55 lines
1.2 KiB
Markdown
55 lines
1.2 KiB
Markdown
```
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FUNCTION "FC_DewPoint" : Void
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{ S7_Optimized_Access := 'TRUE' }
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VAR_INPUT
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Temperature_C : Real; // Air temperature in degrees Celsius
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Humidity_RH : Real; // Relative humidity in percent: > 0 to 100
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END_VAR
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VAR_OUTPUT
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DewPoint_C : Real; // Calculated dew point in degrees Celsius
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Valid : Bool; // TRUE when calculation is valid
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END_VAR
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VAR_TEMP
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Gamma : Real;
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END_VAR
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VAR_CONSTANT
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B : Real := 17.625;
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C : Real := 243.04;
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END_VAR
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BEGIN
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// Validate the input range.
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// RH must be greater than zero because LN(0) is undefined.
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#Valid :=
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(#Temperature_C >= -40.0) AND
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(#Temperature_C <= 50.0) AND
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(#Humidity_RH > 0.0) AND
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(#Humidity_RH <= 100.0);
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IF #Valid THEN
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// Gamma =
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// LN(RH / 100) + (B * Temperature) / (C + Temperature)
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#Gamma :=
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LN(#Humidity_RH / 100.0)
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+
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((#B * #Temperature_C) /
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(#C + #Temperature_C));
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// Dew point =
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// (C * Gamma) / (B - Gamma)
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#DewPoint_C :=
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(#C * #Gamma) /
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(#B - #Gamma);
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ELSE
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#Gamma := 0.0;
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#DewPoint_C := 0.0;
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END_IF;
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END_FUNCTION
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``` |