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Air temperature sensors come in various types, including thermocouples, RTDs, infrared sensors, thermometers, and silicon diodes. They are used in various environments to measure temperature changes and regulate fuel flow in automobiles. Each type has its own advantages and limitations.
Air temperature sensors include a variety of devices often designed to measure the surrounding ambient or atmospheric temperature. They can also be used in common industrial environments, such as in fuel-injected automobile engines to calculate air density so that the car’s on-board computer can regulate fuel flow, lowering it as air density decreases to prevent power loss. Some of the more common types of temperature sensors in general are thermocouples, resistive temperature devices (RTDs), infrared sensors, thermometers, and silicon diodes.
Thermocouples work by the contact of two different types of metals which expand and contract at different rates depending on the temperature. They are often used as thermostats, temperature sensitive switches, but can also be employed as air temperature sensors due to their ability to measure over a wide range of temperatures, from 32° Fahrenheit (0° Celsius) up to 2.102° Fahrenheit (1,150° Celsius) or more. Thermocouple air temperature sensors are used in high-temperature environments such as ovens, but are not very good at measuring minute fractional-degree environmental changes.
Resistive temperature devices and thermistors measure the increase or decrease in the electrical resistance of matter as the temperature increases. RTDs are more accurate than thermocouples at small temperature changes, but they cannot measure temperatures as high or low as thermocouples can. They also tend to be more durable and stable than thermocouples, although their readings can be erroneous due to design and environmental conditions. Because they are unaffected by surrounding electrical noise, RTDs are often used as air temperature sensors in industrial environments where there are powerful motors or high voltage current.
Infrared temperature sensors are indirect measurement devices, requiring no physical contact with the material to be measured, as they measure the thermal radiation it produces, often referred to as black body radiation. They are therefore useful in extreme environments where other sensors could fail, such as in furnaces where temperatures can exceed 5,000° Fahrenheit (2,760° Celsius) and in industrial processes where heating and cooling occur rapidly. Silicon diode sensors operate at the other end of the spectrum from infrared sensors and are useful for measuring extreme cold. While they have applications such as air temperature sensors in high school science experiments, they are more often used to measure cryogenic gases in liquid form, such as liquid nitrogen, oxygen, hydrogen, and helium.
Thermometers, thermistors and RTDs are the most commonly used types of air temperature sensors and are also often used to measure the temperatures of liquids and solids. Typical thermometers work by measuring the expansion and contraction of liquid mercury or an organic red liquid in a sealed tube as the outside air temperature changes. Automotive air temperature sensors are usually thermistors. Whether they are measuring exhaust gases, intake air flow or engine coolant, they all work on the same principle that as the temperature of the surrounding fluid heats up, resistance in the sensor builds up and the voltage it sends to the computer controls is reduced.
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