The SHT21 utilizes a capacitive sensor element to measure humidity, while the temperature is measured by a band gap sensor. Both sensors are seamlessly coupled to a 14-bit ADC, which then transmits digital data to the Arduino over the I2C protocol. Because of the sensor’s tiny size, it has incredibly low power consumption, making it suited for virtually any application.  

To optimize accuracy of temperature and humidity readings, the SHT21 sensor is placed at the tip of the board, isolating it from heat producing circuitry. Two of the sensor's four pins are ground and +5V; the other two are clock and data pins that make up the 2-wire serial interface. The board is 5V tolerant and designed to operate directly off the supply voltage coming from the Arduino.

The sensor can be placed in soil to measure moisture and temperature, which makes it ideal for using it in a garden or greenhouse. The humidity sensor is also especially useful in building remote weather stations. 

The board ships a with a 4-pin male header which has not been soldered on.

* Energy consumption: 80 uW (at 12 bit, 3V, 1 measurement/s)
* Relative Humidity operating range: 0-100% RH
* Relative Humidity resolution of 0.03%
* Relative Humidity Response Time of 8 sec (tau 63%)
* Temperature operating range: -40 to +125°C
* Temperature resolution of 0.01 C
* 4 pins: +5V, GND, Clock (SCL), Data (SDA)
* Bidirectional communication over a single pin on I2C protocol
* Board is 5V tolerant, allowing sensor to run from a 5V supply on Arduino I/O pins

Applications

* Homes, basements and HVAC systems for measuring humidity
* People with physical conditions sensitive to humidity
* Home ventilating, heating and air conditioning systems
* Meteorology stations to predict or check weather temperatures
* Gardens or greenhouses to check humidity and temperatures

Resources

* SHT21 sensor Datasheet

* LibHumidity Arduino library