Relative humidity is a measure of water steam concentration in a gas. The measurement units are the %RH that indicates the concentration of the mentioned steam relative to saturation.
For example, a 0%RH indicates It indicates a total absence of water and 100% indicates that the concentration is saturated and therefore the condensed water (under these conditions of temperature and pressure, the air is not able to withstand a higher concentration of steam).
Humidity vary when increasing pressure or decreasing temperature. Thus, with the same mass of water steam, increasing the pressure will increase the relative humidity to a point that will saturate.
The same happens when decreasing temperature.
The measurement of relative humidity is done by capacitive sensors that vary their characteristics depending on the environmental humidity, So from the measurement of the capacity of the sensor we can know the relative humidity. Amplifying and linearizing this measure we get an analog output of 4-20 mA or 0 to 10 Vdc proportional to the relative humidity.
The most common models are the HX-830, HT 834 y THS.