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What is a hygrometer and how does it work?

Last Updated: 30.01.23


A hygrometer is a type of weather instrument used to measure the amount of humidity in the atmosphere, in soil, or in enclosed spaces.

It might come as a surprise, but the first hygrometer was invented in 1480 by the Renaissance genius Leonardo da Vinci. Science has made progress since then and nowadays we have two main types of hygrometers — the dry and wet bulb psychrometer and the mechanical hygrometer.

Humidity – what is it?

Humidity is the amount of water vapor present in the atmosphere. It appears as a result of condensation and evaporation. There are three main measurements of humidity — absolute humidity, relative humidity and specific humidity.

Absolute humidity represents the amount of water vapor in a unit volume of air, and it’s expressed in grams per cubic meter or grams per kilogram. Relative humidity is the ratio of moisture that can be found in the air to the maximum moisture the atmosphere can hold and it’s expressed as a percentage.

Specific humidity is the ratio of the mass of moisture to the total mass of the moist atmosphere parcel.

Leaving science aside, humidity is what gives you that irritating sticky feeling on a hot day or when you’re on holiday in a tropical climate.


Humidity – how it can affect you?

Humidity is important for both your health and your comfort. People feel most comfortable with relative humidity somewhere between 30% and 60%.

Overexposure to high humidity has been linked to low energy, lethargy, loss of observation skills, irritability and drowsiness.

Our bodies naturally regulate our internal temperature by sweating. When we are hot, we sweat and that sweat is evaporated, thus cooling our bodies. This cooling process is rendered inefficient by exposure to high humidity because the evaporation of moisture from the skin is reduced.

There are some serious health risks which can result from overexposure to humidity — dehydration, muscle cramps and even heat stroke and heat exhaustion.

Humidity doesn’t only affect you, but it can affect your possessions and your house, as well. Too little humidity can deteriorate furniture. On the contrary, too much humidity can cause swelling, moisture stains, condensation, and mold.

How does a hygrometer work?

The easiest and the most common way to measure humidity is using wet and dry bulb psychrometers. This type of instrument uses two very basic mercury thermometers, one with a wet bulb and the other one with a dry bulb. Water evaporation on the wet bulb causes its temperature reading to drop, therefore showing a lower temperature compared to the dry bulb.

To calculate relative humidity, we have to measure the readings using a calculation table that compares the difference in temperatures between the two thermometers to the ambient temperature. Ambient temperature is the temperature given by the dry bulb.

A mechanical hygrometer or a hair tension hygrometer uses a somewhat more intricate system. It’s based on a hygrometer designed in 1783 by the Swiss physicist and geologist Horace Bénédict de Saussure.

This system uses organic material — more precisely human hair, because it’s hygroscopic, which means it tends to retain moisture. The hair expands and contracts as a result of the humidity in the atmosphere. That, in case you didn’t know, is why your hair is frizzy every time it’s hot and humid.

The organic material (the hair) is held under a slight tension with the help of a spring that is linked to a needle gauge that indicates the level of humidity according to how the hair has moved. The instrument can be made more sensitive (or more accurate) by removing the oil from the hair which is usually done by soaking the hair in ether.


Hygrometer maintenance

These instruments must be recalibrated at least once a year to guarantee you the most accurate readings possible.

You don’t need a lab bench to calibrate your hygrometer; all you have to do is place it in a tightly closed container along with a cup of salt water, and leave it for 10 hours in a room where the temperature is maintained relatively unchanged throughout the day.

When those 10 hours are over, the hygrometer should read a relative humidity of 75% (which is the standard). If it doesn’t happen, you need to adjust the display.



Ioana Moldovan

Ioana’s professional experience in the optics field has helped her understand the value of passing her knowledge forward. Her curious personality helps her gather useful information for her readers and her goal is to make technical information fun and accessible to everyone.

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