Concept, Types, and Operating Principles of Temperature Sensors

In our daily lives, we all frequently encounter thermometers, water heaters, microwave ovens, refrigerators, and the like. All of these devices rely on an important component. --Temperature sensors: This article will introduce temperature sensors, their operating principles, and the different types available.

What is a temperature sensor? ?

A temperature sensor is a device that measures the hotness or coldness of an object and provides temperature readings in a readable format via electrical signals. Common types include thermocouples and resistance temperature detectors.

Temperature sensor type

In practical applications, numerous temperature sensors are available, each exhibiting distinct characteristics depending on the specific use case. Temperature sensors are broadly classified into two fundamental physical types:

Types of Contact Temperature Sensors — These types of temperature sensors require physical contact with the object being measured and rely on conduction to monitor temperature changes. They can be used to detect solids, liquids, or gases over a very wide temperature range.

Types of Non-Contact Temperature Sensors — These types of temperature sensors utilize convection and radiation to monitor temperature changes. They can be used to detect both liquids and gases, which emit radiant energy as they heat up and cool down, with the cooler portions sinking to the bottom through convection, or to measure radiant energy transmitted from objects in the form of infrared radiation (such as solar radiation).

Contact and non-contact temperature sensors are further classified into the following types; the operating principles of these sensors will now be explained.

Principle of Temperature Sensors

I. Operating Principle of Temperature Sensors --Thermostat

A thermostat is a contact-type temperature sensor composed of two different metals. It is composed of a bimetallic strip made up of materials such as aluminum, copper, nickel, or tungsten.

The difference in the linear thermal expansion coefficients of the two metals causes them to undergo mechanical bending when heated.

I. Operating Principle of Temperature Sensors --Bimetallic thermostat

A thermostat consists of two metals with different coefficients of thermal expansion bonded back to back. When the temperature drops, the contacts close, allowing current to flow through the thermostat. As the temperature rises, one metal expands more than the other, causing the bimetallic strip to bend upward. (Bend it downward) to open the contacts and prevent current flow.

There are two main types of bimetal strips, primarily distinguished by their response to temperature changes. One type produces an instantaneous action on electrical contacts at a set temperature point. The “snap-action” type for “on/off” or “off/on” operations, as well as the slower “creep” type that gradually shifts position, vary with temperature.

Diagram of the operating principle of a bimetallic thermostat

Quick‑acting thermostats are commonly used in our homes to regulate the temperature setpoints of ovens, irons, and immersion water heaters; they can also be found on walls, where they control residential heating systems.

Crawler-type devices typically consist of bimetallic coils or spirals that slowly expand or coil in response to temperature changes. In general, crawler‑type bimetal strips are more sensitive to temperature variations than standard snap‑action switches. /It is more sensitive to temperature changes because the strip is longer and thinner, making it ideally suited for applications such as thermometers and dial indicators.

II. Operating Principle of the Temperature Sensor --Thermistor

Thermistors are typically made from ceramic materials, such as nickel, manganese, or cobalt oxides coated on glass, which makes them prone to damage. Compared with snap-action types, their main advantage lies in their faster response to changes in temperature, accuracy, and repeatability.

Most thermistors have a negative temperature coefficient. (NTC), which means their resistance decreases as temperature rises. However, some thermistors have a positive temperature coefficient (PTC), and their resistance increases with rising temperature.

The rated value of a thermistor depends on its resistance at room temperature. (Typically at 25 °C), their time constants (the time it takes to respond to temperature changes), and their rated power relative to the current flowing through them. Like resistors, thermistors exhibit resistance values ranging from 10 megohms down to a few ohms at room temperature; however, for sensing applications, types with values in the kiloohm range are typically used.

Temperature Sensor Example No1

The following thermistors are in At 25°C, the resistance is 10 kΩ; at 100°C, it is 100 Ω. When this thermistor is connected in series with a 1 kΩ resistor, calculate the voltage drop across the thermistor and, consequently, the output voltage (Vout) at both temperatures, with respect to a 12 V supply.

