Relative Humidity; Dew Point/ Frost Point; Volumetric Measurement - Ametek 5000 Instrument Manual

Numerical values of water vapor pressure above water and ice are slightly different if the system
contains air or another gas. For this application the difference is small and can be ignored.

Relative Humidity

The relative humidity of a gas or air pressure is the ratio of the actual or measured water vapor pres-
sure to the saturation water vapor pressure (or absolute humidity in the gas at the same tempera-
ture) expressed as a percentage:
P meas x 100
RH = ———————
v
P saturated
v
where RH = percent relative humidity
P meas = water vapor pressure measured
v
P saturated = absolute water pressure at temperature
v
When water vapor pressure or absolute humidity is plotted against temperature, a curve is obtained
showing the relationship at equilibrium. If the water vapor is not in equilibrium with water or ice in
the system, then the relative humidity at the particular temperature is not 100 percent.
Relative humidity must be expressed at a given temperature referred to as the dry bulb tempera-
ture, which is the temperature of the system. The dry bulb temperature sets the value of the maxi-
mum water vapor holding capacity of the gas at a given temperature. The actual amount of water
vapor in the gas may be calculated from the rate of evaporation of a water-saturated wick covering
a thermometer, referred to as a wet bulb thermometer. Readings of wet and dry bulbs are used to
calculate relative humidity at a given temperature.
Dew Point
Dew point is the temperature at which water begins to condense out of a gas. Dew point is very de-
pendent on pressure, so pressure must be specified before conversion from dew point to other units
of moisture is possible.
Figure B1 gives approximate conversions between gas dew point temperatures (at 1030 kPa [14.7
psi] absolute pressure) and moisture contents expressed on both weight and volume bases.
Frost Point
When the dew point is below the freezing point of liquid water, the water vapor may freeze without
going through the liquid phase. A dew point below 0°C is called from the frost point.

Volumetric Measurement

A plot of dew point and frost point versus water vapor content in parts per million by volume at
1030 kPa (14.7 psi) absolute is shown in Figure B2.
B-2 5000 Multi Point Moisture Analyzer