How Does a Hygrometer Work?

Our indoor environment is essential in keeping our health and comfort, especially in our own homes. Some environmental factors that affect the quality of the indoor air include temperature and humidity. To monitor the conditions in your home, these factors can be measured using different devices. Various home appliances also give you the ability to control the temperature and relative humidity in certain rooms or for the whole home. 

Before you purchase any product for indoor air quality or humidity control, it’s important to monitor the current conditions in your home. If you know the exact problem, you can find the appropriate solution. There are appliances for high humidity and those for low humidity. To make sure you get the right product(s) for your needs, you must first invest in a measuring device for humidity. This way you’ll know which one you need at what time of the year and what area of your house. One such device is the hygrometer, which measures the relative humidity in the air. If you’re curious about how it works, we’ll address your questions below.

What is Humidity?

How Does a Hygrometer Work featured photo

First, you must understand exactly what humidity means. Humidity is simply the amount of water vapor in the air. What you often see in weather reports is the relative humidity or RH (in percentage). This refers to the amount of water that is actually present in the air compared to the maximum amount the air could hold at a given temperature. Meanwhile, absolute humidity is the actual amount of water vapor present in the atmosphere.

While the temperature does not directly impact relative humidity (and vice versa), either can have some effect on how we sense the other. As the air temperature increases, the air is able to hold more water molecules. Thus, hot air allows more moisture to evaporate, leading to higher relative humidity.

Conversely, with high relative humidity, high temperature feels even hotter. Meanwhile, in the winter season, the cold air can hold less water molecules. In consequence, low humidity will make it feel even colder.

The ideal indoor humidity level is around 30-50% RH. Anything above 50% promotes the growth of mold and bacteria and attracts pests to your home, which can trigger allergies. Likewise, humidity that is too low can cause dry, scratchy throat, nasal congestion, and other respiratory conditions. 

What is a Hygrometer?

A hygrometer is simply an instrument that measures the relative humidity in the atmosphere. Appliances for humidity control such as dehumidifiers often have an onboard hygrometer measuring humidity in the area.

Acurite Hygrometer

Hygrometer vs. Humidistat

In contrast to a hygrometer, a humidistat does not only measure humidity levels. The latter works just like a thermostat, but instead of adjusting the temperature, a humidistat monitors and adjusts the humidity according to the target level set on the device. Both a humidistat and hygrometer may be bought as a standalone unit or built into a dehumidifier or other air quality appliances.

Honeywell Home Humidity Control

A dehumidifier or humidifier humidistat works by automatically sensing and controlling the humidity in the room. After you select your desired humidity level, the unit will work to achieve this setting, then stop when the target is reached, thus reducing your energy consumption. After a certain period, the unit will again measure the room humidity. If it’s higher than your setting, then it will start working again. 

Aside from mini Peltier units, most dehumidifiers have this basic feature, although some may be more accurate. If you compare with a separate hygrometer, you may find a difference of ±1% up to ±10% depending on the brand and model of the dehumidifier. Nevertheless, this isn’t usually a big deal when choosing a dehumidifier for your home. However, the same may not be true for industrial applications.

How Does a Hygrometer Work?

Hygrometers measure the relative humidity in a given area, either indoors or outdoors. There are different kinds of hygrometers, using analog and digital controls. 

different kinds of hygrometers


A psychrometer is an old device used to measure outdoor relative humidity. It determines the humidity in the atmosphere using a dry-bulb thermometer (which measures the ambient temperature) and a wet-bulb thermometer. When the wet-bulb loses moisture due to evaporation, the wet-bulb temperature drops. The ratio of the two temperatures (from the dry and wet thermometers) is how relative humidity is calculated. 

Capacitive hygrometer

Electronic hygrometers can be capacitive or resistive. Both rely on the amount of condensation resulting in the change of temperature in the sensor. Capacitive hygrometers use two metal plates then run an alternating current between the two plates to test capacitance or its ability to store an electrical charge. As the amount of moisture increases, the capacitance also increases.

Resistive hygrometer

Resistive hygrometers measure the electrical resistance in a sensor made of ceramic. The sensor absorbs water from the air, and as more water vapor condenses, it causes the ceramic to contract. The hygrometer measures the sensor’s resistance.

Dewpoint hygrometer

Dewpoint hygrometers measure the amount of moisture in a gas. They are considered the most precise hygrometers and can be used to measure water vapor in places with the least humidity. The dew point is the temperature at which air is fully saturated with water vapor (100% relative humidity). When the temperature reaches past the dew point, it results in condensation.

Why Do You Need a Hygrometer?

Too low and too much humidity can cause discomfort and also have many different negative effects to your health and everyone in your home. To prevent humidity issues before they become worse, it’s important to monitor humidity levels.

If you want to know more about how humidity affects your health and how humidity affects your home, you may want to check out our article here about humidifiers vs dehumidifiers.


To solve either low or high humidity problems, humidifiers and dehumidifiers help control indoor humidity levels. Humidifiers are commonly used in winter when the air is dry and cold, while dehumidifiers are common during summer days or heavy rains. In any case, it’s important to monitor the relative humidity with a proper hygrometer to make sure you’re using the right device for humidity control.

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