WHAT IS RELATIVE HUMIDITY ?

Anyone who has suffered the discomforts of hot, humid summer weather understands that it is not just the heat, but also the humidity that makes the air feel so miserable. That "muggy" feeling comes from the relative humidity or saturation level– that is, the amount of water contained by a pound of air at a specific temperature and atmospheric pressure.

When air has 50 percent relative humidity (rh), we say it is 50 percent saturated (the terms are numerically so close that we use them interchangeably). The air contains about half the water it could hold at the same temperature and pressure. Obviously, as air approaches 100 percent saturation, it can take on less and less water until at 100 percent rh, the air cannot hold more water.

Relative humidity is determined by comparing the "wet-bulb" and "dry-bulb" readings of a humidity measuring device- a hygrometer (see the table below). Once known, these values identify a point on the psychometric chart (see Appendix-1) where air vapor mixture properties can be read directly.

The following hygrometers can measure the humidity or hygrometric state of the air:

MEASUREMENT OF HUMIDITY

HYGROMETER	APPLICATION	RANGE, °F	PRECISION, °F	LIMITATIONS
Psychrometer	Room of building, outside air, air moving in ducts	20° to 140°wb	+.4° wb	Should be used in air stream moving about 1000 fpm, small diameter wet bulbs may be used at lower velocities; difficult to use at subfreezing temperatures
Dew-point; Condensation type	Automated systems in industrial processes, meteorological observations, remote locations	-150° to 200° dp	0.2° to 2° dp	Expensive
Fog-type	Wide range, method for sampling	-80°to ambient dp	+2° dp	Manual, series of readings needed for measurement
Salt-phase transition	Meteorological measurements laboratory; simple to use	0° to 160° dp 56° dp DEP	+2° dp	Not usable below approximately 15% rh; susceptible to some atmospheric contaminants
Dimensional change: Mechanical	Control, measurement where air motion is slight	20 to 100% rh -40° to 125° dp	+3% rh	Frequent calibration required when used at extremes of range; hair has considerable lag, low sensitivity, and is adversely affected by temperature above 125°F and rh below 20%
Electrical conductivity	Measurement, control	-40° to 120° dp	+1.5 to 3% rh	Susceptible to damage by air contaminants-some to water; require frequent calibration checks.
Electrolytic	Measurement	-60° to -5° dp	5% of scale range	Ordinarily limited to low humidities
Gravimetric	Measurement, standard		+0.1 to 2%	Special equipment and extreme care required for high accuracy

A psychrometer consists of two thermometers (matched in type, scale and range), one of which has a cloth wick-a "sock"-applied to its bulb. To use, the wick is wetted with distilled water and ventilated with air moving at a recommended 900 to 1000 feet per minute (fpm) or more at right angles to the instrument.

Dew-point hygrometers visually note when humidity-that is, water in the air-condenses on a cooled metallic surface. The temperature at which this condensation or dew-point occurs can help determine other air properties via charts and tables. Several types of dew-point hygrometers are widely used.

Some organic materials such as human hair, animal membrane, animal horn, wood, paper or nylon change in dimensions as the humidity changes. These hygrometers may be mechanical, electrical, electrolytic or gravimetric in nature. However, no organic material consistently reproduces its action over an extended time, especially in extremes of humidity or temperature. So this category is of limited value.