate readings and are widely used in medical and household applications.
- Infrared (Pyrometer) Thermometers: These measure temperature from a distance by detecting the thermal radiation an object emits. They are essential for measuring very high temperatures (like in furnaces) or surfaces that cannot be touched.
3. What are the fixed points used to calibrate a thermometer on the Celsius scale?
To ensure accuracy, a thermometer is calibrated using two standard fixed points as defined on the Celsius scale. These are:
- Lower Fixed Point (Ice Point): This is the temperature at which pure ice melts at standard atmospheric pressure, defined as 0°C.
- Upper Fixed Point (Steam Point): This is the temperature at which pure water boils at standard atmospheric pressure, defined as 100°C.
4. How do the three main temperature scales—Celsius, Fahrenheit, and Kelvin—differ?
The three main temperature scales differ in their reference points and the size of their degree units.
- Celsius (°C): Based on the freezing (0°C) and boiling (100°C) points of water. It is widely used for everyday and scientific measurements.
- Fahrenheit (°F): Primarily used in the United States. It sets the freezing point of water at 32°F and the boiling point at 212°F.
- Kelvin (K): This is the SI unit of temperature and is an absolute scale. Its zero point, 0 K (absolute zero), is the theoretical temperature at which all molecular motion ceases. It is directly related to Celsius by the formula: K = °C + 273.15.
5. Why have alcohol and digital thermometers largely replaced mercury thermometers in many applications?
Mercury thermometers have been replaced primarily due to safety and practical limitations. Mercury is a toxic heavy metal, and a broken thermometer poses a significant health and environmental risk. Furthermore, mercury freezes at -39°C, making it unsuitable for measuring very low temperatures. In contrast:
- Alcohol thermometers can measure much lower temperatures (down to -114°C) and are non-toxic.
- Digital thermometers are safer, provide faster and more precise readings, are more durable, and do not pose a risk of toxic spillage.
6. What is the significance of the Kelvin scale in physics compared to Celsius or Fahrenheit?
The Kelvin scale is fundamentally important in physics because it is an absolute thermodynamic scale. Unlike Celsius or Fahrenheit, its zero point (0 K or absolute zero) represents a true physical limit—the lowest possible temperature. Temperature in Kelvin is directly proportional to the average kinetic energy of the particles in a system. This direct relationship makes it essential for scientific laws and formulas, such as the Ideal Gas Law (PV = nRT) and concepts in thermodynamics and quantum mechanics, where relative scales like Celsius would be inappropriate.
7. How is a clinical thermometer different from a standard laboratory thermometer?
While both may work on the same principle, a clinical thermometer and a laboratory thermometer are designed for different purposes. A clinical thermometer is specialized for measuring body temperature and has a narrow range (typically 35°C to 42°C) for high accuracy. It also features a constriction or 'kink' in the tube that prevents the reading from falling immediately after removal, allowing time to read it. A laboratory thermometer has a much wider temperature range (e.g., -10°C to 110°C) to suit various experiments and does not have a constriction, allowing it to show temperature changes in real-time.
8. How can a pyrometer measure the temperature of a star, which is millions of kilometres away?
A pyrometer, or an infrared thermometer, works on the principle of thermal radiation. Every object with a temperature above absolute zero (0 K) emits electromagnetic radiation. The intensity and wavelength of this radiation are directly related to the object's temperature. A pyrometer has a sensor that detects this radiation, specifically in the infrared spectrum. By analysing the properties of the incoming radiation (as per laws like the Stefan-Boltzmann Law), it can calculate the object's temperature without any physical contact. This makes it ideal for measuring extremely hot or distant objects like molten metal or stars.
9. Can a single type of thermometer be used to measure all possible temperatures? Why or why not?
No, a single type of thermometer cannot measure all possible temperatures. The reason lies in the limitations of its thermometric substance and principle. Each material has a specific temperature range in which it behaves predictably. For instance:
- An alcohol thermometer boils at 78°C and cannot measure temperatures above that.
- A mercury thermometer freezes at -39°C, making it useless for very cold climates.
- A platinum resistance thermometer becomes less accurate at extremely high temperatures where the metal itself might be affected.