Example diagram of a temperature sensor

25 degrees Celsius

100 degrees Celsius

By combining The fixed resistor value of R2 (1 kΩ in our example) can be replaced with a potentiometer or a preset resistor to obtain a voltage output at a specified temperature setpoint—for instance, 5 V at 60°C—and by adjusting the potentiometer, a particular output voltage level can be achieved over a broader temperature range.

However, it should be noted that thermistors are nonlinear devices, and the nominal resistance values of different thermistors at room temperature vary, primarily because they are made from semiconductor materials. Thermistors exhibit exponential changes with temperature, thus possessing The beta temperature constant (β) can be used to calculate the resistance at any given temperature.

However, when used in conjunction with a series resistor—such as in a voltage divider network or a Wheatstone bridge configuration—the response to the applied voltage across the voltage divider… The current obtained from the voltage of the bridge network exhibits a linear relationship with temperature. Consequently, the output voltage across the resistor also varies linearly with temperature.

III. Operating Principle of the Temperature Sensor --Resistance Temperature Detector (RTD)

RTDs are precision temperature sensors made by winding high-purity conductive metals—such as platinum, copper, or nickel—into a coil. The resistance of an RTD varies in a manner similar to that of a thermistor. Thin-film RTDs are also available; these devices feature a thin layer of platinum paste deposited on a white ceramic substrate.

Resistance temperature detectors have a positive temperature coefficient. (PTC), but unlike thermistors, their output is highly linear, enabling very accurate temperature measurements.

However, their thermal sensitivity is very poor, meaning that temperature changes produce only very small output variations, for example: 1 Ω/°C.

More common RTD types are made of platinum and are referred to as platinum resistance thermometers or PRTs; the most common is the Pt100 sensor, which has a nominal resistance of 100 Ω at 0 °C. A drawback is the high cost of platinum, and one of the primary disadvantages of this type of device is its expense.

Like a thermistor, RTDs are passive resistive devices; by passing a constant current through the temperature sensor, they produce an output voltage that increases linearly with temperature. A typical RTD has a nominal resistance of approximately 100 Ω at 0 °C, which rises to about 140 Ω at 100 °C, with an operating temperature range spanning from –200 °C to +600 °C.

Because RTDs are resistive devices; we must pass current through them and monitor the resulting voltage. However, when current flows through the resistance wire, any change in resistance due to self-heating—governed by the I²R term (Ohm’s law)—can lead to measurement errors. To mitigate this, RTDs are typically connected to a Wheatstone bridge circuit, which includes additional leads for lead‑wire compensation and/or connections to a constant‑current source.

IV. Operating Principle of the Temperature Sensor --Thermocouple

One of the most common types of temperature sensors is the thermocouple, owing to its wide operating temperature range, reliability, accuracy, simplicity, and sensitivity—largely due to its compact size. Thermocouples also boast the broadest temperature range of all temperature sensors, spanning from below… -200 ℃ to well above 2000 ℃.

Thermocouples typically consist of different metals joined together by welding or crimping. It consists of two junctions, typically made of materials such as copper and constantan. One junction is called the cold junction and is maintained at a fixed temperature, while the other is the measuring junction, known as the hot junction.

When subjected to temperature changes, a voltage drop is generated across the junction.

A thermocouple is a thermoelectric sensor, essentially composed of two different metals joined together by welding or crimping. It consists of two junctions made of materials such as copper and constantan. One junction is kept at a constant temperature and is called the reference (cold) junction, while the other is the measuring (hot) junction. When the two junctions are at different temperatures, a voltage is generated across them, which is used in temperature sensors, as shown below.

Thermocouple Structure

The operating principle of a thermocouple is very simple and fundamental. When two different metals… (For example, copper and constantan) when fused together, they generate a “thermoelectric” effect, resulting in a steady potential difference of only a few millivolts (mV) between them. The voltage difference between the two junctions is known as the “Seebeck effect,” as it arises from a temperature gradient along the conductors, which in turn generates an electromotive force. Consequently, the output voltage of a thermocouple is a function of the temperature change.

