Adding dewpoint calc and example

Author: nseidle

examples/Example10_DewPoint/Example10_DewPoint.ino

@@ -0,0 +1,54 @@
+/*
+  Calculating dewpoint on the BME280
+  Nathan Seidle @ SparkFun Electronics
+  November 3rd, 2018
+
+  Feel like supporting our work? Buy a board from SparkFun!
+  https://www.sparkfun.com/products/14348 - Qwiic Combo Board
+  https://www.sparkfun.com/products/13676 - BME280 Breakout Board
+
+  This example shows how to calculate dew point based on humidity and temperature.
+  Comes from Pavel-Sayekat: https://github.com/sparkfun/SparkFun_BME280_Breakout_Board/pull/6
+
+  Hardware connections:
+  BME280 -> Arduino
+  GND -> GND
+  3.3 -> 3.3
+  SDA -> A4
+  SCL -> A5
+*/
+
+#include <Wire.h>
+
+#include "SparkFunBME280.h"
+BME280 mySensor;
+
+void setup()
+{
+  Serial.begin(9600);
+  Serial.println("Example showing dewpoint calculation");
+
+  mySensor.setI2CAddress(0x76); //Connect to a second sensor
+  if (mySensor.beginI2C() == false) Serial.println("Sensor connect failed");
+}
+
+void loop()
+{
+  Serial.print("Humidity: ");
+  Serial.print(mySensor.readFloatHumidity(), 0);
+
+  Serial.print(" Pressure: ");
+  Serial.print(mySensor.readFloatPressure(), 0);
+
+  Serial.print(" Temp: ");
+  //Serial.print(mySensor.readTempC(), 2);
+  Serial.print(mySensor.readTempF(), 2);
+
+  Serial.print(" Dewpoint: ");
+  //Serial.print(mySensor.dewPointC(), 2);
+  Serial.print(mySensor.dewPointF(), 2);
+
+  Serial.println();
+
+  delay(50);
+}

src/SparkFunBME280.cpp

@@ -495,6 +495,36 @@ float BME280::readTempF( void )
 	return output;
 }
 
+//****************************************************************************//
+//
+//  Dew point Section
+//
+//****************************************************************************//
+// Returns Dew point in DegC
+double BME280::dewPointC(void)
+{
+  double celsius = readTempC(); 
+  double humidity = readFloatHumidity();
+  // (1) Saturation Vapor Pressure = ESGG(T)
+  double RATIO = 373.15 / (273.15 + celsius);
+  double RHS = -7.90298 * (RATIO - 1);
+  RHS += 5.02808 * log10(RATIO);
+  RHS += -1.3816e-7 * (pow(10, (11.344 * (1 - 1/RATIO ))) - 1) ;
+  RHS += 8.1328e-3 * (pow(10, (-3.49149 * (RATIO - 1))) - 1) ;
+  RHS += log10(1013.246);
+         // factor -3 is to adjust units - Vapor Pressure SVP * humidity
+  double VP = pow(10, RHS - 3) * humidity;
+         // (2) DEWPOINT = F(Vapor Pressure)
+  double T = log(VP/0.61078);   // temp var
+  return (241.88 * T) / (17.558 - T);
+}
+
+// Returns Dew point in DegF
+double BME280::dewPointF(void)
+{
+	return(dewPointC() * 1.8 + 32); //Convert C to F
+}
+
 //****************************************************************************//
 //
 //  Utility

src/SparkFunBME280.h

@@ -158,7 +158,7 @@ struct SensorCalibration
 
 };
 
-//This is the man operational class of the driver.
+//This is the main operational class of the driver.
 
 class BME280
 {
@@ -213,6 +213,11 @@ class BME280
     float readTempC( void );
     float readTempF( void );
 
+	//Dewpoint related methods
+	//From Pavel-Sayekat: https://github.com/sparkfun/SparkFun_BME280_Breakout_Board/pull/6/files
+    double dewPointC(void);
+    double dewPointF(void);
+	
     //The following utilities read and write
 
 	//ReadRegisterRegion takes a uint8 array address as input and reads