Pt100 - Sensors

-Designs of Pt100 resistance thermometers-

About the site

This is a private information page about electrical temperature measurement with resistance thermometers (e.g. Pt100, Pt500, Pt1000). My experiences of many years in the field of measurement and control technology are recorded. I do not take over any liability for incorrect information and would be pleased about further suggestions and requests for additions.
Harald Peters

Suppliers of Pt100

Links to selected manufacturers and suppliers of Temperature Sensors.

Designs of Pt100 resistance thermometers

The most common structure of a Pt100 resistance thermometer for industrial application consists of:

Pt100 - sensor elements

Depending on the application, a distinction is made between 3 designs

Thin-film sensors

An approx. 1 mm thick platinum layer is vapor-deposited onto an aluminum oxide plate. The platinum layer is structured and aligned in a spiral with the aid of a laser beam. A glass layer is then applied to protect the platinum layer.
The decisive advantage of the thin-film Pt100 sensor is its inexpensive large-scale production and its fast response time due to its low mass.

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Wire wound Pt100ceramic resistors

A coiled platinum wire is fed into a ceramic tube. For better heat transfer and to fix the winding, the tube is filled with aluminum oxide. By means of a glass plug, the ceramic tube is closed and the connecting wires are fixed. Since there is no connection between the aluminum oxide powder and the platinum filament, the Pt100 wire can expand freely when the temperature changes. The wire is only slightly stressed and therefore shows no hysteresis between rising and falling Temperatur.
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Wire wound Pt100 glass resistors

A platinum wire is wound onto a glass rod and fused into it. After the resistance has been adjusted, a second glass protection tube is slid over and fused to the glass rod. The wire is completely surrounded by the glass. Therefore, these glass resistors are particularly insensitive to vibration. With the glass tube fused on, Pt100 glass resistors are also used directly in aggressive media as fast-responding sensors.
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Protection tubes or protective sleeve for Pt100 sensors with mechanical connection

For most industrial applications, the actual sensing element is installed in a so-called thermowell. This protects the sensor element from mechanical damage or chemical attack. In order to obtain an optimal measurement, some things have to be considered when selecting the protection tube and installing the Pt100 sensor.

The thermowell should have a high thermal conductivity (e.g. metal) to ensure the heat transfer between the medium and the Pt100 sensor element. The thermowell must be chemically resistant to the medium and must be sufficiently pressure-resistant.
The construction of thermowells is recommended in DIN standards or often also produced in manufacturer/customer-specific designs.
The Pt100 sensor element is installed in different ways:

Connection of the Pt100 sensor to a cable or stranded wire

Pt100-Sensor mit Kabel und Schutzhuelse.png

The Pt100 sensor element is welded to a connecting cable or stranded wires and installed directly in a protection sleeve. For optimum heat transfer, the space between the sensor element and the protection sleeve is filled with so-called heat-conducting paste. The maximum temperature to be measured is determined by the insulating material of the connecting cable/stranded wire.
For mechanical connection, threaded fittings, flanges, etc. can be connected. The use of compression fittings allows the sleeve to be moved and the installation length to be changed variably.
Advantage: inexpensive design

Connection Pt100 sensor to mineral insulated metal-sheathed cables

Pt100-Sensor mit Mantelrohr.png

The Pt100 sensor element is connected to a so-called metal sheathed cable in which the bare connecting wires are electrically insulated in magnesium oxide. After welding the connecting wires to the sensor, the Pt100 sensor is also embedded in magnesium oxide and the protection sleeve is tightly welded. For the mechanical connection, threaded fittings, flanges, etc. can be connected. The use of compression fittings allows the sleeve to be moved and the installation length to be varied.
Advantage: Use at higher temperatures.

Measuring inserts

Pt100-Sensor als Messeinsatz.png

In industrial plants, the sheath elements described above are used as measuring inserts with electrical connection socket. These are then plugged into the protection tubes mounted in the plant (see DIN).
This enables defective sensors to be replaced even while the plant is in operation. The length of the measuring insert must be adapted to the protection tubes.

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