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Humidity... It's Relative


July 16,2010
by: Steve Fowler, Fowler Associates, Inc.

sfowler@sfowler.com
www.sfowler.com

Relative Humidity: % RH , what does it mean?

Relative humidity is the cooling effects of one level of moisture in the air to another level. In other words, it is relative. It is not a definite level of moisture vapor.

It is measured by a sling psychrometer. This device has two thermometers. One is a standard thermometer. The other is one with a cotton sock over the bulb. The sock is saturated with water before the test begins.

The thermometer with the sock is called the wet bulb or saturated temperature and the other is called the dry bulb.

The sling psychrometer is spun around until the two thermometers reach stability. The two temperatures are read and compared to a chart to determine the RH - relative humidity.

The wet bulb will be cooler by the amount of evaporative cooling of the air on the wet sock versus the dry bulb. At 100% RH the dry bulb will have the same temperature as the wet bulb. In other words, the moisture in the air has the same cooling effects on the uncovered bulb as the air on the saturated bulb.

Moisture or water vapor is not water. It is a gas. The amount of moisture in the air is measured in grains per pound of dry air.

Grains is converted to grams by the following relationship: 1 grains = 0.06479891 grams

The relationship of relative humidity to the absolute amount of moisture in the air is determined using a psychrometeric chart. Click on the chart to see a full size version.

The left hand curved axis is the wet bulb temperature. The bottom axis is the dry bulb temperature. The dry bulb temperature is the temperature of the air in question. The relative humidity lines are drawn in the chart. These show that the amount of moisture - grains - in the air at any relative humidity- RH - is reduced as the temperature gets lower.

 

The ESD Association sets the standard low humidity test conditions as 72 degrees F, 12 % RH. By the chart this is seen as an absolute moisture level of 14 grains per pound of dry air of water vapor.

Now, if we wish to see how much moisture is in the air at any other temperature, we must use the chart. For example:

30 degrees F, 12 % RH equals 4 grains of moisture per pound of dry air. This amount of moisture in the air at 72 degrees F would equal 4% RH.

30 degrees F, 60% RH equals the same moisture content as 72 degrees F, 12% Rh = 14 grains of moisture per pound of dry air.

So the ESDA standard is really 14 grains of moisture not 12% at any temperature level.

The following table gives some of the relationships of %RH and temperature variations

72 degrees F, 12% RH
14 grains/lb dry air
equals

72 degrees F, 50% RH
58 grains/lb dry air
equals
90 Degrees F, 6.5% RH
90 Degrees F, 27% RH
80 Degrees F, 9% RH
80 Degrees F, 38% RH
60 Degrees F, 18% RH
60 Degrees F, 76% RH
50 Degrees F, 27% RH
50 Degrees F, @ 100 % RH
40 Degrees F, 40% RH
40 Degrees F, @ 100 % RH
30 Degrees F, 60% RH
30 Degrees F, @ 100 % RH
20 Degrees F, 80% RH
20 Degrees F, @ 100 % RH
Below 20 Degrees F - Near 100% RH
Below 20 Degrees F - @ 100 % RH

 

Static electricity or charging and accumulation of charges is related to the amount of moisture in the air not the RH.

you see it is relative.

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