Types of Temperature Sensors

Measurement of temperature is one of the most common requirements in the field of engineering and science and in industrial applications. The temperature of any process is measured to control and ensure its proper working. A variety of temperature sensors are employed for these purposes; however, all these sensors function differently due to their construction and working mechanisms.

It is important to use an appropriate type of sensor for measuring a certain temperature. Otherwise, faulty results can be obtained or even dangerous situations might arise. The following article describes some major categories of temperature sensors along with their basic principle of operation and practical uses.

Thermocouples: Wide Range, Industrial Strength

Thermocouples could be regarded as some of the most common temperature sensing devices in industries, and there are reasons for that. The devices are simple in design and operation, durable, and not expensive, as well as able to measure the largest temperature ranges from approximately -200°C up to 2300°C, depending on the type of thermocouple.

The principle of operation lies in the Seebeck effect. If the two different metals are connected at one end (measuring junction) and the other ends are connected to a device that measures the voltage (reference junction), a voltage is generated. It is proportional to the temperature difference between the two junctions. With knowledge of the temperature of the reference junction, it is possible to calculate the temperature of the measuring point by measuring the generated voltage.

There are several standard types of thermocouples, but the most commonly used in general industries is a type K, which consists of a chromel-alumel alloy and covers the approximate temperature range of minus 200°C up to 1260°C. Thermocouples with types R, S, and B are applied at higher temperatures, especially in precious metal and glass production.

Applications:

Metal and steel manufacturing furnaces
Gas turbine exhaust gas monitoring
Boiler flue gas temperature measurement
Automotive engine testing
Food industry ovens

One of the main drawbacks of the thermocouple sensor is the limited generation of electricity, which makes it highly vulnerable to external electrical disturbances. The other drawback associated with thermocouples is the requirement for reference junction correction.

Resistance Temperature Detectors (RTDs): Accuracy Above All

Unlike the thermocouple, RTD operates on completely different principles where they depend on variations of the resistance value at varying temperatures. The materials commonly used in most RTDs are platinum as the sensing component due to its predictable resistive property. The most popular RTD standards are the Pt100 that measures 100 ohms at 0°C.

With increasing temperature, there will be a linear increase in platinum resistance. Such an increase is recorded, converted, and then read as the temperature value. Because of platinum’s high stability, it ensures that RTDs are more accurate with accuracy ranging from ±0.1°C to ±0.5°C, and such an accuracy level remains constant for a considerable period.

However, RTDs have higher costs and fragility when compared with thermocouples, and their maximum operating range can only go up to 600°C.

Applications:

Pharmaceutical production processes where regulatory accuracy is required
Food and beverage manufacturing
HVAC air and water temperature control systems
Laboratory instruments and calibration equipment
Power generation plants

RTDs are the sensor of choice wherever precision matters more than cost and wherever the process temperature stays within their working range.

Thermistors: High Sensitivity in a Compact Package

Just like RTDs, thermistors are also resistance-type sensors, though their behavior is not quite similar to the latter. Where the resistance of an RTD increases gradually as the temperature rises, the resistances in thermistors show considerable fluctuations over the range of temperatures from –50° C to +150° C. Thermistors consist of semiconductor ceramics.

Thermistors come in two kinds. An NTC thermistor shows the effect of a fall in resistance due to an increase in temperature, whereas a PTC thermistor shows the effect of a sudden surge in resistance at a certain temperature point. While PTC thermistors are less common, NTC thermistors are more preferred due to their high sensitivity.

The issue that has to be taken into consideration in order to design a circuit using a thermistor is that of self-heating. This means that, since thermistors work based on measurements of resistances, there should be a certain current flow for the measurement of temperature. In case the current excitation is high, the thermistor measures wrong temperature values due to heating effects caused by current flow.

Applications:

Medical devices, including digital thermometers and incubators
Battery management in laptops and mobile devices
Home appliances such as refrigerators, air conditioners, and water heaters
Cabin temperature sensing in vehicles

Infrared Temperature Sensors: Measurement Without Contact

All the previously discussed sensors need direct contact with the object or medium under consideration. Infrared sensors operate on a totally different principle; they sense the thermal radiation of the object and measure the temperature without coming into contact with it in any way.

In fact, all objects radiate infrared energy if their temperatures are above absolute zero. The amount of radiation is proportional to the temperature of the object. The infrared sensor collects the energy emitted by the object through a lens and focuses it onto a receiver, which measures the temperature based on the Stefan-Boltzmann law.

These sensors respond almost instantaneously, are safe for use with rotating or moving objects, and are the only practical option when contact measurement would disturb or damage the process. One limitation is the need to know the emissivity of the target material, surface conditions like dust, moisture, or steam between the sensor and target can also introduce measurement error.

Applications:

Rotating machinery and electrical panel monitoring
Glass and paper manufacturing where contact sensing is not practical
Medical fever screening
Food safety checks in commercial kitchens
Metallurgical plants where surface temperatures are extremely high

Semiconductor IC Temperature Sensors: Plug-and-Play for Electronics

A semiconductor temperature sensor is an integrated circuit that generates either an analog voltage output or a digital signal corresponding to the temperature being measured. This technology utilizes the property of the forward voltage of a silicon p-n junction to vary predictably with temperature.

They are manufactured pre-calibrated and ready for plug-and-play integration into electronic systems without further processing. The LM35 produces a linear analog voltage relative to temperature changes, while the TMP36 and DS18B20 use a digital interface for single-wire communication.

The disadvantages are similar in nature. For instance, the sensing temperature ranges are -55°C to 150°C; they are less accurate than RTDs in challenging applications, and their fragility makes them unsuitable for harsh industrial settings. Nevertheless, when used in electronics and IoT platforms, they provide optimal results.

Applications:

Consumer goods such as laptops, mobile phones, and tablets for thermal control purposes
Thermal management of circuit boards
Weather monitoring
Monitoring physiological parameters in wearable medical devices
IoT applications

The Key to Selecting the Correct Temperature Sensor

Choosing the appropriate temperature sensor requires the consideration of many aspects as a whole rather than in isolation. One aspect to look at first is the temperature range since a thermocouple works well at extremes, an RTD and a thermistor at moderately hot temperatures, and an IC sensor at low temperatures common in electronics systems.

The physical environment plays a significant role, as vibrations, moisture, chemicals, and electrical disturbances can affect your sensor’s performance. The ability to make a contact measurement, the time the sensor should react, the budget, and output format are also important aspects in this regard.

A correctly chosen temperature sensor should work efficiently without any noticeable effort on your part by providing correct measurements. In contrast, a wrongly chosen sensor will become a frequent cause of mistakes and replacements, as well as process disturbances.

Conclusion

Temperature sensing is an area that cannot be generalized. The existence of thermocouples, RTDs, thermistors, infrared detectors, semiconductor sensors, or bimetals proves there are actual situations where they provide better results than other types.

Grasping the mechanism of operation of each type makes it much easier to choose the best device for each specific application, and this selection is what distinguishes a successful system from an unreliable one that requires constant maintenance. With increasing automation of processes in different industries, temperature sensors are also developing correspondingly, providing ever more precise measurements.

Take a look at our complete range of industrial heating products to see how we can help your facility. You can also head back to the Wattco to learn more about our engineering team and custom solutions.