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How Does a Hygrometer Work?

A Lesson in the Weather Instruments HOW-TO Series



A wet bulb hygrometer used to measure atmospheric humidity.

Image courtesy of www.weatherhut.com 2007. Used with permission.
Contributed by guest writer Jenni Worboys

What is a Hygrometer?

A hygrometer is an easy to use tool for measuring the amount of humidity in the atmosphere. There are two main types of hygrometers – a dry and wet bulb psychrometer and a mechanical hygrometer.

What is Humidity?

Humidity is the amount of water vapor in the atmosphere caused by condensation and evaporation. It can be measured as absolute humidity (the amount of water vapor in a unit volume of air), or as relative humidity (the ratio of moisture in the atmosphere to the maximum moisture the atmosphere can hold). It is what gives you that horrible sticky feeling on a hot day and can cause heat stroke. We feel most comfortable with relative humidity between 30% and 60%.

How do Hygrometers Work?

Wet and dry bulb psychrometers are the most simple and common way of measuring humidity. This type of hygrometer uses two basic mercury thermometers, one with a wet bulb one with a dry bulb. Evaporation from the water on the wet bulb causes its temperature reading to drop, causing it to show a lower temperature than the dry bulb.

Relative humidity is calculated by comparing the readings using a calculation table that compares the ambient temperature (the temperature given by the dry bulb) to the difference in temperatures between the two thermometers.

A mechanical hygrometer uses a slightly more complex system, based on one of the first hygrometers designed in 1783 by Horace Bénédict de Saussure. This system uses an organic material (usually human hair) that expands and contracts as a result of the surrounding humidity (that also explains why you always seem to have a bad hair day when it’s hot and humid!). The organic material is held under slight tension by a spring, which is linked to a needle gauge that indicates the level of humidity based on how the hair has moved.

How Does Humidity Affect Us?

Humidity is important for our comfort and our health. Humidity has been linked to sleepiness, lethargy, lack of observations, lower observation skills and irritability. Humidity also plays a factor in heat stroke and heat exhaustion.

As well as effecting people, too much or too little humidity can effect your possessions. Too little humidity can dry out and damage furniture. In contrast, too much humidity can cause moisture stains, condensation, swelling, and mold.

Hygrometers and Modern Technology

Most weather stations come with hygrometers to measure relative humidity. Many also come with alarms that you can set to a predetermined level to let you know when the humidity in your home is getting too high or low.

Getting the Best Results from a Hygrometer

Hygrometers must be calibrated at least once a year to ensure they provide the most accurate results possible. Even the best, most expensive hygrometer’s accuracy is likely to alter over time.

To calibrate, place your hygrometer in a sealed container alongside a cup of salt water, and place it in a room where the temperature stays relatively constant throughout the day (e.g. not by a fireplace or front door), then leave it to sit for 10 hours. At the end of the 10 hours, the hygrometer should display a relative humidity level of 75% (the standard) - if not, you need to adjust the display.

Hygrometers are widely used and a good way to help keep your family and possessions healthy. To find out more about using weather instruments and weather stations for measuring and forecasting the weather, visit The WeatherHut.com. You can also find out more from the Weather Calculators page.

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