Temperature probes

Whenever temperatures need to be measured, monitored, controlled, or regulated, temperature probes or temperature sensors are used. Depending on the type of sensor, measurements can be performed in temperature ranges from -200°C to well above 1000°C.

Operating principle of temperature probes

The temperature sensing element is a fundamental component of every temperature probe. It detects and monitors temperature changes in the environment, solids, gases, and liquids and transmits the measured values to systems such as measuring instruments, computers, or other monitoring devices. In most cases, temperature sensors maintain continuous contact with the medium being measured (invasive measurement).

In addition to invasive probes, clamp-on probes are also available. Since these sensors are merely attached to the surface of the medium rather than being inserted into it, they do not have direct contact with the process medium, making them a non-invasive measurement method. This allows, for example, temperature monitoring of pipelines without affecting the flow inside the pipe.

Although non-invasive measurement offers several advantages, it is generally less accurate because there is no direct thermal contact with the medium itself. For this reason, clamp-on or surface probes are often used primarily for monitoring applications. Ambient temperature also significantly influences surface-mounted probes, which is why adequate insulation and shielding are required. Alternatively, infrared thermometers or pyrometers can be used for surface temperature measurements.

Mineral-insulated sheathed sensors, manufactured from highly durable stainless-steel cable, are particularly suitable for high-temperature applications.

Cable probes

A cable probe represents the most basic and flexible design in temperature measurement technology. In this configuration, the sensing element—such as a Pt100, Pt1000, or NTC sensor—is directly connected to the lead wire and secured within a protective sleeve. To achieve good thermal coupling and high mechanical strength, thermal compound is typically applied inside the sleeve, and the cable transition is mechanically crimped or rolled.

The operating limits of a cable probe are largely determined by the insulation material of the lead wire. For standard applications in building services and heating systems with moderate temperatures, cost-effective PVC insulation is commonly used. When higher thermal and chemical resistance is required, such as in mechanical engineering or the chemical industry, PTFE or FEP insulation is preferred. These materials offer excellent resistance to acids, alkalis, and oils.

For applications involving significant mechanical stress and medium temperatures, silicone-insulated cables provide outstanding flexibility. When high-temperature measurement is required, for example on motors or heating elements, fiberglass insulation is used. It withstands very high temperatures but is inherently not resistant to hydrolysis.

An important quality criterion for cable probes is the IP protection rating. Sensors used in humid environments or refrigeration technology must be moisture-resistant. Additional heat-shrink tubing or specialized potting techniques are often employed to achieve protection classes such as IP67 or IP68.

Screw-in probes

For permanent installation in pipelines, vessels, or machine components operating under pressure, screw-in probes are the standard design used in process engineering. These sensors feature an integrated process thread positioned directly above the sensing sleeve. Common thread types include G1/2", G1/4", and various metric thread sizes.

The outer diameter and immersion length of the screw-in probe must be matched to the flow conditions and pipe diameter. If the insertion length is too short, the sensing element remains too close to the pipe wall, causing measurement errors due to heat conduction. If the sleeve diameter is excessively large relative to the pipe diameter, flow resistance increases and may lead to turbulence.

Screw-in probes are widely used in industrial plants, hydraulic systems, and process engineering applications. Signal transmission is achieved either via a permanently attached cable or through standardized connectors such as M12 or DIN connectors.

Immersion probes

Immersion probes are specifically designed for measuring temperatures in liquid and gaseous media. In many industrial applications, the sensor is installed in conjunction with a permanently mounted thermowell. This arrangement allows the sensor to be removed for maintenance or calibration without draining or depressurizing the process.

Immersion sensors must provide an optimal balance between mechanical stability and thermal response time. Since the thermowell introduces an additional thermal barrier, the air gap between the sensor and the thermowell is often filled with thermal compound to improve heat transfer and minimize response times.

Typical applications include large thermal storage tanks in solar heating systems, district heating substations, heat pump components, and process vessels in industrial plants.

Surface probes and temperature probes for pipes

When process penetration is either impractical or undesirable for safety or design reasons, surface sensors are used. These sensors measure temperature through direct contact with the external surface of an object. Since metals are excellent heat conductors, the temperature of the medium flowing inside a pipe can often be accurately inferred from the pipe surface temperature.

A specialized variant is the pipe clamp sensor. It features a mounting surface adapted to the pipe radius, often prism-shaped, and is secured using metal straps, hose clamps, or cable ties. To minimize measurement errors caused by ambient influences such as drafts, appropriate insulation should be applied.

Magnetic probes are frequently used in test benches and for temporary monitoring of control cabinets and steel structures. Integrated permanent magnets allow the sensor to be attached without tools and without damaging the surface.

Surface probes are a key component in heating and HVAC systems for cost-effective monitoring of supply and return temperatures.

