s7-SCL-scripts/dew-point-calulator.md

FUNCTION "FC_DewPoint" : Void { S7_Optimized_Access := 'TRUE' }

VAR_INPUT Temperature_C : Real; // Air temperature in degrees Celsius Humidity_RH : Real; // Relative humidity in percent: > 0 to 100 END_VAR

VAR_OUTPUT DewPoint_C : Real; // Calculated dew point in degrees Celsius Valid : Bool; // TRUE when calculation is valid END_VAR

VAR_TEMP Gamma : Real; END_VAR

VAR_CONSTANT B : Real := 17.625; C : Real := 243.04; END_VAR

BEGIN // Validate the input range. // RH must be greater than zero because LN(0) is undefined. #Valid := (#Temperature_C >= -40.0) AND (#Temperature_C <= 50.0) AND (#Humidity_RH > 0.0) AND (#Humidity_RH <= 100.0);

IF #Valid THEN

    // Gamma =
    // LN(RH / 100) + (B * Temperature) / (C + Temperature)
    #Gamma :=
        LN(#Humidity_RH / 100.0)
        +
        ((#B * #Temperature_C) /
        (#C + #Temperature_C));

    // Dew point =
    // (C * Gamma) / (B - Gamma)
    #DewPoint_C :=
        (#C * #Gamma) /
        (#B - #Gamma);

ELSE
    #Gamma := 0.0;
    #DewPoint_C := 0.0;
END_IF;

END_FUNCTION