Overview Dew point temperature

This function block calculates the dew point temperature – the temperature at which the air is saturated with the water vapour it contains and condensation forms. The calculation uses the Magnus formula from temperature and relative humidity. The block recalculates whenever one of the two inputs changes.

The typical application is dew point monitoring of a chilled ceiling or a cooling circuit: the output Dew point reached blocks the cooling before condensation forms. So that this happens in good time, the calculated dew point temperature is raised by the Offset; it is this raised threshold that is present at the output Dew point temp. and switches the output Dew point reached. It is only reset once the temperature has exceeded the threshold by the Hysteresis.

Without a valid humidity no dew point can be determined. If the input Humidity % is not linked, its value is 0 or the temperature sensor is missing, the block does not calculate and the outputs remain unchanged.

Switching behaviour

14 16 18 20 22 24 °C Time Offset Hysteresis TE 1 0 0 Output dew point reached Temperature (input TP) Switching threshold = dew point + offset (output TP) calculated dew point Release threshold = switching threshold + hysteresis

Inputs

TP
 Temperature
 Actual air temperature in °C. Not linked: no dew point is calculated, the outputs remain unchanged.
FP
 Humidity %
 Relative humidity in percent; meaningful values are above 0 up to 100. Larger values are limited to 100. Not linked or 0: no dew point is calculated, the outputs remain unchanged.

Outputs
TP
 Dew point temp.
 Calculated dew point temperature in °C, raised by the parameter Offset dew point temp. This value is at the same time the switching threshold of the output Dew point reached.
TE
Dew point reached
Becomes 1 as soon as the temperature at the input Temperature reaches or falls below the switching threshold (dew point + Offset). Becomes 0 again as soon as it exceeds the switching threshold by the Hysteresis. In between, the last state is kept.

Parameters
Hysteresis
 Hysteresis for the output Dew point reached. It acts only on the release: the output only returns to 0 once the temperature exceeds the switching threshold by this value. This keeps the output from chattering at a temperature close to the threshold.
Offset dew point temp.
 Safety margin in kelvin added to the calculated dew point temperature. It shifts the output Dew point temp. and therefore the switching threshold upwards, so the output Dew point reached comes that much earlier. Default is 0, the calculated dew point is then used as is. A negative value shifts the threshold downwards.



See also common parameters of all function blocks.