Hygrometer
Hygrometers are instruments that
measure the humidity (amount of moisture content) present in air or gas.
Hygrometers consists of two mercury thermometers, one of which has a wet base
(to measure wet-bulb temperature) while the other one has a dry base.
Genesis of Hygrometers
The first basic hygrometer was
invented in 1400s by Leonardo da Vinci. However, a more practical hygrometer
was built by Francesco Folli in 1664. British chemist and meteorologist, Sir
John Frederic Daniell made the dew point hygrometer in 1820. It was used with
cold water and the temperature (or dew point) of the vessel was recorded to
measure humidity.
How do Hygrometers work?
At a particular temperature and
velocity, the cooling effect of air is directly related to humidity. This
phenomenon is used in hygrometers. When air is circulated on the wet bulb
thermometer, it will show a lower temperature (as compared to the dry bulb
thermometer). The differential in these two readings is then translated to
percent humidity, using established charts.
Calibration of Hygrometers
Precise calibration of the
thermometers is critical to ensuring that the measurement of humidity is also
accurate. Additionally, thermometers need to be protected from any radiant heat
as well as a good airflow has to be maintained over the wet bulb. Further, for
accurately measuring humidity when the air temperature is below the freezing
point, temperature is actually raised by heating the air (since the vapor
content remained unchanged even when the air is heated).
Different types of Hygrometers
There are several types of
hygrometers used by meteorologists. The simplest and best known form of
hygrometer is “psychrometer”. It is uses the principle of evaporation to
determine humidity. Rapid rotation of both the thermometers and blowing air
over the bulbs reduces the temperature of the wet bulb. This temperature
difference between the thermometers is used to calculate the moisture content
in the air.
An electronic hygrometer deploys
electronic means of recording the information, using capacitors or resistors
for the purpose. The capacitive sensors can detect the presence (and quantum)
of water, by passing an electric current and measuring the change in
capacitance caused by the presence of water. Alternatively, resistive sensors
use a polymer membrane whose conductivity changes, based on the amount of water
that is present.
Other hygrometers include
gravimetric hygrometers (primarily used for calibration and setting national
standards), and hair tension hygrometers use human or animal hair under
tension.
Application of Hygrometers
Hygrometers are used in
greenhouses, industrial setups that are sensitive to vapor content (example
being paints, food and beverages and pharmaceutical), saunas, humidors, museums
and residences, primarily for the purpose of measuring and controlling
humidity.
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