In this article, we will learn about hygrometers and its types. Let’s begin with the definition of the hygrometer.
Hygrometer
A hygrometer is an instrument that measures the humidity directly. Now discuss some terms.
Humidity
It is the measure of water vapour present in a gas or environment. It is measured in relative, absolute humidity or dew point temperature.
Absolute humidity (AH) is the mass of the water vapour present per unit volume.
Relative humidity (RH) is the ratio of water vapour pressure actually present to water vapour pressure required for saturation at a given temperature. RH is dependent upon temperature. A hygrometer is used to indicate RH.
Hygrometer instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. These properties are used in transducers that are designed and calibrated to read RH directly.
Types of Hygrometer
- Aluminium Oxide Hygrometer
- Capacitive Hygrometer
- Crystal Hygrometer
- Thermal Hygrometer
- Resistive Hygrometer
- Gravimetric Hygrometer
1. Aluminium Oxide Hygrometer
There is a Aluminium Oxide coating on anodized aluminium. There is a change in dielectric constant and a change in resistivity with the change in humidity of aluminium coating.
2. Capacitive Hygrometer
This hygrometer is based on the fact that there is a change in dielectric constant with a change in humidity. But the changes are small. So a change in capacitance is measured by including it as the frequency-determining element in an oscillator, heterodyning this signal with a beat frequency oscillator, and measuring the resulting difference frequency.
3. Crystal Hygrometer
In this hygrometer, a hygroscopic material coated on a crystal is used as frequency determination elements in electronic oscillators. There is a frequency shift with humidity as the mass of the crystal changes with the amount of water absorbed by the hygroscopic coating and this frequency shift is measured electronically.
4. Thermal Hygrometer
In thermal hygrometers, the change in thermal conductivity of air due to humidity is measured. These sensors measure absolute humidity rather than relative humidity.
5. Resistive Hygrometer
In resistive hygrometers, the change in electrical resistance of a material due to humidity is measured. Some hygroscopic salts exhibit a change in resistivity with humidity. The common example of a hygroscopic salt is lithium chloride. The resistance is measured by either with a Wheatstone bridge or by a combination of current and voltage measurements.
6. Gravimetric Hygrometer
A Gravimetric hygrometer measures the mass of an air sample compared to an equal volume of dry air. This is considered the most accurate primary method to determine the moisture content of the air.
Applications of Hygrometer
Hygrometers are also used in
- Greenhouses and industrial spaces.
- Incubators, saunas, humidors and museums.
- Coating industry because the application of paint and other coatings may be very sensitive to humidity.
- Care of wooden musical instruments such as pianos, guitars, violins, and harps which damaged by improper humidity.
- batteries to measure SOC.
