What is the Difference Between a Hygrometer and a Psychrometer?

What is the Difference Between a Hygrometer and a Psychrometer?

What’s the difference between a hygrometer and a psychrometer? How do they measure air moisture content? Lots of information, many questions, and so few answers? We are ready to sort it all out.

In short, they both measure relative humidity but differ fundamentally in the way they work. Hygrometers and psychrometers are equally popular on the market for those who need to make such measurements.

What Is Hygrometer & How Does It Work

A hygrometer is a device that is used to measure the amount of water vapor in the air, soil, or confined spaces. This equipment generally depends on measurements of aspects such as temperature, pressure, weight, and a mechanical or electrical change in essence as moisture is concentrated.

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The hygrometer includes a pair of thermometers that stand side by side to each other. The first thermometer has a bulb that is opened to the air and the other one includes a bulb that is covered in a wet cloth. The evaporation and loss of heat from the bulb are caused by the water on the cloth, making the measurements lower than they are on the thermometer with a dry bulb.

The hygrometer works on the evaporate cooling phenomenon. Its principle shows that when the water evaporates from any area, it becomes cool as water molecules grab heat energy from the surface while evaporating. Because of this cooling phenomenon, the wet-bulb always shows a lower temperature than the dry bulb does.

What Is Psychrometer & How Does It Work

United Scientific Supplies SPCRKIT-PK/15 Sling Psychrometer

The principle of psychrometer is based on the relationship between the rate of moisture evaporation and the decrease/increase in the relative humidity of the air.

General Tools EP8706 Digital Psychrometer

The simplest psychrometer is a system of two bulb thermometers (mercury or alcohol), one of which is dry, and the second is wet (moistened), i.e. wrapped in a damp cotton cloth. One end of this fabric is dipped into a reservoir of water. The water evaporates and cools the wet thermometer. 

At the same time, the lower the humidity of the ambient air, the more intensive the evaporation process takes place. Thus, the drier the air, the lower will be the readings of the "wet" thermometer, and the greater the difference between the readings of dry and wet thermometers.

Having signed the readings of dry- and wet-bulb thermometers, you may use the psychrometric table supplied with the device to determine the relative humidity.

Why and how to use a psychrometer you may find here.

Hygrometer VS Psychrometer

Fat-free human hair or its artificial analog is used as the main component. Under the influence of changing humidity, its original length changes. With a decrease in moisture, the hair becomes shorter, with an increase, it lengthens.

Sper Scientific 736920C Dial Hygrometer/Thermometer

It is the principle of operation that the hygrometer differs from the psychrometer. A psychrometer is a more simple device, therefore it is more protected from various extraneous influences and, as a result, more reliable.

However, at the same time, hygrometers are more functional. Some models of hygrometers can measure moisture content not only of air but also of solids. For example, using a capacitive hygrometer, you can measure the moisture content of tablets.

The main similarity between a hygrometer and a psychrometer is in their purpose. Both of them are needed to determine the relative humidity of the air. The difference between devices is in the way they carry out these measurements.

Their field of application is another difference. Hygrometers are more relevant in domestic use while psychrometers - due to the greater accuracy of measurements and their smaller error - in medical institutions and industrial premises.

Mechanical & Digital Devices

Mechanical & Digital Devices

Mechanical or Analog Hygrometer measures and records the humidity with the aid of material that contracts and expands as the humidity changes. Like a bimetallic strip - a strip made of two metals joined face to face along their lengths. The mismatch in thermal expansion coefficients between them will cause the strip to bend slightly as the temperature changes. This method has been in use for many years. They are often used in microclimate systems, as well as for humidity control of industrial, warehouse, and household premises and control of the composition of protective atmospheres.

Digital Hygrometers are industrial devices that work based on resistive or capacitive sensors. For example, a digital hygrometer has silicon transistors instead of two mercury or alcohol temperature sensors. Sometimes this device is equipped with a thermometer, which makes it an ideal device for use in a variety of industries such as Agriculture.

Nowadays, there are various types of psychrometers and hygrometers, such as dual, sling, pen, pocket, twirling, and more. Both types of devices (psychrometers and hygrometers) are in demand by consumers. It should be noted that a hygrometer is more often used at home, and a psychrometer is applied in factories, laboratories, and sanitary stations. However, it depends only on you which one to choose to suit your specific needs.

Which Device Should You Use

The last and obvious question is which device should you use for taking measurements. Generally, it will depend on your special needs. Meteorologists and those who think that accuracy is the most critical aspect of the measurements should better use a psychrometer.

HVAC technicians and people who work in certain instances and painters will use hygrometers, as they appreciate comfort during the use rather than total accuracy.

Generally, an electronic hygrometer is a better option due to easier measurement reading and faster work. A lot of users don’t need a maximum accurate measurement, that is why the electronic version of the hygrometer will be okay.

Nevertheless, if you are interested in weather conditions or maybe you are a meteorologist, both devices are perfect for you. So, have you already chosen your device?

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