What is Humidity?
Humidity is the amount of moisture or water vapour present in the atmospheric gas. The more water vapour in the air, the higher the humidity. Humidity depends on the temperature and pressure of the system of interest. Humidity depends on the temperature and pressure. In this article, Humidity measurement is explained.
The same amount of water vapour results in higher relative humidity in cool air than warm air. At high temperature more water vapour can be held since the movement of molecules at higher temperatures prevents condensation from occurring.
There are three main measurements of humidity
- Relative humidity
- absolute humidity
- specific humidity
Types of Humidity
Absolute humidity is the total mass of water vapor present in a given volume or mass of air. It does not take temperature into consideration.
Absolute humidity is a measure of the actual amount of water vapor in the air, regardless of the air’s temperature.
Its unit is grams of water vapour per cubic meter volume of air. The higher the amount of water vapor, the higher the absolute humidity.
Absolute humidity in the atmosphere ranges from near zero to roughly 30 g per cubic meter when the air is saturated at 30 °C. Means a maximum of about 30 grams of water vapour can exist in a cubic meter volume of air.
Relative humidity, expressed as a percent, is a measure of the amount of water vapour that air is holding compared the amount it can hold at a specific temperature. This type of humidity is basically a ratio of the absolute humidity and the potential amount of water saturation that the air could possibly hold.
The amount of moisture the atmosphere can hold totally depends on the temperature. Warm air can possess more water vapour (moisture) than cold air, so with the same amount of absolute, air will have a higher relative humidity.
Relative Humidity= ( Actual amount of water in air)/(Saturated amount of vapour in the air can be hold at that temperature)
A relative humidity of 50% means the air holds on that specific temperature holds 50% of water needed for the air to be saturated.
Saturated air has a relative humidity of 100%.
Specific humidity refers to the weight of water vapor contained in a unit weight (amount) of air. Specific humidity is equal to the ratio of water vapour mass and the air parcel’s total air mass. It is expressed as grams of water vapor per kilogram of air.
Absolute and specific humidity are quite similar in concept.
Device used for the measurement of humidity is called a hygrometer. A hygrometer is an instrument used to measure the amount of water vapor in air, in soil, or in confined spaces.
Mechanical hygrometers make use of the principle that organic substances contract and expand in response to the humidity. The organic substance may be particularly finer substances such as goldbeater’s skin [ox gut] and human hair. Contraction and expansion of the this substance in a mechanical hygrometer causes the spring to move the needle on the dial.
Electrical hygrometers measure the change in electrical resistance of a thin layer of lithium chloride, or of a semiconductor device, as the humidity changes.
Dew point hygrometers are used to measure the saturation of moist air in a gas. These hygrometers typically consist of a polished metal mirror that is cooled at a constant pressure and constant vapor content until moisture just starts to condense on it. The temperature of the metal at which condensation begins is the dew point.
Dew point hygrometer is used where the smallest amounts of moisture need to be found. This is the most precise of all the hygrometers.
The oldest method for measuring relative humidity is the psychrometric method. The simplest hygrometer is a sling psychrometer. A sling Psychrometer measures relative humidity of the air.
It consists of two thermometers mounted together with a handle attached on a chain. One thermometer is ordinary and is called a dry-bulb thermometer. It measures the actual air temperature.
The other has a cloth wick over its bulb and is called a wet-bulb thermometer. The sling psychrometer is shown below.
As water molecules evaporate from the surface of the wet-bulb, they will take heat with them. It results in lowering the reading on the thermometer. The rate of evaporation depends on the vapor pressure or amount of water vapor in the air.
When the air temperature is below freezing, the wet-bulb must be covered with a thin coating of ice, in order to be accurate. As a result of the heat of sublimation, the wet-bulb temperature will eventually be lower than the dry bulb.
At 100% relative humidity no water will evaporate from the wet bulb and the readings on both thermometers will be the same.
Using the wet and dry bulb temperatures, the relative humidity can be observed on a psychrometric chart. Comparing the two temperatures in a chart will give the relative humidity.
The psychrometric chart is shown in the following figure.
Looking up the percentage relative humidity on a chart for every measurement is time-consuming and cumbersome. Nowadays, psychrometric charts and dew point equations can be stored in a microprocessor, thus making this a direct sensing method for relative humidity and dew point.
- Psychrometer is inexpensive.
- It doesn’t need a power source.
- It is portable and easy to use.
- Its maintenance is low.
- Hygrometers are generally used in laboratory to determine precise measurements for experiments and chemical reactions.
- It is used in some forms of industrial equipment.
- It is also used in the coating industry because the application of paint and other coatings may be very sensitive to humidity and dew point.
- Hygrometers are also used in some incubators, humidors and museums.