BASIC INSTRUMENT FOR HUMIDITY

What is Relative Humidity:
It is a ratio that compares the amount of water vapor in the air with the amount of water vapor that would be present in the air at saturation. It can also be stated as the ratio of the actual mixing ratio to the saturation mixing ratio. Relative humidity is given as a percentage: the amount of water vapor is expressed as a percentage of amount of water vapor at saturation. If 20 grams of water vapor were present in each kilogram of dry air, and the air would be saturated with 40 grams of water vapor per kilogram of dry air, the relative humidity would be 20/40=50%. What is Dew Point Temperature (Td): It is the temperature at which the water vapor pressure in gas mixture equals the saturation water vapor pressure and is usually expressed in °F or °C. Any plane surface in a given environment which is colder than the dew point temperature of the air or gas surrounding it will accumulate a dew or frost layer.

Instruments to measure humidity:

Psychrometer

A psychrometer is one of the most effective instruments used to measure the water vapor content-or the amount of moisture or relative humidity-in the air.
How does it work?
The psychrometer consists of two Celsius thermometer tubes. The red tube measures the actual ambient temperature-referred to as the dry bulb temperature. The base of the blue tube is immersed in a vessel containing a muslin wick soaked in water. This tube measures the temperature caused by the effects of evaporation from the saturated wick. This temperature reading is referred to as the wet bulb temperature. The relative humidity (%) of the air is then calculated by crossing the dry bulb and the wet bulb temperature readings on the easy to read chart beneath the tubes.
Why use the Psychrometer?
It is one of the most easy and accurate methods of determining the relative humidity. The psychrometer is very durable and affordable.
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Hair hygrometer

A hygrometer in which the sensitive element is a strand or strands of human hair, the length of which is a function of the relative humidity of the air.
Hair is made from keratin, a protein that is wound into a coil. The turns of the coil are held together by a type of chemical bond called a hydrogen bond. Hydrogen bonds break in the presence of water, allowing the coil to stretch and the hair to lengthen. The bonds re-form when the hair dries, which allows people to style their hair simply by wetting it, shaping it, then drying it.
A hygrometer measures humidity or the amount of moisture in the air. This is done by measuring the change in length of an organic fibre (e.g. human hair) brought about by the absorption of moisture.
The Hair Hygrometer works on the principle that human (and horse) hair changes its length in accordance with the relative humidity of the atmosphere and not the vapour pressure.
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Electronic humidity sensor or capacitive sensor:

Electronic sensors, mostly capacitive sensors, are usually relatively cheap and easy to install and maintain. There exists a relationship between capacitance and the humidity to be measured. Capacitance is transferred into a signal that is converted by the computer to RH or another humidity reading. They have a major disadvantage: they easily become unreliable particularly at high humidity levels. They are usually good when new, but they quickly deteriorate under rough conditions like changing humidity and spraying. In tests, most electronic devices, even the more expensive ones, became inaccurate within a short period of time. Keep in mind that calibration under wet conditions with another electronic device is very dubious.