Humidity Meters (Hygrometers) - Types and Uses

Simple brass wall hygrometer measuring 77% humidity.
Simple brass wall hygrometer measuring 77% humidity. | Source

In a situation where the amount of moisture in the air is crucial to good health, efficient production, or is an indicator of future circumstances, how does one keep track and keep control of humidity, without having to measure levels constantly? By metering, i.e. measuring and calibrating the existing humidity, while automating corrective action.

There are many different kinds of water-related meters. Water meters measure characteristics of water as a liquid, and moisture meters measure the amount of water in a solid or semi-solid. Hygrometers, covered here, measure the third main water category - water vapor in the air, i.e. humidity.

Some of the different technologies used in hygrometers to measure humidity are: The tensile property of hair (which stretches in high humidity), the dew point of water (condensation on a chilled surface), and the evaporation differential between a dry vs. wet surface. More on this below.

As with other types of meters, the more complex meters measure more accurately, so the one you choose to use will depend upon how sensitive your reading needs to be. In a life and death situation, like an ICU unit, the hygrometer used will be more complex than will a wall unit for your office.

Hygrometer Considerations

Those hygrometers where the amount of money to be earned does not pay for the time and complexity of a high quality manufacture, will necessarily be made with fewer controls and will need higher maintenance by the buyer. In addition, certain atmospheric conditions can distort a measurement's accuracy.

In all of the uses below, the following characteristics of hygrometers need to be kept in mind. Some will be more important than others, depending on the project at hand:

Calibrating Your Hygrometer

  • Most handheld hygrometers will need regular calibration and cleaning in order to provide accurate measurements. (See video at right for a simple calibration process.)

  • Humidity is dependent upon temperature, which is why many hygrometers come together with a thermometer. High temperatures increase humidity more than do lower temperatures, and water vapor will condense more quickly at lower temperatures than at higher ones, which especially affects dew point meter readings. Most hygrometers, as a result, measure "relative humidity" or RH.

  • Humidity readings can be contaminated by any existing dust. This is a special consideration for the coating industry.

  • Readings can also be altered by condensation of gases other than water, since different gases condense at different temperatures - a special consideration for the petroleum and natural gas industries.

How Hygrometers are Used

Hygrometers are used in any situation where the air needs to be especially moist or especially dry, or where the relative moisture in the air indicates an upcoming situation that will affect human activity or safety. A steady environment invites the use of a wall-mount or plug-in device, whereas a technician measuring many things in many locations will want a portable device. The lists below show different kinds of humidity meters and their uses.

This old fashioned humidity meter uses the tensile strength of hair to determine which figure to rotate to the front. Right now the gauge is reading 70ºF with high humidity, hence the potential for rain.
This old fashioned humidity meter uses the tensile strength of hair to determine which figure to rotate to the front. Right now the gauge is reading 70ºF with high humidity, hence the potential for rain. | Source

Weather Instrument

Weather hygrometers are used to predict weather-related events that could prove dangerous or inconvenient for humans. The purpose in gathering this type of information is to take preventive action ahead of time, so any disaster is minimized. Farmers anticipating a rainstorm will want to protect their cut grain and animals. Drivers anticipating fog will want to make sure their vehicle's low beams work and the tires have treads.

Weather House - This is an early, decorative weather predictor made in the shape of a miniature Alpine chalet. Inside the chalet is a hair-tension hygrometer that measures moisture absorbed by the hair. Since hair contracts with dryness, dry air will tighten the hair strand/s and cause a circular platform to rotate. Fixed to the platform is a figure representing rain and another representing dry weather. The platform rotates to put one or the other on the outside of the chalet, telling the viewer it will be a rainy or a rain-free day.

Psychrometer - This device compares readings from two thermometers, one dry and one wet (see video at right). The difference is used to determine relative humidity. This device is used for measuring humidity in weather readings, predicting icy conditions along an aircraft's flight path, and alerting forest fighters to the possibility of mountain fires when the air is especially dry.

This type of hygrometer is also used in residential and commercial HVAC systems to make sure the refrigerant is charging the air conditioning properly. And it is often the type of hygrometer used with humidistats (see below).

Research Hygrometers

The standard hygrometer used in laboratories is a "chilled mirror hygrometer" or "dewcheck." This kind takes a reading the moment water vapor starts to condense on a mirror kept at a certain level of frigidity.

Frost Point Hygrometer - This is the more complicated (and hopefully accurate) hygrometer used by the Earth System Research Laboratory of the National Oceanic & Atmospheric Administration in the United States. This laboratory measures the distribution of atmospheric water vapor around the world, monitoring those elements especially related to climate change.

Scientists have postulated that with the increase of carbon dioxide in the atmosphere, water and ice have been evaporating more rapidly, adding to the heat level or greenhouse effect. Now, with years of humidity callibrations on hand, they're realizing that may not be true - that vapor levels may not have been measured accurately in the beginning, and may actually be much lower than suspected.

Diagram of Frost Point Hygrometer

The cryogen and heater work together to keep the thermistor at a steady level of frigidity. When outside air flows in and condenses on the thermistor, lit by the light source, the detector notices and sends a reading to the electronics board.
The cryogen and heater work together to keep the thermistor at a steady level of frigidity. When outside air flows in and condenses on the thermistor, lit by the light source, the detector notices and sends a reading to the electronics board. | Source

HygroPort Portable Hygrometer - This is a relatively new high quality, sensitive scientific calibrator that measures humidity from -148F to +65F with a variance of only 2º. It has several measurement units to select from. This hygrometer uses microprocessor technology and is resistant to contamination. It is used for monitoring humidity in compressed air, environmental test chambers, bottled gases, and more.

