Temperature sensors and thermostats

Temperature sensors and thermostats are technical components that provide precise temperature monitoring and control in industry, households, and automation. From thermocouples, RTDs, or bimetallic switches, each system has specific strengths and applications. This article provides an overview of the various components for accurate temperature measurement and temperature monitoring and shows differences and typical applications of the most important temperature sensors and thermostats. Find the right solution for your temperature measurement needs.

What are temperature sensors and temperature switches?

The variety of terms may initially be confusing. In principle, temperature can be measured by electrical or mechanical means.

Temperature sensors is the generic term for electrically measuring sensors that convert physically detectable temperature values into electrical signals. This also includes the thermocouple, which consists of two different metals and measures the resulting thermo-electric voltage. Thermistors are made up of one or more temperature sensors.

Temperature sensor, also called a thermal sensor, is in turn a generic term for a component that converts temperature into measurable signals using electronic/electrical components. They are used wherever circuits are controlled depending on temperature, e.g. as overheating protection. Thermal sensors are further differentiated according to their measurement principle: They measure either resistance or voltage. For example, there is also the RTD, which measures temperature changes by changes in the electrical resistance in a platinum wire.

Temperature switch is a generic term for components that change their switching state depending on the temperature. Unlike thermocouples and temperature sensors that operate permanently, temperature switches only know the ON or OFF switching state. For this purpose, two strips made of different metal (e.g. zinc, steel) that are permanently connected to each other are installed in a bimetallic switch (PTO). These metals expand to different degrees when the temperature changes and thus bend the strip and trigger the switch.

Temperature switches are usually divided into the categories of temperature regulators, temperature limiters or temperature monitors according to their switching function. A temperature regulator, also called a thermostat, readjusts the temperature in the event of a deviation from the setpoint (above or below the setpoint) so that the setpoint is reached again. A temperature limiter protects against overheating by stopping any further heating when a setpoint value is reached. A temperature monitor starts fans or heaters when a temperature exceeds or falls below a previously defined temperature, e.g. to protect against frost or overheating. Temperature switches are used e.g. in everyday use as thermostats in heating systems, but also especially in the IT area for cooling electrical devices.

Temperature measurement via electrical resistance

As noted above, the temperature can be determined using thermal sensors by measuring resistance or voltage. Temperature sensors or thermistors that determine the temperature due to the change in resistance are so-called resistance thermometers (also RTD = Resistance Temperature Detector) such as the Pt100 temperature sensor, NTC hot conductor and PTC cold conductor. All three variants take advantage of the fact that the resistance in metals changes depending on the temperature. When the resistance value is mapped to a temperature value, the resistance can be used to measure the temperature.

But what is the difference between NTC / PTC and RTDs like the Pt100?

NTC, PTC and Pt100

Hot conductors, also called NTC temperature sensor or thermistor (NTC = Negative Temperature Coefficient), employ negative temperature coefficients. This means that the electrical resistance is high in the cold state and decreases as temperatures increase or the electrical current is better conducted at high temperatures.

On cold conductors PTC (PTC = Positive Temperature Coefficient), this works exactly the other way around: Electrical resistance is low in the low temperature range and increases as temperatures increase. The NTC thermistor and PTC thermistor are both made of semiconductor materials.

The temperature sensor Pt100 is an RTD made of pure metal. Typically, platinum is used as its resistance increases linearly as temperature increases. It therefore acts similarly to the PTC thermistor. Compared with the semiconductor elements of NTC and PTC thermistors, the platinum elements of the Pt100 comply with international standards. The Pt100 also has better long-term stability and can be used in higher temperature ranges of up to 500 °C. In general, however, RTDs are usually used in the medium and low temperature ranges.

Temperature measurement via electrical voltage

Thermocouples are used to measure temperature via electrical voltage. A thermocouple consists of two different metallic conductors connected to each other at one end and using the so-called thermoelectric effect to measure temperatures. A temperature difference along the conductors creates an electrical voltage - the so-called thermo-electric voltage - which is proportional to the temperature difference. This feature makes thermocouples into reliable sensors for a wide range of applications. Compared to RTDs, thermocouples can be used at high temperatures. There are various thermocouples, such as Type J, K, T or E thermocouples. Below is a brief overview of Types J and K, as well as an insight into thermocouple color coding.

Type J thermocouple

The Type J (iron-constantan) thermocouple is suited for use in reducing atmospheres and can be used up to a continuous operating temperature of 800 °C, and for short periods up to 1,000 °C. However, in oxidizing environments above 550 °C and at temperatures below the ambient temperature, there is an increased risk of material damage such as rusting and embrittlement.

Type K thermocouple

The Type K (nickel-chromium-nickel) thermocouple is the most commonly used thermocouple and is suited for a wide temperature range from -250 °C to momentarily 1,200 °C, with a continuous operating limit of approximately 1,100 °C. The appropriate thermal insulation also plays a crucial role here. The Type K thermocouple is cost-effective and versatile, but exhibits pronounced temperature change hysteresis at 250-600 °C. Delayed or erratic behavior can lead to inaccuracy and deviation errors in this range, which can increase significantly at temperatures above 800 °C and at temperatures above 850 °C can then also lead to irreversible (permanent) measurement deviations due to oxidation. In reducing atmospheres, it may only be used with an appropriate protective sheath; for nuclear applications, type N is considered the more stable alternative.

