Humidity Levels

Humidity Levels is the amount of water vapour in an air sample.

There are three ways to measure humidity. Absolute humidity which measures how many grams of water are in a cubic meter of gas or air. It is the most frequently used measurement as it is often used in weather forecasts because it can help estimate the likelihood of rain, dew or fog. Secondly relative humidity measures the percentage of maximum water vapour currently in the air. The greater the relative humidity hotter it may appear to be as it reduces the effectiveness of sweating to cool the body. Finally specific humidity is the ratio of water vapour to air in a particular volume of air.

Dew Point

Dew point is the temperature at which a sample of moist air (or any other water vapour) at constant pressure reaches water vapour saturation As temperature falls the ability of air or gasses to hold water is reduced.

Dew point can be measured using a chilled mirror as further cooling at saturation temperature will result in the condensation of water. An optoelectronic mechanism is used to detect condensation on the mirror surface and dew point temperature is closely measured by looking at the dynamic equilibrium between evaporation and condensation on the mirror,

Effects of Humidity

The human body sheds heat by a mixture of perspiration and evaporation to the surrounding air. In conditions of high humidity evaporation of sweat decreases and the body’s attempts to remain cool can be significantly reduced. Therefore the body may use other methods to increase heat loss such as redirecting blood nearer to the surface of the skin, but because so much of the blood is being transported to the extremities it can starve more vital organs such as the muscles and the brain thus resulting in fatigue and a decline in physical strength. If this is severe it can lead to heat stroke which can affect alertness and mental capacity which could cause an accident.

It is recommended that relative humidity should be kept between 30 — 60% and preferably below 50% to control dust mites.

Measuring Humidity

Levels of humidity are measured using an instrument called a hygrometer.

A simple hygrometer is known as a psychrometer which consists of a dry bulb and a wet bulb thermometer. Evaporation from the wet bulb lowers its temperature thus usually reads lower than the dry bulb. However when air temperature is below freezing the wet bulb may be covered in ice giving it a warmer reading than the dry bulb.

Relative humidity is measured using the ambient temperature as shown by the dry bulb thermometer and the difference in the temperature shown on the wet bulb. Relative humidity can also be measured by placing the intersection of the wet and dry bulb on a psychrometric chart. A device called the sling psychrometer uses the wet and dry bulb method to measure humidity.

Ambient humidity can also be measured using other types of common hygrometers. Many of these meters use an animal or human hair placed under tension & others record the humidity on a piece of graduated paper so values can be read off a chart.

More modern instruments use electronic means of recording the information, the 2 most common electronic sensors are capacitive sensors which sense water by applying an AC signal between 2 plates and measuring the change in capitance due to the amount of water present. Resistive sensors use a polymer membrane that changes conductivity according to absorbed water. Temperature must also be measured with these meters as it affects the calibration.

Hygrometers can be used in many places including industry, green houses, saunas and humidors.

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