Temperature is one of the most widely measured variables. It affects almost all physical, biological, and chemical functions. Consequently, temperature sensors are widely used in various applications where temperature needs to be monitored or measured.
Different types of temperature sensors offer different features and depending on the requirement considerations, a suitable sensor can be used. In this article, we will see a few basic types of temperature sensors that are commonly used.
Thermocouples
Thermocouple (TC) sensor is a simple, robust sensor that is also cheaper than most other sensors. It has two wires of different metals or metal alloys connected at two points encased in a ceramic or metal casing. These sensors work on the principle that the difference in temperature between two different conductors results in a voltage difference between them. Thus, the temperature is detected by measuring the voltage between the points. Even though the output voltage is proportional to the temperature, it is non-linear in nature. Hence, a mathematical linearisation is required. Usually, a look-up table comes with the sensor so that the reading the temperature becomes convenient.
TC sensors work for a wide temperature range and can detect temperatures from about -250 °C to 1800 °C. But, they offer low accuracy and sensitivity compared to some other sensors that are more precise. They don’t require an external power source which makes them suitable for use in various applications including industrial usage thermocouple to a thermocouple found in everyday appliances. The most common type of TC sensor is the K-type sensor which uses the metal alloys nickel-chromium and nickel-aluminium.
Different thermocouples include:
- Beaded wire thermocouple
- Thermocouple probe
- Surface probe
Thermistors
A thermistor sensor is basically a thermally sensitive resistor hence where the name has derived. The resistance changes when it is exposed to changes in temperature. These sensors are usually made using a mix of metal oxide material and metals. The materials can then be formed once mixed.
There are two types of thermistors - Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC). NTC thermistors show a reduction in the resistance as temperature increases while the resistance of PTC thermistors increases with the increase in temperature. NTC thermistors are more commonly available.
Thermistors are highly sensitive and respond to temperature changes quickly. Even though they come with decent accuracy, they are prone to errors due to self-heating. They are inexpensive but are vulnerable to physical damage. They offer limited temperature sensing range between -40 °C and 250 °C.
Resistance Temperature Detector (RTD)
These sensors, too, work on the principle of resistance changes due to temperature changes. Unlike thermistors which usually come with a negative temperature coefficient, RTDs have a positive temperature coefficient. They are made from wires of high-conducting materials wound into coils. High-cost RTDs are made from platinum and are commonly known as Platinum Resistance Thermometers (PRTs). They offer a higher accuracy than the RTDs made from economical materials such as copper or nickel.
These sensors are quite stable and provide a linear resistance change in response to temperature changes. Hence, the temperature readings are highly accurate. Just like thermistors, RTDs too are susceptible to self-heating, but the error produced is minimal, giving them greater accuracy. The major drawback of RTDs is that they are expensive. The operating range of a typical RTD is between -200 °C and 600 °C.
These sensors, too, work on the principle of resistance changes due to temperature changes. Unlike thermistors which usually come with a negative coefficient, RTDs have a positive temperature coefficient. They are made from wires of high-conducting materials wound into coils. High-cost RTDs are made from platinum and are commonly known as Platinum Resistance Thermometers (PRTs). They offer a high accuracy than the RTDs made from economical materials such as copper or nickel.
Semiconductor temperature sensors
Semiconductor temperature sensors are electronic devices that are manufactured in a completely identical way to electronic semiconductor devices like for example, microprocessors. In the form of integrated circuits (IC), they are commonly referred to as IC temperature sensors. Hundreds or even thousands of devices are formed on silicon wafers before being cut into individual chips, usually being laser trimmed.
Available from different manufacturers, there are no generic types compared with RTDs and thermocouples, although, the sensors do share a number of key factors and characteristics. Typically, available in small sizes, they provide a reasonably linear output, they offer small interchangeability, they give accurate readings when calibrated correctly and are considered perfect for embedded applications where they are installed within equipment itself.
In comparison to RTDs and thermocouples, their electrical and mechanical performance is not very robust. They are also not capable of measuring high temperatures. From -40°C to 120°C, their range is limited.
There are five different types of semiconductor sensors:
- Voltage output temperature sensors
- Current output temperature sensors
- Digital output temperature sensors
- Resistance output silicon temperature sensors
- Diode temperature sensors
Other temperature sensors not mentioned in this article include IR sensors, and Thermometers.
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