The humidity sensor or hygrometers differ in sensitive element and measurement method. According to the principle of operation, sensors are divided into different types: capacitive, where the sensor element  is a capacitor, resistive with sensor element changing resistance depending on humidity, thermistor, which uses resistance changes comparison of two thermistors located in one sealed thermal chamber, and the second one is in the open chamber. Optical sensors, which are used as a method to determining humidity relatively to the dew point, which changes the light transmission coefficient between the light source and photodiode.