Accuracy Classes for Thermocouples and RTDs

Thermocouples and RTDs are each divided into different accuracy classes. The accuracy class gives an indication of the permissible deviation of the measurand from the actual value.

It should be noted that the accuracy classes refer only to the sensor element and not to errors due to connections, cables, etc.

Example using a thermocouple:

At a temperature of 500 °C measured by a type K / accuracy class 2 temperature sensor, a possible measurement error of ±3.75 Kelvin results. Since this value is higher than the alternative direct numerical value (±2.5 Kelvin), it is assumed to be the valid value for this temperature.

Color coding of thermocouple compensating cables

Compensating cables for thermocouples are color coded differently according to the respective regionally recognized standards. The color coding is used for the practical purpose of quickly identifying the connection cables and avoiding connection errors.

Today, the EN IEC 60584-3 color coding is the most common in Europe. According to this standard, the jacket of the negative conductors is always white and the jacket of the positive conductors is given a defined color.

In addition to EN IEC 60584-3, which is widely used in Europe, there are also other standards with color coding for thermocouples. The ANSI MC 96.1 color coding assigns different colors to each type. The negative conductor jacket is always red according to ANSI MC 96.1.

MISUMI, as a company with Japanese roots, also offers thermocouples and compensating cables with color coding according to the Japanese standard JIS C 1610, which also deviates from EN IC 60584-3.

Mounting options and positioning aids

For a safe and maintenance-friendly connection between the thermocouple and the evaluation unit, thermo-voltage-free connectors should be used, as these influence the values determined by the temperature sensor as little as possible. The MISUMI web shop has a variety of connectors, such as the type K temperature sensor connector (MSNDC).

In order to prevent falsification of the measurands, a compensating connection cable should be used as a connection cable for jacketed thermocouples or as an extension for a temperature sensor (type K). This can be used at an operating temperature between 0 °C and 150 °C. They only function correctly within the defined temperature range. The compensating connection cable or extension cable ensures that the signal transmission works without temperature falsification, i.e. the thermo-electric voltage arriving at the analysis device corresponds to the voltage at the measuring point.

MISUMI offers the MSPL and MSNFG mounting brackets, for example. For the MSPL mounting bracket, a 1/8 R(PT) thread is cut into the object being heated.

Application: Position the parts 1 (see Fig.) in place, then insert the sensor after initial tightening of 2 and 3. Then tighten part 3 and position 2 and 3. After tightening, parts 2 and 3 are connected and can no longer be removed. The temperature sensor cannot be removed. If airtightness is required, a screw with conical thread should be used.

Mounting bracket MSPL is very well suited for precisely adjusting the sensor position to the respective situation and insertion depth. The sensor is firmly clamped to the holder and cannot be moved after assembly.

Mounting bracket MSNFG allows a subsequent correction of the sensor position, since the sensor can be moved inside the bore after loosening the set screw. By retightening the set screw, the sensor will be securely held in position again at the desired insertion depth.

Temperature sensors can also be provided directly with a threaded joint, with the help of which, for example, they can be screwed into pipelines, containers or machine components. This creates a permanent and vibration-resistant joint. The threaded joint is fastened to the object to be heated by cutting a thread.

For measuring cylindrical objects, it is also recommended to use a temperature sensor with a strap connector. The strap is tightened around the cylinder.

For moving parts, it is recommended to use a ring cable lug that is screwed into the heated object using a screw and provides a greater range of motion.

Another option for fastening is a spring contact, e.g. an MSNBB with spring contact. The contact between the heated object and the tip of the thermowell is held securely by the spring force. By loosening and moving the screw, the spring tensile force can be adjusted.

Application: Cut the 1/8 thread Rc(PT) into the object to be heated and secure with (1). Insert the sensor, then hook latch (2) into the protrusion of part (1).

The MSNDL temperature sensor with L-shape (1 = limit) is suited for tight installation spaces.

Jacketed or free temperature sensor?

On a sensor with a jacket, the measuring tip is protected by a sheathed cable filled with a mineral insulator. This allows the measuring tip to be used even at higher temperatures. During assembly, however, care must be taken regarding the lower temperature resistance of the connection cables and plugs. They are usually significantly more robust than free temperature sensors, but are also more sluggish in reacting to rapid temperature changes.

For the temperature sensor, e.g. the MSEN thermocouple, the temperature measuring point is exposed, which makes the reaction faster than with a jacketed design. The temperature measurement can be carried out at the measuring point of the device under test. This measuring point is created in advance by turning and welding the exposed Alumel or chrome part.

How are temperature sensors used in industrial plants?

Industrial thermometers such as temperature sensors are essential in industrial plants to monitor processes, ensure product quality, and protect equipment against damage. Temperature sensors are used, for example, to monitor bearing temperatures in electric motors in order to protect them against premature wear or failure due to overheating. Even in battery storage systems, the use of temperature sensors can protect against overheating. Temperature sensors can also control drying processes, e.g., in the paper and pulp industry, and ensure consistent product quality. Temperature sensors also ensure stable process conditions in industrial 3D printing.