Humidity Measurement Unit PDF – Instruments, Sensors and Application
Humidity of air refers to the content of the water vapour present in the air at a particular time and place. Humidity is measured by an instrument called hygrometer. Another instrument used for the same purpose is sling psychrometer (dry and wet bulb). It helps in cooling and heating of the atmosphere.
The actual amount of water vapour in a given volume of air is known as Total Humidity or Absolute Humidity. The ratio between the actual amount of water present in a given volume of air and the water vapour, which the same volume of air can contain till its saturation level, is called ‘Relative humidity’. Humidistat is controller for humidity. Instruments : Hair Hygrometer, Torsion Hygrometer, Electrical Absorption Hygrometer, Diffusion Hygrometer, Spectral Hygrometer, Psychrometer, Satellite, Analog and Digital Hygrometer.
Types of Humidity
The atmospheric humidity is expressed in a number of ways such as
| Way of Measurement of Humidity | Definition |
| Humidity Capacity | The capacity of air of certain volume at certain temperature to retain maximum amount of moisture content. |
| Absolute Humidity | The total weight of moisture content per volume of air at definite temperature is called absolute humidity. Its unit is gram/ cubic meters. In British unit, grains/ cubic foot, 1 grain = 0,0648 gm |
| Specific Humidity | The mass of the water vapour in grams contained in a kilogram of air and it represents the actual quantity of moisture present in a definite air. w/ (1+w) where w = mixing ratio |
| Relative Humidity | It is the ratio of the amount of water vapour actually present in the air having definite volume and temperature (i.e. absolute humidity) to the maximum amount the air can hold (i.e. humidity capacity) |
Relative Humidity = Absolute humidity/Humidity capacity × 100
-> Condensation is the change of physical state of matter from gaseous phase into liquid phase and is the reverse of vaporisation.
-> Dew On cool nights during winter, when the earth radiates heat and gets cooler, then the air which comes in contact with the cool ground also becomes cooler and water vapour starts condensing over the grass, trees, leaves etc. which could be seen in the early morning. This is called Dew. When the relative humidity reaches 100%, the air is completely saturated. The air temperature is said to be dew-point.
-> Smog (Smoke+Fog) is a form of fog that occurs in areas, where the air contains a large amount of smoke.
-> Fog is made from the droplets of water suspended in the lower of the atmosphere. Fog is not considered as a form of precipitation. Visibility of less than 1 km is the internationally recognised definition of fog.
-> Haze is formed by water particles that have condensed in the atmosphere and visibility lie between 1 km to 2 km.
-> Frost is the moisture on the ground surface that condenses directly into ice i.e. when condensation occurs below freezing point. In other words, when the temperature falls down to 0° C, the water drops forms ice crystals which are known as frost.
-> Clouds are a mass of minute droplets of water formed by the condensation of water vapours in the upper atmosphere. On the basis of this appearance and the altitude at which they occur, the clouds are classified into the following categories – Nimbus, Cumulus, Stratus, Cirrus clouds.
-> Rainfall is the falling down of moisture from the atmosphere in the form of liquid water drops. Rainfall is of three types:
Convectional Rainfall : When the air comes in contact with the hot surface of the earth, it gets heated, becomes lighter and starts rising up. Thus convectional currents are formed. On reaching the upper layer of atmosphere, the air starts cooling. Then condensation takes place and clouds are formed. And if a critical temperature is reached, then the rain occurs. This is called convectional rainfall.
Relief Rainfall : When the relief features, such as mountains, come in the way of a moist wind, then the wind is forced to rise up the mountain. The rising wind starts cooling and then condensation takes place and the clouds are formed.
Cyclonic rainfall : Cyclones are the centres of low pressure. The air converges from all direction towards the low pressure. It is forced to rise up. The rising air gets cooled, condensed and clouds are formed which ultimately bring rainfall.
Role in Climatology
1. Humidity Coefficient (given by A J Angstorm) : Precipitation/ (1.07)t where t is mean temperature in degree Celsius.
2. Humidity Index (given by Thomwrite) : 100s/n where s is surplus water, n water require.
3. Humidity Province : Two classification – 5 provinces.
Moisture Index = [100 * water surplus] – [100 * water deficit] / PET
Factors Reflecting Relative Humidity
- Temperature
- Amount of Vapour
- Altitude
- Distance from the sea
- Latitude
- Vegetation
Humidity Sensor
A humidity sensor (i.e. hygrometer) senses, reports and measures both moisture and air temperature. There are 3 types of humidity sensor.
1. Capacitive
This a capacitive humidity measures relative humidity by keeping a thin strip of metal oxide between 2 electrodes. Electrical capacity of metal oxide changes with the atmosphere’s relative humidity. Its major application areas are industries, weather and commercial.
This type of sensors are linear and can measure relative humidity from 0% to 100%.
2. Resistive
These sensors utilize ions in salts to measure electrical impedance of atoms. As humidity changes, the resistance of the electrodes on either side of the salt medium.
3. Thermal
Two thermal sensors conduct electricity based upon the humidity of the surrounding air. One sensor is encased in dry nitrogen while other measures ambient air. The difference between the two is called humidity.
Optical Humidity Sensor
Absorption Measurement. Active fiber core optical sensor (AFCOS). Carbon Nanotube Gas and Vapor sensors. FBG glass fibre with strain coating. Fibre coasting based sensors (evanescent wave EW). Guided mode resonance sensors. Hybrid sensors – grating and interferometric. Interferometric sensor. LPFBG sensors. Photonic Crystal fiber based sensors. Polymer Fiber FBG based sensors. Porous optical fiber with fluorescent reagent. Porous optical fiber with colorimetric reagent. Reflectance change based sensors.
Acoustics Sensors
Surface acoustic waves humidity sensors. Quartz crystal microbalance. Bulk Acoustics wave. Quartz tuning fork.
Electronic sensors
Capacitive humidity sensors. Carbon nanotube. MEMS, magnetoelastic humidity sensors, Peizoresistive humidity sensor, RFID Sensors, Resistive humidity sensor, Surface Plasmon resonance.
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