Relative humidity depends on temperature and the pressure of the
system of interest. The same amount of water vapour results in higher
relative humidity in cool air than warm air. A related parameter is that of
dew point. It is normally expressed as %.
Relative Humidity (RH or φ) of an air-water mixture is defined as the
ration of the partial pressure of water vapour in the mixture to the
equilibrium vapour pressure of water over a flat surface of pure water at
a given temperature.
𝑃𝑎𝑐𝑡 𝑉 𝑅 𝑠𝑎𝑡 𝑇
𝑃𝑠𝑎𝑡 𝑉 𝑅 𝑠𝑎𝑡 𝑇
Absolute humidity is the mass of water vapour divided by the mass of
dry air in a certain volume of air at a specific temperature. It is defined
as the mass of vapour contained in 1 kg of dry air.
Specific humidity, (mass of water vapour) is a unit mass of moist air,
usually expressed as grams of vapour per kilogram of air, or, in air
conditioning, as grains per pound. The specific humidity is an extremely
useful quantity in meteorology.
At 100% RH, the air is saturated and is at its dew point.
Dew point: The temperature (depending on pressure and humidity)
below which water droplets begin to condense and dew can form.
Psychrometry is the science of studying the thermodynamic properties
of moist air and the use of these properties to analyse conditions and
processes involving moist air.
Dry Bulb Temperature (𝑇𝑑𝑏): The dry-bulb temperature (DBT) is the
temperature of air measured by a thermometer freely exposed to the air,
but shielded from radiation and moisture.
Wet Bulb Temperature (𝑇 𝑤𝑏): The wet-bulb temperature (WBT) is the
temperature read by a thermometer covered in water-soaked cloth (wet-
bulb thermometer) over which air is passed.
At 100% relative humidity, the wet-bulb temperature is equal to the air
temperature (dry-bulb temperature) and it is lower at lower humidity.
By using both wet-bulb temperature and dry-bulb temperature the
relative humidity can be calculated.
𝑃𝑝𝑎𝑟𝑡𝑖𝑎𝑙 = 𝑃𝑠𝑎𝑡 − 0.5(𝑇𝑑𝑏 − 𝑇 𝑤𝑏)
𝑃𝑝𝑎𝑟𝑡𝑖𝑎𝑙 = partial pressure of water vapour.
𝑃𝑠𝑎𝑡 = saturation water vapour pressure.
For ideal gas behaviour, 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐻𝑢𝑚𝑖𝑑𝑖𝑡𝑦 = 0.622
Relative Humidity – Measurement
A hygrometer is an instrument used to measure the amount of humidity
and water vapour in the atmosphere, in soil, or in confined spaces.
Relative Humidity can be measured using the following types of
1. Resistive Hygrometer
2. Capacitive Hygrometer
3. Crystal Hygrometer
Relative Humidity – Resistive Hygrometer
The resistance-type hygrometer utilizes the principle that electrical
resistance varies in a material that absorbs moisture.
Special resistive sensors measure the electrical resistance between the
Suitable in mass production and for use in home appliances.
Accurate is a concern.
Relative Humidity – Resistive Hygrometer
The conducting film of the resistive hygrometer is made by the lithium
chloride and the carbon.
The resistance of the conducting film varies with the change in the value
of humidity present in the surrounding air.
If the relative humidity is high, the lithium chloride will absorb more
moisture and their resistance decreases.
The change in the value of resistance is measured by applying the AC to
The DC is not used in the bridge as they breakdown the layer of lithium
Relative Humidity – Capacitive Hygrometer
A capacitance-type hygrometer measures humidity by detecting the
change in capacitance of a thin polymer film.
This type of sensor is sufficiently accurate in most cases and is used
frequently in industry.
It is made by placing the hygroscopic material between the metal
electrodes, quickly absorbing water and decreasing the capacitance
Relative Humidity – Crystal Hygrometer
In crystal hygrometer, the hygroscopic crystal or the crystal having the
coating of hygroscopic material is used.
When the crystal absorbs the drops of the water, the mass of the crystal
The change in mass is proportional to the total water absorbed by the
Relative Humidity – Other Methods
Thermal - The change in thermal conductivity of air due to humidity is
measured. These sensors measure absolute humidity rather than relative
Gravimetric - A Gravimetric hygrometer measures the mass of an air
sample compared to an equal volume of dry air. This is considered the
most accurate primary method to determine the moisture content of the
Optical - An optical hygrometer measures the absorption of light by
water in the air. A light emitter and a light detector are arranged with a
volume of air between them. The attenuation of the light, as seen by the
detector, indicates the humidity, according to the Beer–Lambert law.