If the two junctions are at the same temperature, the potential difference between them is zero; in other words, there is no voltage output, because V1 = V2. However, when the junctions are connected in a circuit and maintained at different temperatures, a voltage output proportional to the temperature difference between the two junctions—V1 – V2—will be detected. This voltage difference increases with rising temperature until it reaches the junction’s peak voltage level, which is determined by the properties of the two distinct metals used.

Thermocouple Amplification

It is essential to carefully select the amplifier type—whether discrete or operational‑amplifier‑based—since excellent drift stability is required to avoid frequent thermocouple recalibration. Consequently, chopper‑stabilized and instrumentation‑type amplifiers are better suited for most temperature‑sensing applications.

Thermocouple Amplification Diagram

V. Semiconductor-Based Temperature Sensors

Semiconductor-Based Temperature Sensors and Dual-Integrated Circuits (IC) They operate together. They comprise two diodes with temperature‑sensitive voltage and current characteristics, enabling accurate measurement of temperature changes.

However, they provide a linear output, but... Accuracy is lower between 1 °C and 5 °C. They also exhibit the slowest response times—ranging from 5 to 60 seconds—within the narrowest temperature range (-70 °C to 150 °C).

V. Semiconductor-Based Temperature Sensors --0V-type vibrating-wire temperature sensor

The Type 0V vibrating-wire thermometer is used to measure internal temperatures in concrete structures or in water. It offers a resolution better than 0.1°C and operates on a principle similar to that of thermocouple temperature sensors. It also features a wide operating temperature range from –20°C to 80°C.

V. Semiconductor-Based Temperature Sensors --ETT-10TH Type Resistive Thermistor Probe

The ETT-10TH resistance temperature probe is a low-mass, waterproof temperature sensor designed to measure temperatures between –20 and 80°C. Thanks to its low thermal mass, it offers a fast response time.

The ETT-10TH resistance temperature probe is specifically designed for measuring the surface temperature of steel and the surface temperature of concrete structures. The ETT-10TH can be embedded in concrete to measure the overall internal temperature, and it is even capable of operating underwater.

The ETT-10TH resistance temperature probe is fully interchangeable. Within the specified operating temperature range, the difference in temperature readings does not exceed 1°C. This allows a single indicator to be used with any ETT-10TH probe without the need for recalibration.

How does the ETT-10TH resistance thermistor probe work?

The ETT-10TH temperature probe features a thermistor with a resistance–temperature curve that is matched to the sensor, encapsulated in epoxy within a copper tube to ensure faster thermal response and environmental protection. The tip of the tube is flattened, allowing it to be securely mounted on virtually any flat metal or concrete surface for measuring surface temperature.

Using an easily obtainable two-part epoxy adhesive, the probe’s flat tip can be secured to most surfaces. If necessary, the probe can also be bolted to a structural surface.

V. Semiconductor-Based Temperature Sensors --ETT-10PT Type RTD Temperature Probe

The ETT-10PT RTD (Resistance Temperature Detector) temperature probe consists of a ceramic resistance element (Pt. 100) calibrated to the European curve specified in DIN IEC 751 (formerly DIN 43760). The resistance element is housed in a robust stainless steel tube with a closed end, providing protection against moisture.

How does the ETT-10PT RTD temperature probe work?

The operating principle of a resistance temperature probe is that the sensor’s resistance varies as a function of temperature. Platinum. RTDs exhibit excellent accuracy, linearity, stability, and repeatability.

The ETT-10PT resistance temperature probe is equipped with a three‑core shielded cable. The red wire provides one connection, while the two black wires together provide another. This configuration compensates for lead‑wire resistance and its temperature‑induced variations. Resistance temperature sensor readings can be easily obtained using a digital RTD temperature indicator.