Technical Information

10. Self heating error

In order to be able to determine the resistance at an appropriate temperature the sensor resistance must be attached according to the respective diagram, i.e. a current must flow through the sensor resistance. This current produces a heat capacity in the sensor resistance, which is described as follows:

Nw = I2 * Rt

this power is converted into heat in the sensor resistance and thus falsifies the measurement result.

In the DIN EN 60751 it is stated that the yielded energy dissipation should be = 0,1mW . This error is generally called "self-heating coefficient" (EK) and indicated in K/mW.

This coefficient amounts according to manufacturer data measured in water, for

  • glass - sensor resistances: between 0,04 and 0,4 K/mW
  • ceramic sensor resistances: between 0,06 and 0,21 K/ mW
  • sheath sensor resistances: 0,1 and 0.3 K/ mW

These large spreads indicate that this value is strongly coupled to the design of the sensor resistance. This means however, that during the subsequent treatment of the sensor resistances, e.g. by installation into measurement inserts, attention must be paid that only materials that guarantee very good heat coupling and/or thermoelectric conduction are used, as otherwise this EK clearly increases.

With a measurement insert, e.g. execution according to DIN 43762, this self heating coefficient for simple - resistances lies between 0,015 and 0,038 K/mW and for double - resistances between 0,02 and 0,045 K/ mW.

It is to be further noted that through the still necessary protection fitting a mean value of approx.
0,15 K/ mW is reached.

By converting in DJ:

EK= DJ/ NW (K/mW)

through the formula

DJ=EK * NW ((K*mW)/ mW)

for each expected signal current, measuring errors which are released by the self-heating can be calculated.

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