Mineral insulated thermocouples

For applications requiring extremely fast response times, exceptional vibration resistance, and a very wide temperature range, mineral insulated thermocouples represent one of the most advanced technical solutions. Unlike resistance temperature detectors (RTDs), these sensors operate based on the thermoelectric effect.

A mineral insulated thermocouple consists of a thin, flexible sheath made of stainless steel or a high-temperature nickel alloy. Inside the sheath, the thermocouple wires are embedded in highly compacted magnesium oxide powder. This material electrically insulates the conductors while simultaneously providing excellent thermal conductivity.

Thanks to this construction, mineral insulated thermocouples are highly resistant to severe mechanical vibration, making them ideal for engines, turbines, and chemical processing equipment.

With sheath diameters of less than 1 mm, they are suitable for both high-temperature measurements in furnaces and exhaust systems and low-temperature measurements in cryogenic applications. Electrical connections are typically made using standardized thermocouple connectors, available in both standard-size and miniature versions.

Bayonet probes

In machinery and equipment exposed to strong cyclic vibrations and pressure loads, conventional mounting methods often prove inadequate. Bayonet probes are specifically designed for such environments. They feature a spring-loaded cap that locks into a matching bayonet adapter.

By rotating the bayonet fitting, the internal spring is compressed, forcing the sensor tip against the measuring point with a defined and constant contact force. This ensures reliable thermal contact even when thermal expansion or vibration occurs.

Bayonet probes are commonly used in toolmaking, plastic extrusion systems, and injection molding machines, where precise monitoring of molten polymer temperatures is essential.

Penetration and core temperature probes

Whenever the internal temperature of a solid material or a viscous medium is critical for quality assurance, core temperature probes are employed. Their most distinctive feature is a needle-shaped probe tip that allows easy penetration into the product being measured.

To withstand insertion forces, the probe is designed with thick walls and high mechanical strength. The rear section typically includes an ergonomic handle made from robust plastic or PTFE, providing safe handling while acting as a thermal barrier for the operator.

Core temperature probes are the primary measurement tool in the food industry for monitoring cooking, cooling, and storage processes, as well as in pharmaceutical applications. Since these sensors come into direct contact with the product, they must meet stringent hygienic requirements. Therefore, the probe is generally manufactured from stainless steel with a hygienic, crevice-free design that allows thorough cleaning.

Temperature probes with connection heads

For heavy-duty industrial applications, chemical process engineering, and large-scale plant construction, temperature probes with connection heads provide the most robust solution. In this design, the sensing element is installed within a rigid protection tube that terminates in a sturdy connection head.

The connection head, manufactured from die-cast aluminum, stainless steel, or high-performance plastic, protects the electrical connections from harsh environmental conditions. One major advantage is the ability to install a compact transmitter directly inside the head. The transmitter converts the sensor signal—such as the resistance value of a Pt100 RTD or the thermoelectric voltage of a thermocouple—into a standardized industrial signal, such as 4–20 mA, 0–10 V, or a digital fieldbus protocol.

This significantly improves immunity to electromagnetic interference and allows long transmission distances to centralized control systems.

Ambient air temperature probes

Ambient air temperature probes are designed specifically for outdoor temperature measurement. Unlike process sensors, they are intended for wall mounting rather than insertion into a medium.

The housing is typically made from weather-resistant and UV-resistant plastic and provides at least IP65 protection against rain, snow, and splash water. The sensing element is often mounted in a small external sleeve or slightly protrudes from the housing to ensure adequate airflow and rapid response to temperature changes.

Outdoor sensors are key components in building automation, HVAC systems, and heating control systems. They provide the data required for weather-compensated control strategies, allowing heat pumps and boiler systems to adjust supply temperatures according to changing environmental conditions.

Practical selection criteria

When selecting a temperature probe for building services or industrial applications, engineers and purchasing professionals should systematically evaluate several key criteria.

The medium to be measured and its physical state are particularly important. Aggressive liquids require chemically resistant sheath materials and insulation systems such as PTFE or FEP. For solids, penetration probes or surface sensors are generally preferred.

Another important factor is thermal dynamics. If rapid temperature changes must be detected within seconds, a slim mineral insulated thermocouple or a direct-contact cable sensor with a small diameter is usually the best choice. For slower systems such as large water storage tanks, conventional immersion sensors installed in thermowells are generally sufficient.

Mechanical stress must also be considered. Strong vibrations near motors or pumps can damage delicate sensing elements over time. In such environments, bayonet sensors and mineral-insulated thermocouples offer superior operational reliability due to their robust internal construction.

Finally, the electrical interface must be selected appropriately. For short distances between sensor and measuring device, Pt1000 or NTC sensors in a two-wire configuration are often sufficient. For longer transmission distances in extensive industrial facilities, sensors with connection heads and integrated transmitters are recommended, or alternatively RTDs connected using three-wire or four-wire circuits to minimize measurement errors.

Non-contact alternatives such as infrared thermometers should be considered whenever physical contact with the measuring surface is technically impractical or prohibited.