Relative Humidity Measurement
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Relative humidity is defined as the ratio of the partial pressure of water vapor in an air-water mixture to the equilibrium vapor pressure of water at a given temperature. It can be measured using various devices, including resistive, capacitive, crystal, thermal, gravimetric, and optical hygrometers. Resistive hygrometers measure changes in the electrical resistance of moisture-absorbing materials as humidity varies. Capacitive hygrometers detect changes in capacitance of a polymer film. Crystal hygrometers measure changes in mass of a hygroscopic crystal as it absorbs water from the air.
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Relative Humidity Measurement
- 1. Relative Humidity Measurement ER. FARUK BIN POYEN FARUK.POYEN@GMAIL.COM
- 2. Content: Definition Coefficient Units Measuring Devices 2
- 3. Relative Humidity 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. 𝑅𝐻 = 𝑚 𝑣 𝑚 𝑠𝑎𝑡 = 𝑃𝑎𝑐𝑡 𝑉 𝑅 𝑠𝑎𝑡 𝑇 𝑃𝑠𝑎𝑡 𝑉 𝑅 𝑠𝑎𝑡 𝑇 = 𝑃𝑎𝑐𝑡 𝑃𝑠𝑎𝑡 3
- 4. Relative Humidity 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. 4
- 5. Relative Humidity 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. 5
- 6. Relative 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 𝑃 𝑎𝑐𝑡 𝑃𝑠𝑎𝑡 6
- 7. 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 devices. 1. Resistive Hygrometer 2. Capacitive Hygrometer 3. Crystal Hygrometer 4. Thermal 5. Gravimetric 6. Optical 7
- 8. 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 electrodes. Suitable in mass production and for use in home appliances. Accurate is a concern. 8
- 9. 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 bridge. The DC is not used in the bridge as they breakdown the layer of lithium chloride 9
- 10. 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 value. 10
- 11. 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 changes. The change in mass is proportional to the total water absorbed by the crystals. 11
- 12. 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 humidity. 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 air. 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. 12