<< Click to Display Table of Contents >> Navigation:  Software Overview > RMS Monitoring Software > Working with the RMS Software > Using RMS > Calculations used in RMS > Relative Humidity (%RH)

Relative humidity (%RH) is the ratio of two pressures:

 

%RH = 100 x p/ps

 

p is the actual partial pressure of the water vapor present in the ambient.

ps the saturation pressure of water at the temperature of the ambient.

 

100% RH corresponds to the maximum amount of water vapor a gas mixture can contain at constant pressure and constant temperature. At constant water vapor partial pressure and changing ambient temperature, the water vapor saturation pressure changes and consequently the relative humidity also changes (see Water Vapor Saturation Pressure).

 

To obtain useful measurements of relative humidity, it is extremely important that the measurement probe and measured material have the same temperature.

 

Relative humidity sensors are usually calibrated at normal room temperature (above freezing). Consequently, it generally accepted that this type of sensor indicates relative humidity with respect to water at all temperatures (including below freezing).

 

Ice produces a lower vapor pressure than liquid water. Therefore, when ice is present, saturation occurs at a relative humidity of less than 100 %. For instance, a humidity reading of 75 %RH at a temperature of -30°C corresponds to saturation above ice.