Types of Temperature Sensors

Various types of temperature sensors are available. You can choose from Thermocouple, RTD, NTC thermistors, or Semiconductor-based sensors. Each has its advantages and disadvantages, so it’s important to choose the correct one for your application. Let’s look at some of them.

RTD

The RTD temperature sensor market is highly competitive, with several manufacturers operating in the domestic and international markets. Major players are focusing on mergers and acquisitions to further strengthen their positions in the market. For example, STMicroelectronics recently entered into an alliance with Hyundai Autron to provide eco-friendly automotive sensor solutions. Moreover, the company also focuses on powertrain controllers.

An RTD temperature sensor works by changing its resistance as the temperature increases. This relationship between resistance and temperature is well-known. In order to measure the temperature, an external electronic device passes a small electrical current through the sensor and generates a voltage. The measuring current used in RTDs is typically low, up to one milliamp (mA), and the resistance tolerances vary based on the type.

Thermocouple

A thermocouple temperature sensor is a device that detects changes in temperature. Its sensitivity and temperature range depend on its type and alloy. There are three types of thermocouples: platinum resistance type, thermoelement type, and thermistor type. The thermoelement type is the most common thermocouple sensor. It is suitable for most general applications.

Thermocouples can be made of various materials and are able to measure extreme temperatures between -200oC and 2000oC. There are standards for the range of temperatures that thermocouples can measure, and colour coding is available to help users select the correct sensor for their application.

A thermocouple temperature sensor has two wires that connect to the electrical circuit. The wires should be made of the same metal as the thermocouple. Otherwise, the wires will create different measuring junctions, giving inaccurate readings.

NTC thermistors

NTC Thermistors are a type of temperature sensor that has a resistance that changes with temperature. The decrease in resistance is proportional to an increase in temperature and is related to a constant beta in degrees Kelvin (degK). This characteristic makes NTCs a good alternative to conventional semiconductor circuitry. They are available in many shapes and lead types, allowing them to used for a variety of applications.

NTC resistance of a NTC thermistor measured by using an ohmmeter setting on a multimeter. The resistance value displayed on the multimeter should match the ambient temperature near the thermistor. The resistance will change with temperature, as shown in Figure 2. The change in resistance is not linear over the temperature range of the thermistor.

Semiconductor-based sensors

Semiconductor-based temperature sensors are based on integrated circuits (ICs) and provide high accuracy and linearity over a wide temperature range. These sensors can also contain extensive signal processing circuitry. They can produce digital or analog output. There are various types of semiconductor-based temperature sensors, and they can also further divided into subgroups. The semiconductor-based temperature sensors with analog output typically contain a temperature sensor, an ADC, a two-wire digital interface, and registers.

One type of sensitive temperature sensor is the thermistor. This type of sensor has a large resistance at low temperatures, and its resistance drops rapidly as temperature rises. It also has a negative temperature coefficient, which means that small changes in temperature displayed with high accuracy. These sensors have a wide range of temperature, and their working temperature is usually between -50 and 250 degC. Semiconductor-based temperature sensors have a dual integrated circuit that contains two diodes that have voltage and current characteristics based on the temperature of the materials.

Bi-metal thermometers

Bi-metal temperature sensors used to detect temperature. The strip on the sensor made of two different metals – red and blue. Red metal expands at a faster rate than blue metal, and this difference causes the strip to bend upward as the temperature increases. This metal strip connected to a wire by a small gap. These temperature sensors can calibrated to detect high and low temperatures accurately.

The bi-metal thermometer works by measuring the difference in the coefficients of thermal expansion. Two metals attached together to form a strip, and the movement of the strip correlated to temperature indicated on a scale. These sensors are simple, inexpensive, and reliable.

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