Temperature Measurement: A Comprehensive Guide to Thermometers and Pyrometers
Temperature Measurement
Understanding Temperature
Temperature defines the thermal state of a body and its ability to transfer or receive heat from another body. Thermodynamics defines temperature as a measure of the average velocity of a substance’s molecules.
Thermometric Scales and Reference Points
Temperature measurement relies on established scales based on fixed reference points. The Celsius scale uses the freezing point of water (0°C) and the boiling point of water (100°C) at standard atmospheric pressure. The Fahrenheit scale uses 32°F and 212°F for the same reference points. The Kelvin scale, used in scientific contexts, is an absolute temperature scale where absolute zero represents the theoretical state of zero molecular velocity in an ideal gas.
According to the kinetic theory of ideal gases, as a gas cools, molecular velocities decrease, causing the gas to contract. Conversely, heating a gas accelerates its molecules, causing expansion.
Thermometers
A thermometer (from Greek θερμός meaning “hot” and metro meaning “measure”) is an instrument used to measure temperature. Thermometer design has evolved significantly, particularly with the advent of digital electronic thermometers.
Early thermometers relied on the principle of thermal expansion, utilizing materials with high expansion coefficients. Mercury, housed in a glass tube with a graduated scale, was commonly used.
Types of Thermometers
Mercury Thermometer
A mercury thermometer, often used to measure ambient temperature, features a reflective bulb (typically white) to minimize absorption of surrounding radiation, ensuring accurate air temperature readings.
Bimetallic Strip Thermometer
A bimetallic strip thermometer utilizes two different metals joined together. Temperature changes cause uneven expansion, resulting in bending of the strip. This motion is amplified mechanically to indicate temperature on a scale. These thermometers are known for their ease of use and wide measurement range.
Gas Thermometer
A gas thermometer, often used for industrial applications, consists of a pressure gauge connected to a gas-filled bulb. Temperature changes affect the gas pressure, which is measured and converted to a temperature reading. The bulb volume is significantly larger than the rest of the system to minimize errors due to temperature differences.
Resistance Thermometer
Resistance thermometers (or resistance temperature detectors) operate on the principle that a material’s electrical resistance changes with temperature. This change in resistance is measured and correlated to temperature.
Thermocouple
A thermocouple consists of two dissimilar metals joined together. A temperature difference between the junction and the open ends generates a voltage (Seebeck effect), which is proportional to the temperature difference. Thermocouples are widely used in industrial settings due to their low cost and wide temperature range.
A series of thermocouples is called a thermopile.
Thermistor
A thermistor is a semiconductor-based temperature sensor whose resistance changes with temperature. There are two types: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient). NTC thermistors exhibit decreasing resistance with increasing temperature, while PTC thermistors exhibit increasing resistance within a limited temperature range.
Bulb Thermometer (Psychrometer)
A psychrometer uses two thermometers: a dry bulb thermometer and a wet bulb thermometer (covered in a water-soaked cloth). The difference in readings helps determine humidity. Modern hygrometers use changes in electrical resistance to measure humidity.
Digital Thermometers
Digital thermometers utilize transducers like thermistors to convert temperature variations into voltage changes, which are then processed electronically and displayed digitally.
Globe Thermometer
Similar to other thermometers, the globe thermometer measures temperature based on the principle of thermal expansion.
Wet Bulb Thermometer
Used in conjunction with a dry bulb thermometer in a psychrometer, the wet bulb thermometer measures temperature influenced by evaporation, providing information about humidity.
Dry Bulb Thermometer
A standard thermometer used to measure air temperature without the influence of humidity.
Maximum and Minimum Thermometer
These thermometers record the highest and lowest temperatures reached over a period. The maximum thermometer uses a constriction in the capillary tube to prevent mercury from retreating after expansion. The minimum thermometer uses a small index within the alcohol column that is dragged to the lowest temperature.
Pyrometers
A pyrometer measures temperature remotely, without contact. They are often used for temperatures above 600°C, with ranges typically spanning from -50°C to +4000°C.
Operating Principle
Pyrometers measure infrared radiation emitted by objects. The amount of energy emitted is proportional to the fourth power of the object’s absolute temperature. The emissivity factor, which represents the ratio of radiation emitted by an object compared to a perfect black body, is also considered.
Types of Pyrometers
There are three main types: radiation, infrared, and optical.
Radiation Pyrometers
These measure the total radiation emitted by an object and determine temperature based on the calorific effect.
Optical Pyrometers
These compare the brightness of an object to a reference source (e.g., a filament lamp) to determine temperature.
Infrared Pyrometers
These measure infrared radiation within a specific wavelength range.
Pyrometers are chosen over thermocouples in situations involving harsh environments, moving objects, high temperatures, or the need for rapid response times.
Features of Resistance Thermometer Materials
- High temperature coefficient of resistance for sensitivity.
- High resistivity for greater sensitivity.
- Linear resistance-temperature relationship.
- Stiffness and ductility for manufacturing.
Common materials used in probes include platinum, nickel, and copper.