| Hygrometer
, instrument used to measure the moisture content of a gas,
as in determining the relative humidity of air. The temperature
at which dew or frost forms is a measure of the absolute humidity—the
weight of water vapor per unit volume of air or other gas
at the temperature before cooling.
Knowing absolute humidity and air temperature,
the observer can calculate relative humidity. The most common
type of hygrometer is the dry- and wet-bulb psychrometer.
It consists of two identical mercury or electrical
thermometers , one of which has a wet cotton or linen wick
around its bulb. Evaporating water from the wick absorbs heat
from the thermometer bulb, causing the thermometer reading
to drop.
The difference between dry-bulb and wet-bulb
temperatures are compared on psychrometric charts. The temperature
at which dew forms, called the dew point, is determined by
a dew-point, or condensation-type, hygrometer, which is basically
a mirror, usually of polished metal, cooled until dew or frost
forms on it.
Various cooling methods employ compressed
carbon dioxide, dry ice, liquid air, or mechanical refrigeration.
When dew or frost forms on the mirror, the temperatures between
when dew appeared and when it disappeared is the dew point
and is read with optical, electrical, or nuclear techniques.
Because human observation varies, photoelectric
cells are sometimes used to register the time at which the
mirror fogs. Another type of hygrometer works on the principle
that electrical resistance varies in a material that absorbs
moisture, with the resistance to a current passing between
wires measured by special sensors.
The most accurate way to measure humidity
is with an electric hygrometer, in which a known volume of
gas passes over a hygroscopic, or moisture-absorbing, material
such as phosphorus pentoxide. It is weighed before and after
to determine how much water it took out of the gas. The older
types of mechanical hygrometer use human hair, which stretches
as it absorbs moisture, with a linkage connecting the center
of a bundle of hairs under tension to a pointer.
Another uses goldbeater's skin, a membrane
from the intestines of oxen. Some materials can determine
relative changes in humidity because they change color as
they absorb moisture.
What
are Hygrometers? | How
to Calibrate a Hygrometer | Hair
Hygrometer | Recording
Hygrometer | The
use of The Hygrometer |