Readings graphed out from a laboratory thermohydrometer.
Readings graphed out from a laboratory thermohydrometer. | Source

Standard Laboratory Hygrometers - Heavy duty hygrometers and humidistats are made for laboratory walls, incubators, refrigerators, and freezers. They measure the full range of humidity from 0%-100% with an accuracy reading of plus or minus 6%. They come in both wall mount and manual models, and many are water resistant. The wall mounts are usually made of stainless steel. The hand-helds are heavy duty plastic, and are used to measure relative humidity in different spots of a large lab or outside doing field research.

Controlling Device

Whereas weather hygrometers are used to gather information for public dissemination, and research hygrometers are used to gather data to better understand how a thing works, controller devices are used to keep a specific level of humidity steady in a designated space. The following types of hygrometers are all used as forms of control.

Carob tree growing in a giant, humidity controlled greenhouse. Taken at the Arboretum in Albuquerque NM.
Carob tree growing in a giant, humidity controlled greenhouse. Taken at the Arboretum in Albuquerque NM. | Source
Humidity-controlled ICU unit from the 1980's.
Humidity-controlled ICU unit from the 1980's. | Source
Wall humidistat unobtrusively doing its work.
Wall humidistat unobtrusively doing its work. | Source

Humidistats - Some hygrometers are built into a device with a humidistat that is part of a humidifier, dehumidifier, or wall unit. The hygrometer measures humidity and sends a signal to the humidistat, which triggers the machine to increase or decrease humidity within the enclosure where it is located. This is used with refrigerators, incubators, greenhouses, microwave ovens (e.g. the "smart cooking" selection, like for microwave popcorn), climate controlled warehouses, room humidifiers, museums, precision factories, and saunas.

In large greenhouses there are sensors scattered all around the enclosure, each reading the humidity in that area and sending wireless signals that trigger misting or the greenhouse's drying systems (vents or fans), depending on the need for more or less moisture.

In egg incubators for chickens and ducks, ICU units for premature babies or sick animals or people, warm/hot humidity is required. All need to be carefully regulated to keep up the health of their inhabitants. High quality humidistats are used for this purpose.

Wall Hygrometers - These are often, but not always, built into a wall unit with a thermometer (thermohygrometer), where you get both temperature and humidity readings on the same dial. You can also get wall clocks with humidity and temperature dials included in them. These can be used for a home, office, food storage area, or small greenhouse.

You can also get wireless units where the hygrometer is wall-mounted and sends its readings to a manual unit that you carry around with you (or keep by your desk in the home office). And you can get a nautical wall unit for outside the building with temperature and humidity, plus a barometer measuring air pressure. This is good for locations prone to tornadoes or hurricanes.

Humidity Controllers - Of great assistance to people who cannot monitor rooms all the time are plug-in humidity controllers. These hygrometers plug into an electrical socket in your wall and provide a socket that you can, in turn, plug your humidifier or dehumidifier into. They measure from 30-90% relative humidity within a 5% variance and turn the machine on and off as needed. There is a battery inside that takes over when electricity goes out.

Manual Hygrometers

These are used for non-precise field work or small applications. They are hand carried and read. They come in a clock face dial, a narrow thermometer-type stick, or a digital reading. These hygrometers, generally of lower cost as well, are less reliable than the hygrometers above and therefore need to be calibrated regularly (see calibration video).

Here are some of their uses:

  • Instrument Cases - Wooden musical instruments, like violins or guitars, will be ruined if humidity goes below 40% in their storage cases. To measure humidity, place the manual hygrometer on a soft cloth on top of the instrument in its case and close the lid. After 4-6 hours, open the lid and quickly read the hygrometer.

  • Humidors - Cigars need to be kept at a steady humidity of 70% in their cases (humidors) in order to stay fresh and strong. Place the hygrometer in one of the trays in the humidor, close the lid for 4-6 hours, open and read.

  • Terrariums - Similar to storage cases, a digital hygrometer is used to measure humidity inside and around the plant or reptile terrarium. Some come with a suction cup to affix the humidifier directly to the glass.

Hygrometers are indicators. They are readers of a current condition that allow a person to decide whether or not to change that condition. If humidity in a terrarium reads low, a water tray can be added to the inside or water sprayed into the tank to keep plants thriving. If humidity in a semi-conductor factory is high, the air conditioning or another dehumidifying machine can be turned on to prevent the conducters from corroding. When a humidifier indicates an upcoming storm, grain can be stored in the barn or buckets set out under leaky roofs indoors. When weather forecasters read and then announce high humidity and high winds, trips can be cancelled. These little gadgets have all kinds of uses, have enabled the savings of billions of dollars, and continue to save lives as well.

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Comments 2 comments

Ruchira profile image

Ruchira 4 years ago from United States

Interesting read, water geek.


CWanamaker profile image

CWanamaker 4 years ago from Arizona

I'm glad that you discussed calibration. Many people seem to forget this. Though I haven't spent much time with hygrometers, I find that other devices like gauge tipping buckets and pressure transducers need frequent calibration. Another consideration should be the accuracy of the calibration equipment as well. Thanks for the detailed and thorough hub - I learned a few new things. Oh, and thanks for the link.

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