Temperature sensors for process installation

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Temperature sensors for process installation measure where the temperature originates: in the medium, in the pipe, in the vessel, or in the bearing. Available as screw-in probes, modular compact sensors, mineral-insulated or cable probes, with Pt100 and Pt1000 resistance thermometers or thermocouples, with process connections in G and NPT threads, and stainless steel thermowells.

Temperature sensors for process installation: In the process or from the outside?

The first decision does not concern the probe, but rather the access to the object being measured.

In the process, you measure most accurately and quickly because the probe has direct contact with the medium. This requires an opening, a process connection, and the ability to shut down the system for installation and replacement.

If that is not possible, there are two alternatives: magnetic surface sensors that are attached to the outside of a pipe or vessel, or infrared sensors that measure without contact. Both are less accurate than measuring in the medium, but can be retrofitted without intervention.

This category covers installation into the process.

Immersion depth: the most common installation error

If a probe is immersed too shallowly, the thermowell conducts heat to the outside, and the sensor measures not the medium temperature, but an average of the medium and the ambient temperature. The error can amount to several Kelvin and is usually blamed on sensor accuracy, even though it originates from the installation.

As a rule of thumb, an immersion depth of at least ten to fifteen times the thermowell diameter, measured from the inner pipe wall, should be maintained. For a 6 mm thermowell, this is 60 to 90 mm into the medium. In thin pipes, this can only be achieved with angled installation, installation in a bend against the flow, or with a thinner mineral-insulated element.

Further installation rules: The probe should be positioned in the flow and not in a dead zone; in horizontal pipes, it should ideally not be installed from below, as deposits can collect there.

Thermowell diameter: response time versus robustness

A thin thermowell reacts quickly, a thick one lasts longer. You must consciously decide on this trade-off.

For control tasks and fast processes, response time is paramount; here, mineral-insulated elements with a diameter of 1.5 to 3 mm are the right choice. In cases of high flow velocity, vibration, or abrasive media, mechanical strength is the deciding factor, and a thermowell from 6 mm or an additional protection tube is appropriate.

Thermowells are available from 1.5 to 15 mm in diameter and nominal lengths from 50 to 1000 mm, in stainless steel 1.4571 and 1.4401, and for mineral-insulated elements also in Inconel 2.4816 for high temperatures.

Designs

Screw-in sensors with integrated electronics combine a probe and transmitter in one device. They provide 4…20 mA output directly, some with displays and limit contacts, and require no separate head transmitter. Process connections from 1/4" to 1", also in NPT.

Modular compact sensors are designed for tight installation spaces, are fully encapsulated and made entirely of stainless steel, with a transmitter as an option. Spring-loaded versions press the sensing element against a surface and are suitable for boreholes, bearing points, and mountings without medium contact.

Probes with a connection head are the classic process design. Their advantage lies in service: the measuring insert can be replaced without removing the thermowell and thus without opening the process. In systems that cannot be easily shut down, this is a strong argument.

Mineral-insulated and cable probes are flexible, thin, and fast. They are suitable for small spaces, for measurements at multiple points, and wherever no threaded connection is provided.

Temperature switches do not provide a measured value, but rather a switching contact. If only limit value monitoring is needed, this is the more cost-effective and simpler solution.

Hygienic designs with appropriate process connections and surfaces for food, beverage, and pharmaceutical applications.

Selecting the process connection

Fixed threads in G or NPT are standard and determine the installation length. A compression fitting, by contrast, allows for infinitely variable adjustment of the immersion depth during installation. This is useful wherever the exact depth is only determined on-site, and it regularly saves projects where the drawing deviates from reality.

Additionally, check the permissible system pressure and the sealing type, as well as the required surface quality for hygienic applications.

Measuring principle and accuracy class

Resistance thermometers with Pt100 or Pt1000 are the standard for the medium temperature range; they are very accurate and long-term stable. Available in 2-, 3-, and 4-wire configurations as well as in classes AA, A, and B, up to 1/10 DIN if required.

Thermocouples of types J, K, and N cover significantly higher ranges, up to 1,200 °C, and are robust and fast-reacting. The trade-off is lower basic accuracy and the necessity of cold junction compensation in the evaluation unit.

As a rule of thumb: Everything up to around 400 °C favors the Pt100, everything above that favors the thermocouple. You can find more about signals, displays, and HART under temperature transmitters, and more about high-temperature applications on our page for high-temperature measurement technology.

Approvals

ATEX and IECEx versions are available for potentially explosive areas; see explosion protection for details. For shipbuilding and offshore applications, variants with DNV and EU-RO approval are available. Calibration can be performed upon request according to ISO 17025 in our own laboratory.