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How to Use a Hygrometer and How Does It Work?

how to use a hygrometer and how does it work

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To fully understand how to use a hygrometer, it helps to know when and where you can use it. A hygrometer is an essential instrument in meteorological science. It has numerous commercial uses, but the primary purpose of the device is to measure the amount of water vapor (humidity) in the air.

The primary use of the instrument is to provide the amount of moisture in the air and compares that number to the maximum amount of moisture that the air, confined space, or soil can potentially contain. If you have this question, “How does a hygrometer work?” here is the answer. Just like other instruments used for measuring humidity, hygrometers work by relying on other quantities or measurements of temperature, mass, and pressure among others as these things absorb moisture.

What is a Hygrometer?

Hygrometers are considered weather instruments that can measure humidity. Over the centuries, these instruments have changed, advanced, and have taken different varieties to suit the users’ needs. It was in the 1400s when one of the very first models of hygrometers appeared. Leonardo da Vinci constructed his early version of this device in which his goal was to measure the moisture in the air.

da Vinci-Hygrometer Manuscripter

He had a genuinely clever design, which centered around a gauge that had absorbent and non-absorbent materials on each side. Whenever humidity would increase, the side with the absorbent material would get heavier as it got more saturated. The weight difference was interpreted as a reading for a relative humidity.

After a few hundred years, Horace Bénédict de Saussure, a physicist, and a geologist from Switzerland who loved to explore the Swiss Alps created an improved version of the early hygrometer. Just like what da Vinci developed, the device measured humidity.

de Saussure-Hygrometer

The difference though is that de Saussure used a strand of hair, which was suspended on the device under tension. The moisture in the air had an effect on the hair, causing it to either contract or expand, depending on the humidity level. A needle would point to a number on the dial, which he interpreted as the humidity in the air.

Other versions of the device were created. Most of the early designed replicated thermometer technology during that time. To be specific, the devices borrowed the techniques used in wet and dry bulb psychrometer. It had two mercury thermometers, which as the name implies, had one dry and one wet. The latter had a wet fabric sleeve covering it at the base of the device. To measure relative humidity, the difference in the temperatures of the dry and wet thermometers was calculated.

How the Hygrometer Work

Today, there are different types of a hygrometer and how they work will depend on their design. For instance, one of the types is a capacitive hygrometer. Devices classified under this form use different other types of sensors. Capacitive hygrometers are created for applications where price, fragility, or space is a concern, so they risk the accuracy of reading moisture for the affordability of the device.

Capacitive hygrometers measure the effect of moisture on a metal oxide material. Frequently, they are also used for quantifying humidity’s impact on the dielectric constant (or the relative permittivity) of a polymer, such as synthetic plastics. When calibrated, the sensors have an accuracy rating of ±2% relative humidity within the range of five to 95% relative humidity.

If not calibrated, the accuracy is up to three times worse. The sensors though can withstand high temperatures and the effects of condensation, and they are suitable for various applications. However, these capacity hygrometers have sensors that can easily be contaminated and show signs of aging rapidly.

Another sensor is resistive, which is less sensitive when compared to capacitive sensors. These sensors measure the changes in a material’s electrical resistance, which humidity in the environment causes. Since they are not as sensitive as capacitive, resistive sensors require a more complex circuit board. The accuracy depends on the resistive material used but is usually up to ±3% relative humidity. The material also dictates how robust the hygrometer will be against condensation.

What is a Hygrometer Used For?

Hygrometers are deemed as useful tools, which utilize calculation and calibration of pressure and temperature changes. Combined together, the resulting percentage will be the amount of moisture in the air. A hygrometer is a helpful device in office buildings and homes, as well as in manufacturing and industrial processes. These areas need to ensure that no change in humidity will occur, since it may affect the production of materials.

The use of the hygrometer depends on the type, which includes the following:


Psychrometers As mentioned, this type has two thermometers, one dry and one wet bulb. The difference in temperature is measured when evaporation takes place in the wet bulb, causing its temperature to drop lower than the dry bulb. Psychrometers are useful devices to get the measurement of outdoor humidity. They are also ideal for places that require dry conditions, such as storage areas.

Electrical hygrometers:

Electrical hygrometers

These devices can either be resistive or capacitive. Resistive electrical hygrometers have electricity passing through a ceramic substance that is in the air. When the humidity gets high, more water vapor causes the ceramic to contract, which leads to a modified resistance. Meanwhile, a capacitive electrical hygrometer uses two plates with air between them. The amount of humidity is measured through the ability of these metal plates in storing electrical charges, which moisture affects directly.

Dewpoint hygrometers:

Dewpoint hygrometers

These devices are for measuring the saturation of moisture in a gas. Typically, they are used in places with the least amount of humidity can be found. Currently, they are considered the most precise type of hygrometers.

Hygrometers have become a popular device in industrial spaces and greenhouses. These days, however, they are also used in museums, humidors, saunas, and incubators. Moisture can harm wooden musical instruments, which is why hygrometers are also used in caring for pianos, violins, harps, and guitars among others.

Meanwhile, homes can benefit from hygrometers because they can guide the residents on humidity control. Very low humidity can negatively impact the skin and the body. On the other hand, too high humidity encourages mildew and dust mites to proliferate. With the help of hygrometers, homeowners can figure out a way to control moisture in their surroundings.

The paint industry also sees the importance of hygrometers. Paint applications, for instance, can be quite sensitive to both dew point and humidity.

How to Use a Hygrometer: Reading the Device

Whether you will use a hygrometer for weather observation or measuring conditions for indoor spaces, it makes sense to know how to read one. To get the best and most accurate results, these devices need to be calibrated at least once every year.

Electrical hygrometers

Relative humidity is computed by comparing the numbers in a calculation table, which matches the ambient temperature (in a wet and dry bulb psychrometers, it is the dry bulb) with the difference in the temperatures given by the two thermometers.

On the other hand, a mechanical hygrometer has a slightly more complicated system since it is based on the model that Horace Bénédict de Saussure designed.

Reading the hygrometer is not tricky, but it may vary depending on the type and model you are using. For instance, a wet and dry bulb requires the following numbers:

  • Dry bulb temperature
  • The temperature of the wet bulb
  • Wet bulb depression (the difference between the first and second temperatures)

To read the result, look at the chart that is on the face of the device and read down starting from the wet bulb depression and then the temperature of the dry bulb. Get the spot where the numbers meet, and you will have the relative humidity of the area. Use percentage to express the final answer.

For digital hygrometers, you will only have to place the instrument at about 3.3 feet (one meter) from the ground. Let the device work for at least three minutes so it can adequately detect the temperature of the air. You do not have to perform any other calculations because the instrument will do it for you.

An example of a digital hygrometer is ThermoPro TP63, which is suitable for both indoor and outdoor use. The device can monitor the conditions of the environment in four different locations of up to 200 feet (60 meters) away, making it suitable for greenhouses. It can provide you with the humidity percentage for the past 12 hours.

ThermoPro TP-63 Hygrometer use for both outdoor and indoor

For homes, the ThermoPro TP50 is an easy-to-use device for monitoring humidity and temperature. This digital device will provide you with the current humidity in bold numbers, along with the current temperature. It will also show the minimum and maximum temperature and humidity in the room. The instrument also gives three levels of comfort in which lower than 30% of humidity is dry, and 30% to 60% is considered comfortable. However, the temperature should be between 68F to 79F. Humidity that is higher than 60% will indicate the area is wet.


With these hygrometers, homeowners and businesses can keep their properties or products in prime condition. At the same time, they promote recognition of the ideal humidity in a specific area to increase its overall comfort level.

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