Pt100 - Temperatur Sensors

-Measuring error 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.
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Suppliers of Pt100

Links to selected manufacturers and suppliers of Temperature Sensors.

Measuring error

Besides the basic accuracy of the Pt100 sensor, the following additional errors may occur:

Self-heating of the Pt100

To measure the resistance of a Pt100 sensor, a current must flow through the sensor. The measuring current generates a power loss and thus heat at the sensor. The associated measurement error of a higher temperature indication depends on various influencing variables:

Line error

The line error results from the connection of the Pt100 resistor to the evaluation electronics with a 2-wire connection line (two-wire connection). The resistance of the connecting cable is included in the measurement as an error.
Particularly with long connection lines and small line diameters, this can lead to considerable measuring errors. With modern evaluation devices, the resistance of the connecting cable can be compensated, but temperature fluctuations on the cable cannot be taken into account. The resistances of the Pt100 and the connecting cable add up, resulting in the display of a higher temperature.

This can be remedied by connecting the Pt100 in 3- or 4-wire technology.
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Insulation error

Penetration of moisture into the sensor or into the insulating material between the connecting cables, leads to a measuring error (lower temperature). temperature).
The sensors are normally sufficiently protected against moisture. However, when manufacturing the measuring insert, care must be taken to use absolutely dry insulating materials and careful sealing.
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Parasitic thermoelectric voltages

When different metals are joined, a thermoelectric voltage occurs.
Such metal connections occur, for example, when extending the sensor connection wires with copper wires. Normally, the connection points have the same temperature and thus cancel each other out in terms of value. However, if different temperatures are present at the connection points, e.g. due to different heat dissipation, different thermoelectric voltages occur which lead to a voltage drop which is interpreted by the evaluation device as a change in resistance.
This results in a measurement error.
How high the measuring error is strongly depends on the evaluation device and the selected connection type (2, 3 or 4-wire). Fault detection by reversing the measuring current. The higher the difference of the measured values, the higher the thermoelectric voltage present.
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Response time (T50, T63, T90)

In many applications, the speed with which a temperature sensor reacts to a change in temperature is crucial for precise control and regulation of the process.
As a temperature sensor only reacts with a delay, especially in the event of sudden temperature changes in the medium, this can lead to undesirable process errors or even damage to machines or systems.
How quickly a Pt100 temperature sensor reacts to a change in temperature depends on various factors. In order to obtain a comparable statement about the reaction speed of a Pt100 temperature sensor, the "response time" was defined in DIN 60751 or, in the US, in ASTM E644-11.
This specifies the time a temperature sensor needs to go from a certain initial temperature to 50% (T50), 63% (T63) or 90% (T90) of the temperature difference between the initial and final temperature.

For example:
Rapid increase in medium temperature from 50 °C to 100 °C
This is a temperature jump of 50°C.
The T50 value is therefore: 75 °C = 25 °C (50% of the temperature jump) + 50 °C (initial temperature)
T63 Time required for the temperature sensor to reach 81.5 °C
T90 Time required for the temperature sensor to reach 95 °C

The response time of a Pt100 temperature sensor is determined by the manufacturer and specified in the data sheet.
Correctly, the temperature range in which the measurement was made should also be specified.
Example: T50: 0,5 seconds from 50 to 100 °C

T0,5 is also often used instead of T50, T0,63, instead of T63 or T0,9 instead of T90.

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