Humidity is notoriously difficult to measure. Some of the scientific and electronic considerations to measuring humidity include: analog to digital interface and external circuitry which might require op-amps or oscillator circuitry; temperature compensation adjustments to calculate dew point; calibration against a known humidity source; mounting, protection and response time in the real-world. If you're interested in the details see Tracy Allen's web site (EME Systems) for his discussion.

The Sensirion SHT1x addresses many of these issues head on. It is a smart sensor for both humidity and temperature, and it comes from the factory in a tiny package that incorporates the analog to digital interface. All that your microcontroller has to do is read out the humidity and temperature values through the two-wire digital serial interface. The only math required is a simple scale and offset. The SHT1x is factory calibrated so that it returns temperature with a resolution of 0.01 degrees Celsius and relative humidity with a resolution of 0.03 percent. The accuracy is better than most other sensors too. Worst-case temperature accuracy is +/- 2 degrees C - but in the "room temperature" range the accuracy better than +/- 1 degree C. The relative humidity sensor is similarly accurate: +/- 3.5% in the range 20% to 80%. This is quite remarkable for a low-cost sensor. Excellent for commercial or home-based projects that require such readings.

Features:

• Provides a fully calibrated, digital output
• Measure temperature with a resolution of 0.01 degrees and within +/- 2 degree accuracy
• Measures relative humidity with a resolution of 0.03%and within +/- 3.5% accuracy
• Low power consumption (typically 30 μW)

Specifications:

• Power requirements: 2.4 to 5.5 VDC
• Communication: Two-Wire Serial
• Dimensions: 11 x 21 mm
• Operating temp range: -40 to +254.9°F (-40 to +123.8°C)

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#include 
 
// Specify data and clock connections and instantiate SHT1x object
#define dataPin 10
#define clockPin 11
SHT1x sht1x(dataPin, clockPin);
 
void setup()
{
Serial.begin(38400); // Open serial connection to report values to host
Serial.println("Starting up");
}
 
void loop()
{
float temp_c;
float temp_f;
float humidity;
 
// Read values from the sensor
temp_c = sht1x.readTemperatureC();
temp_f = sht1x.readTemperatureF();
humidity = sht1x.readHumidity();
 
// Print the values to the serial port
Serial.print("Temperature: ");
Serial.print(temp_c, DEC);
Serial.print("C / ");
Serial.print(temp_f, DEC);
Serial.print("F. Humidity: ");
Serial.print(humidity);
Serial.println("%");
 
delay(2000);
}