Thermowells for Pt100 resistance thermometers and thermocouples
Thermowells are hollow bodies that serve as a protective device for thermometers or other temperature measuring devices.They are inserted directly into the medium and separate the thermometer mechanically and thermally from the medium to be measured.
This means that the thermometer itself does not come into direct contact with the liquid, gas or other substances, which both extends the service life of the device and protects the environment and operating personnel.
If the thermowell is installed and designed correctly, the temperature inside the thermowell is the same as in the process. Depending on the design, temperature changes result in a time-delayed temperature adjustment between the medium and the temperature sensor.
Thermowells allow the resistance thermometer to be replaced during operation and thus enable uninterrupted maintenance or replacement of the sensor.
They are usually installed under critical operating conditions, such as aggressive media, high pressures or high flow velocities.
Types of thermowells
- One-piece protective tubes
One-piece thermowells are turned from solid bar stock.
They are usually used for high process-side and high chemical loads.
The maximum length is currently limited to 2 m for production reasons (deep drilling). If longer thermowells are required, several drilled parts are welded together.
One-piece thermowells are used almost exclusively in the petrochemical industry.
- Multi-piece protective tubes
Multi-piece thermowells are made from a tube that is closed at the top by a base section. At the other end of the tube there is usually a thread to accommodate the temperature sensor or other connecting parts such as neck tubes or flanges.
They are used for low or medium process loads.
The maximum length is often several meters. Frequently used in the chemical, apparatus and mechanical engineering industries.
- Thermowells according to DIN
The most common thermowells are defined and described in DIN standard 43772.
This defines manufacturer- and country-independent standards that guarantee a certain level of quality, safety and interchangeability of temperature measuring devices.
By specifying the materials, dimensions and wall thicknesses to be used, the maximum operating pressures at different temperatures can be read off in diagrams.
In many older systems, thermowells are often still installed in accordance with the old DIN standard 43763.
As an aid when renewing the thermowells or replacing the thermometer measuring inserts, you will find below a comparison of the old standard 43763 with the new standard 43772.
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Comparison of DIN 43763 (old) and new DIN 43772DIN 43763 (old)
ShapeDIN 43772
ShapeDesign/connectionInstallation length
L1Total length
L3A1without500517710727100010171400141720002017B12GG 1/2 A160305B2G 1/2 A250395B3G 1/2 A400545C1G 1 A160305C2G 1 A400545D14for welding in65140D2125200D365200D5125260E13conical,
with rim for union nut225307E2285367E3345427F13Fconical,
with flange225307F2285367F3345427G13Gconical,
with G 1 A225307G2285367G3345427
- Thermowells in special designs
Special design thermowells are specially produced protective devices that are used under extreme operating conditions. They are used to protect temperature sensors from mechanical loads, pressure and aggressive media. They are most frequently used when the installation lengths specified in the DIN standard are not sufficient for the process.
Their design requires precise planning and production, whereby the following points should be taken into account:- Material
Die Auswahl des Materials erfolgt hauptsächlich nach der chemischen Beständigkeit gegenüber dem Medium. Häufig werden verschiedene Edelstahllegierungen verwendet.
Bei besonderen Anforderungen werden auch Hochtemperaturlegierungen wie Inconel®, Hastelloy®, Monel® oder Beschichtung aus PTFE, Halar® oder Stelit® verwendet. - Max. temperature
When selecting the material for the protective sleeve, the maximum permissible temperature must be taken into account in addition to the chemical resistance of the material.
High-temperature alloys such as Inconel®, Hastelloy®, Monel® or special stainless steel alloys are often used.
Thermal insulation may also be required to ensure the functionality of the thermometer. - Max. pressure
The pressure calculation is based on standards such as ASME or DIN.
The wall thickness and material properties of the thermowell are decisive for the pressure resistance. It should be noted that the thicker the wall thickness, the longer the response time.
In addition to the operating pressure, peak pressures and a safety buffer in accordance with standard specifications must also be observed.
