NACE Resource Center
|Corrosion - Dehumidification|
Humidity MeasurementAt a constant pressure, the warmer the air the amore water vapor it can retain. If air at a certain temperature has absorbed all the moisture it can hold at that temperature, it is said to be saturated. The amount of water present, or the degree of saturation is evaluated in terms of Relative Humidity or Saturation ratio.
Hence, Relative Humidity is the actual water content in % of that of a saturated volume. When air is 50% saturated, it contains only one half the amount of water than it can contain at the same temperature and pressure. As the relative humidity approaches 100%, the air can take on less and less moisture and at 100% relative humidity, that air cannot hold more water.
If air is saturated with water vapour at a given temperature, a drop in the temperature will lead to condensation of water in the form of droplets. The temperature at which moisture condenses out is the dewpoint temperature.
Measurement of RH
The comparison between the two readings given us the measurement of water vapor in the air.
The readings can be plotted on a chart known as the psychometric chart from where the properties of air vapor mixture like relative humidity, absolute humidity, dewpoint can be directly determined.
Using a Psychrometric Chart
A psychrometric chart is nothing but a graphic presentation of several interrelated air parameters brought together. A simplified psychrometric chart is shown here.
On a particular day if the dry bulb and wet bulb temperatures are known then by plotting these readings on the chart all parameters can be found out. It can be used to plot changes needed to transform an initial state to a final state. Given a required change, the initial and desired conditions are plotted, then the chart can determine the energy, temperature and moisture content changes, that will take place.
The process of heating, cooling and dehumidifying and chemical dehydration can be explained with the help of the psychrometric chart. reference