Update dew-point-calulator.md

dew-point-calulator.md

@@ -53,3 +53,85 @@ BEGIN
 
 END_FUNCTION
 ```
+
+# Buck version
+```
+
+FUNCTION "FC_DewPoint_Buck" : Void
+{ S7_Optimized_Access := 'TRUE' }
+
+VAR_INPUT
+    Temperature_C : Real;   // Air temperature in °C
+    Humidity_RH   : Real;   // Relative humidity in %
+END_VAR
+
+VAR_OUTPUT
+    DewPoint_C : Real;      // Dew point in °C
+    Valid      : Bool;
+END_VAR
+
+VAR_TEMP
+    T             : LReal;
+    RH            : LReal;
+    SaturationVP  : LReal;
+    ActualVP      : LReal;
+    S             : LReal;
+    Discriminant  : LReal;
+    Td            : LReal;
+END_VAR
+
+BEGIN
+    // RH must be greater than zero because LN(0) is undefined.
+    #Valid :=
+        (#Humidity_RH > 0.0) AND
+        (#Humidity_RH <= 100.0);
+
+    IF #Valid THEN
+        #T := REAL_TO_LREAL(#Temperature_C);
+        #RH := REAL_TO_LREAL(#Humidity_RH);
+
+        // Buck saturation vapour pressure over liquid water.
+        // Result is in hPa.
+        #SaturationVP :=
+            6.1121 *
+            EXP(
+                (18.678 - (#T / 234.5)) *
+                (#T / (257.14 + #T))
+            );
+
+        // Actual water vapour pressure.
+        #ActualVP :=
+            (#RH / 100.0) * #SaturationVP;
+
+        // Intermediate logarithmic value.
+        #S :=
+            LN(#ActualVP / 6.1121);
+
+        // Part below the square root.
+        #Discriminant :=
+            ((18.678 - #S) * (18.678 - #S))
+            -
+            ((4.0 * 257.14 * #S) / 234.5);
+
+        IF #Discriminant >= 0.0 THEN
+            #Td :=
+                (234.5 / 2.0) *
+                (
+                    18.678
+                    - #S
+                    - SQRT(#Discriminant)
+                );
+
+            #DewPoint_C := LREAL_TO_REAL(#Td);
+
+        ELSE
+            #DewPoint_C := 0.0;
+            #Valid := FALSE;
+        END_IF;
+
+    ELSE
+        #DewPoint_C := 0.0;
+    END_IF;
+
+END_FUNCTION
+```