One-piece thermowells are often used for applications in high-pressure environments (> 200 bar). - Vibration calculation
Vibration analyses are essential for protective tubes in special designs, as resonant frequencies can jeopardize mechanical stability.
Methods such as finite element analysis (FEA) or according to ASME PTC 19.3 TW-2016 enable precise calculations. The length, diameter and flow velocity of the medium play a central role here.
An optimized geometry and, if necessary, damping systems prevent resonance vibrations and thus breakage of the thermowell.
Some manufacturers offer free calculation tools or supply the vibration calculation for the produced thermowell. - Zeugnisse / Prüfungen
Bei Schutzrohren in Sonderbauform wird oftmals vom Kunden auch ein 3.1 oder 3.2 Werkszeugnis nach DIN EN 10204 gefordert.
Die Zeugnisse bestätigen, dass das gelieferte Material mit den Anforderungen der Bestellung übereinstimmt, und enthalten spezifische Prüfergebnisse (z. B. chemische Zusammensetzung, mechanische Eigenschaften).
Die Prüfungen werden vom Hersteller selbst durchgeführt, und die Ergebnisse werden von dessen Qualitätssicherungsabteilung bestätigt.
Beim 3.2 Werkszeugnis wird die 3.1 Prüfung durch eine unabhängige Stelle oder den Kunden selbst überwacht oder verifiziert. Das wird z.B. bei sicherheitskritischen Anwendungen, wie bei der Luft-/Raumfahrt, Schiffsbau oder Hochdruckanwendungen gefordert.
Der Nachweis einer Innen- oder Außendruckprüfung geben zusätzliche Sicherheit bei kritischen Anwendungen.
- Material
Connection of thermowells
- Screw-in connection
The thermowell is screwed via an external thread (usually BSP, NPT or metric thread) into a corresponding threaded hole or threaded connection piece in the container, pipe or apparatus.
The threaded connection is mainly used for moderate temperatures and pressures.
Sealing is achieved with sealing rings (e.g. copper, PTFE, FKM) or with sealing tape (e.g. Teflon tape) for NPT threads.
Advantages: Easy to install, widely used, good sealing.
- Flange connection
The thermowell is connected to a mating flange system via a flange (e.g. DIN, ANSI or JIS).
The flange connection is mainly used in the chemical and petrochemical industry as well as for high-temperature or high-pressure applications.
Sealing is achieved with flat gaskets (e.g. graphite, PTFE, metal) or with spiral wound gaskets or metal ring gaskets for high-pressure applications.
Advantages: Enables easy disassembly and secure sealing with gaskets.
- Welded connection
The thermowell is welded directly into the system.
Ideal for extreme conditions such as high pressures, aggressive media or strong vibrations.
The welded-in thermowell is standard in the petrochemical industry.
Advantages: Maximum tightness and stability and no risk of leaks due to mechanical connections.
- Sanitary, hygiene or food connection
The sanitary, hygiene or food connection is used wherever hygienic conditions are required.
The food industry, beverage industry and pharmaceutical industry are typical areas of application.
The hygienic connection guarantees easy cleaning without the formation of so-called dead spaces in which product residues can remain, which can then lead to the formation of germs and contamination of the product.
Furthermore, the hygienic connections allow the thermowell to be replaced quickly, often without additional tools. There are many standardized, hygienic closures on the market (e.g. Tri-Clamp, milk pipe thread).
Advantages: Easy cleaning, avoids dead spaces, easy replacement.
Material of thermowells
Thermowells are mainly made of stainless steel or, in the case of mechanical thermometers, brass.If there are special operating conditions, special alloys such as Hastelloy®, Inconel® or Monel® are used.
Partial or full linings made of plastic (e.g. PTFE) are also used for high chemical stresses.
Very high temperatures or certain aggressive gases require the use of ceramic thermowells.
When selecting the thermowell material, the chemical resistance to the medium and the maximum permissible temperature and pressure resistance of the material must always be taken into account.