Added support for Celsius in computeHeatIndex

Author: MyIgel

DHT.cpp

@@ -20,7 +20,7 @@ void DHT::begin(void) {
   _lastreadtime = 0;
 }
 
-//boolean S == Scale.  True == Farenheit; False == Celcius
+//boolean S == Scale.  True == Fahrenheit; False == Celcius
 float DHT::readTemperature(bool S) {
   float f;
 
@@ -76,18 +76,30 @@ float DHT::readHumidity(void) {
   return NAN;
 }
 
-float DHT::computeHeatIndex(float tempFahrenheit, float percentHumidity) {
+//boolean isFahrenheit: True == Fahrenheit; False == Celcius
+float DHT::computeHeatIndex(float temperature, float percentHumidity, bool isFahrenheit) {
   // Adapted from equation at: https://github.com/adafruit/DHT-sensor-library/issues/9 and
   // Wikipedia: http://en.wikipedia.org/wiki/Heat_index
+  if (!isFahrenheit) {
+    return -8.784695 +
+             1.61139411 * temperature +
+             2.338549   * percentHumidity +
+            -0.14611605 * temperature*percentHumidity +
+            -0.01230809 * pow(temperature, 2) +
+            -0.01642482 * pow(percentHumidity, 2) +
+             0.00221173 * pow(temperature, 2) * percentHumidity +
+             0.00072546 * temperature*pow(percentHumidity, 2) +
+            -0.00000358 * pow(temperature, 2) * pow(percentHumidity, 2);
+  }
   return -42.379 + 
-           2.04901523 * tempFahrenheit + 
+           2.04901523 * temperature + 
           10.14333127 * percentHumidity +
-          -0.22475541 * tempFahrenheit*percentHumidity +
-          -0.00683783 * pow(tempFahrenheit, 2) +
+          -0.22475541 * temperature*percentHumidity +
+          -0.00683783 * pow(temperature, 2) +
           -0.05481717 * pow(percentHumidity, 2) + 
-           0.00122874 * pow(tempFahrenheit, 2) * percentHumidity + 
-           0.00085282 * tempFahrenheit*pow(percentHumidity, 2) +
-          -0.00000199 * pow(tempFahrenheit, 2) * pow(percentHumidity, 2);
+           0.00122874 * pow(temperature, 2) * percentHumidity + 
+           0.00085282 * temperature*pow(percentHumidity, 2) +
+          -0.00000199 * pow(temperature, 2) * pow(percentHumidity, 2);
 }
 
 

DHT.h

@@ -33,7 +33,7 @@ class DHT {
   float readTemperature(bool S=false);
   float convertCtoF(float);
   float convertFtoC(float);
-  float computeHeatIndex(float tempFahrenheit, float percentHumidity);
+  float computeHeatIndex(float tempFahrenheit, float percentHumidity, bool isFahrenheit=false);
   float readHumidity(void);
   boolean read(void);
 

examples/DHTtester/DHTtester.ino

@@ -46,16 +46,17 @@ void loop() {
   float t = dht.readTemperature();
   // Read temperature as Fahrenheit
   float f = dht.readTemperature(true);
-  
+
   // Check if any reads failed and exit early (to try again).
   if (isnan(h) || isnan(t) || isnan(f)) {
     Serial.println("Failed to read from DHT sensor!");
     return;
   }
 
-  // Compute heat index
-  // Must send in temp in Fahrenheit!
-  float hi = dht.computeHeatIndex(f, h);
+  // Compute heat index in Celsius
+  float hic = dht.computeHeatIndex(t, h);
+  // Compute heat index in Fahrenheit
+  float hif = dht.computeHeatIndex(f, h, true);
 
   Serial.print("Humidity: "); 
   Serial.print(h);
@@ -66,6 +67,8 @@ void loop() {
   Serial.print(f);
   Serial.print(" *F\t");
   Serial.print("Heat index: ");
-  Serial.print(hi);
+  Serial.print(hic);
+  Serial.print(" *C ");
+  Serial.print(hif);
   Serial.println(" *F");
 }