Humidity is the concentration of water vapour present in the air. The maximum amount of water vapour that can exist in air is depended on temperature. One cubic metre of air at 30°C can hold around 30g of water vapour.
The most common humidity measurement is Relative Humidity (RH) which is a percentage of the actual amount of water vapour in the air compared to the total amount of vapour that could exist at its current temperature.
A reading of 50%RH would indicate the air is holding 50% of water vapour it could hold at its current temperature. I.E., 15g per m³ at 30°C.
So, to indicate relative humidity we need to measure both the concentration of water vapour and the temperature. Most transmitters have outputs for both relative humidity and temperature measurements.
There are many reasons why the measurement and control of relative humidity is important for many aspects including protection of moisture sensitive goods in storage, electrical safety, process control and, of course, ability to measure and control RH not only helps to create comfortable environment inside buildings but also optimizes the efficiency of HVAC systems.