What is mean by Hygrometer?

A Hygrometer measures the Value of Humidity Directly.Generally,the output of Hygrometer is used to indicate Relative Humidity.

Several Material exhibit changes in Electric Properties that are caused by Humidity.These are frequently used in Transducers that are designed and
Calibrated to read Relative Humidity Directly.

01)Resistive Hygrometer:-
Some Hygroscopic Salts exhibit a change in Resistivity with Humidity.The most common is Lithium Chloride.This,with a Binder may be coated on a wire
or on Electrodes.

Resulting Resistance changes cover a wide range for example 10 rase to four to 10 rase to nine ohm,as the Humidity changes from 100 to 0 percent.

This makes it imparcticle to design a single element to operate from 1 to 100 percent Relative Humidity.Instead several elements are used,each in a narrow
range,with provision for switching elements.

Resistance is measured either with a Wheatstone Bridge or by a combination of Current and Voltage Measurments.

Most of these must not be exposed to conditions of 100 percent Humidity as the Resulting Condensation may damage the Device.Either they must be
operated in a costant Temperature Environment or Temperature corrections must be made.

These are Accurate to within plus or minus 2.5 percent or plus or minus 1.5 percent in some cases.Response times are typically of the order of a few
seconds.These are currently the most common Electronics Hygrometers.

A typical Resistive Hygrometer having a mixture of Lithium Chloride and Carbon which acts as Conducting Film.This is put on an Insulating Substrate
between Metal Electrodes.

The Resistance of the element changes when it is exposed to variations in Humidity.The higher the Relative Humidity the more Moisture the Lithium
Chloride will obsorb,and the lower will be it's Resistance.

The Resistance of the Sensing unit is a Measure of the Relative Humidity.Resistance should be Measured by applying Alternative Current (A.C) to the
Wheatstone Bridge.

D.C. Voltage is not applied beacuse it tends to Breakdown the Lithium Chloride it it's lithium and Chlorine Atoms.The Current flow is a Measure of the
Resistance and hence of the Relative Humidity.

There are different types of Hygrometer are as follows.
01)Resistive Hygrometer
02)Capacitive Hygrometer
03)Microwave Hygrometer.
04)Aluminium Oxide Hygrometer.
05)Crystal Hygrometer.

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