Technical Information

9. Leakage resistance

The leakage resistance works as a bypass resistance to the sensor resistance and so leads to a reduction of the temperature display. In the production and use of resistance thermometers attention should always be paid to a sufficiently high insulation resistance between conductors and protection housing as well as between the conductors themselves. The DIN EN 60751 regulates the insulation resistances which must be reached in each case: they must amount to (15 °C to 35 °C) measured at ambient temperature with direct current (10 to 100 V) with a relative humidity < 80% at least 100 M W.

Since with rising temperature the insulation resistance decreases, dependent on the temperature, leakage resistances are predetermined also: These are to be measured with max. 10 V direct current between all conductors and the thermowell. The following minimum leakage resistances must be achieved.

Allowable max. temperature in °C Min. insulation resistance in MΩ
100 - 300 10
301 - 500 2
501 - 800 0,5

Table 2: minimal leakage resistances

Causes for a fall in the leakage resistances are:

  • Humidity within the insulation materials
  • evaporation of conductor materials
  • conductivity of the insulation materials.