Temperature sensors are measurement devices that determine temperature by sensing a corresponding physical characteristic, such as electrical resistance, electromagnetic field (EMF), or thermal radiation.  The way a temperature sensor works depends upon the physical property that is measured. Sensor options include:

• Thermocouples
• Resistance temperature detectors (RTDs)
• Thermistors
• Bimetal switches
• Noncontact infrared temperature sensors

Types of Temperature Sensors

There are five common types of temperature instruments. Each has advantages and disadvantages.

Thermocouples

Thermocouples are accurate, highly-sensitive to small temperature changes, and quickly respond to changes to the environment. They consist of a pair of dissimilar metal wires joined at one end. The metal pair generates a net thermoelectric voltage between their opening and according to the size of the temperature difference between the ends. A temperature reading is made by calibrating the device with known temperatures, then placing one of the metal junctions on ice (or something else of a known temperature) and the other on the object whose temperature needs to be identified. The voltage displayed is read using the calibration formula, and the temperature of the object can be calculated.

Advantages of thermocouples include their high accuracy and reliable operation over an extremely wide range of temperatures. They are also well-suited for making automated measurements both inexpensive and durable.

Disadvantages include errors caused by their use over an extended period of time, and that two temperatures are required to make measurements. Thermocouple materials are also subject to corrosion, which can affect the thermoelectric voltage

Thermocouple temperature sensors are divided into various types, and include devices made of Chromel and Constantan (Type E), iron and Constantan (Type J), Chromel and Alumel (Type K), and copper and Constantan (Type T). Chromel, Alumel, and Constantan are registered trademarks of the Hoskin Manufacturing Company. Nobel metal thermocouples are also available from some temperature sensor suppliers. Each type operates within a specified temperature range.

RTDs

Resistance temperature detectors (RTDs) are wire windings that exhibit changes in resistance with changes in temperature. The hotter they become, the higher the value of their electrical resistance. Platinum is the most commonly used material because it is nearly linear over a wide range of temperatures, is very accurate, and has a fast response time. RTDs can also be made of copper or nickel, but these materials have restricted ranges and problems with oxidation. RTD elements are usually long, spring-like wires surrounded by an insulator and enclosed in a sheath of metal.

Advantages of RTDs include their stable output for long periods of time. They are also easy to calibrate and provide very accurate readings.

Disadvantages include a smaller overall temperature range, higher initial cost, and a less rugged design.

Thermistors

Thermistors are extremely sensitive, but have a limited temperature range. A thermistor is a semiconductor device with an electrical resistance that is proportional to temperature. There are two types of thermistors: negative temperature coefficient (NTC), where the resistance drops nonlinearly with a rise in temperature; and positive temperature coefficient (PTC), where the resistance increases as the temperature rises. There are several advantage and disadvantages to using an NTC thermistor thermometer.

Advantages of thermistors include their small size and high degree of stability. NTCs are also long lasting and very accurate.

Disadvantages include their non-linearity, and unsuitability for use in extreme temperatures.

Bimetal Switches

Bimetal switches use a bimetal spring as their temperature-sensing element. This coil spring is made of two different types of metals that are fastened together. These metals could include copper, steel or brass as long as one has low heat sensitivity while the other metal has high heat sensitivity. Whenever the welded strip is heated, the two metals change length based on their individual rates of thermal expansion. Since the two metals expand to different lengths, the bimetallic strip is forced to bend or curl towards the side with a lower coefficient of thermal expansion. The movement of the strip is used to deflect a pointer over a calibrated scale, which then indicates temperature to the user.

Advantages of bimetal switches include their low cost and toughness. they are also easy to use and install, and accurate over a wide range of temperatures

Disadvantages include that only indicating types are available, the possibility of calibration change due to use or environment, and that bimetal switches are not as accurate as glass stem thermometers.

Noncontact Infrared Temperature Sensors

Noncontact infrared temperature sensors absorb ambient infrared (IR) radiation emitted by a heated surface. They are used in a variety of applications where direct temperature measurement is not possible. With noncontact infrared temperature sensors, incoming light is converted to an electric signal that corresponds to a particular temperature.

Advantages of Noncontact infrared temperature sensors include separation from the source being measured, laser spot aiming and ease of use.

Disadvantages include higher cost and the requirement for electrical power.

Specifications

Buyers should consider these product specifications when selecting temperature sensors

• Temperature measurement range includes the minimum and maximum temperature that can be observed.
• Accuracy refers to how exactly the temperature of the thermal sensor matches that of its targeted measured environment.
• Stability includes the sensor's optimum operating environments, durability, and life expectancy.
• Probe type describes the unit which houses the temperature sensor. There are several different styles available, which are described on the temperature probes page on the Engineering360 site.
• Termination style refers to how the user knows when the reading is completed. Options include alarms, timed readings or a switch set to a specific temperature.

Combination Units

Temperature sensor suppliers may offer both temperature sensor kits and temperature liquid level sensor combination units.

Temperature sensor kits include a thermocouple or RTD with a stainless steel temperature probe that can be connected to a programmable controller or indicator.

Temperature and liquid level sensor combination units are designed for use with liquid level switches. These temperature sensors may incorporate RTD or thermocouple technology.

Image credit:

OMEGA Engineering, Inc.