Dew Point Arduino

what is really dew point ? 

In simple terms, the dew point (dew point temperature or dewpoint) is the temperature at which a given concentration of water vapor in air will form Dew. More specifically it is a measure of atmospheric moisture. It is the temperature to which air must be cooled at constant pressure and water content to reach saturation. A higher dew point indicates more moisture in the air; a dew point greater than 20 °C (68 °F) is considered uncomfortable and greater than 22 °C (72 °F) is considered to be extremely humid. Frost point is the dew point when temperatures are below freezing.

here is my setup using only one dht11 humidity sensor

Here is the result using serial monitor

Here is the code :

// Example testing sketch for various DHT humidity/temperature sensors
// Written by ladyada, public domain

#include "DHT.h"

#define DHTPIN 2 // what digital pin we're connected to

// Uncomment whatever type you're using!
//#define DHTTYPE DHT11 // DHT 11
#define DHTTYPE DHT22 // DHT 22 (AM2302), AM2321
//#define DHTTYPE DHT21 // DHT 21 (AM2301)

// Connect pin 1 (on the left) of the sensor to +5V
// NOTE: If using a board with 3.3V logic like an Arduino Due connect pin 1
// to 3.3V instead of 5V!
// Connect pin 2 of the sensor to whatever your DHTPIN is
// Connect pin 4 (on the right) of the sensor to GROUND
// Connect a 10K resistor from pin 2 (data) to pin 1 (power) of the sensor
DHT dht(DHTPIN, DHTTYPE);

float tC;
float dP;
float dPC;

// Initialize DHT sensor.
// Note that older versions of this library took an optional third parameter to
// tweak the timings for faster processors. This parameter is no longer needed
// as the current DHT reading algorithm adjusts itself to work on faster procs.

void setup() {
 Serial.begin(9600);
 Serial.println("DHTxx test!");

dht.begin();
}
void loop() {
 // Reading temperature or humidity takes about 250 milliseconds!
 // Sensor readings may also be up to 2 seconds 'old' (its a very slow sensor)
 float h = dht.readHumidity();
 float t = dht.readTemperature();

// check if returns are valid, if they are NaN (not a number) then something went wrong!
 if (isnan(t) || isnan(h)) {
 Serial.println("Failed to read from DHT");
 } else {
 Serial.print("Humidity: ");
 Serial.print(h);
 Serial.print(" %\t");
 Serial.print("Temperature: ");
 // Serial.print(t);
 // Serial.print(" *C ");
 tC=((t*9)/5)+32;
 Serial.print(tC);
 Serial.print(" *C ");
 Serial.print(" \t");
 
 Serial.print("Dew Point: ");
 // Serial.print(dewPointFast(t, h));
 // Serial.print(" *C ");
 dP=(dewPointFast(t, h));
 dPC=((dP*9)/5)+32;
 Serial.print(dPC);
 Serial.print(" *C");
 Serial.print(" \t");

Serial.print("Heat Index: ");
 Serial.print(heatIndex(tC,h));
 Serial.println(" *C");


 }
}

// delta max = 0.6544 wrt dewPoint()
// 6.9 x faster than dewPoint()
// reference: http://en.wikipedia.org/wiki/Dew_point
double dewPointFast(double celsius, double humidity)
{
 double a = 17.271;
 double b = 237.7;
 double temp = (a * celsius) / (b + celsius) + log(humidity*0.01);
 double Td = (b * temp) / (a - temp);
 return Td;
}

double heatIndex(double tempF, double humidity)
{
 double c1 = -42.38, c2 = 2.049, c3 = 10.14, c4 = -0.2248, c5= -6.838e-3, c6=-5.482e-2, c7=1.228e-3, c8=8.528e-4, c9=-1.99e-6 ;
 double T = tempF;
 double R = humidity;

double A = (( c5 * T) + c2) * T + c1;
 double B = ((c7 * T) + c4) * T + c3;
 double C = ((c9 * T) + c8) * T + c6;

double rv = (C * R + B) * R + A;
 return rv;
}

sources

http://arduinotronics.blogspot.co.id/2013/12/temp-humidity-w-dew-point-calcualtions.html

https://learn.adafruit.com/dht/overview