Relative Humidity and Dew point…

Relative Humidity is readily checked parameters in pharmaceuticals and most misconceived topic . In pharmaceuticals the Drug is prone to degrade in presence of moisture so , Humidity is critical step in governing the stability of drug moieties . Relative humidity is checked almost in all cubicles in pharmaceuticals .It is defined as amount of water vapor present in air .

Explanation of Relative humidity..

Air is composed of molecules of gasses like Nitrogen , Oxygen etc these molecules have gaps between them filled by water molecules the ratio amount of water molecules present in air when divided with maximum amount of molecules air can hold is called relative humidity .

With increase in temperature these molecules will move apart and consequently the space between molecules increases  and air can accommodate more water molecules and relative humidity decreases.

With decrease in temperature the molecules will come close and space between molecules decreases and it can accommodate less molecules  And subsequently the Relative humidity Increases .

Dew point is another important concept ..

As we will decrease the temperature the molecules in air will come closer and the water holding capacity of molecules decreases as there is less inter-molecular space between the molecules at some point the water holding capacity reaches its limit and air can no more accommodate any more water molecules the excess of water molecules will condensate and appear as water droplets in cubicle . so , Dew point can be defined as temperature at which water molecules will start condensation(Vapors are converted to water because air is saturated) .

Principle of Sling Psychrometer

Sling psychrometer is commonly used in pharmaceuticals  in this psychrometer  2 thermometers are present one thermometer is covered with wet cloth when this psychrometer is rotated

• The Dry bulb thermometer will give Dry bulb temperature .
• The water will evaporate from Wet cloth on wet bulb thermometer and will decrease the temperature of bulb of thermometer .
• The rate of evaporation of water depends upon the concentration of moisture in air . if concentration of moisture in air is more there is less space between the molecules of air to accommodate more moisture so , the rate of evaporation from wet bulb is low and there is less depression in wet bulb.
• By subtracting the Dry bulb temperature – Wet bulb temperature we will get the Depression
• By plotting the Depression against Dry bulb temperature in Psychometric chart we will get the Relative Humidity .

Effects of pressure

When the pressure is increased the inter molecular space will also decrease and relative humidity is increased and when pressure is decreased the space between molecules increases so more water molecules can be accommodated and relative humidity decreases .