Four Categories of Humidity Meters (Hygrometers) and Their Uses
Hygrometers are one of three main kinds of water-related meters. The other two are water meters (measuring water as a liquid) and moisture meters (measuring the amount of water contained in objects). Hygrometers measure water vapor in the air, i.e. humidity.
Hygrometers can both measure and control humidity. When ambient moisture is critical to physical health or to producing certain goods in a factory, automated metering keeps track of its levels - by measuring the existing humidity, then automatically adding or subtracting water vapor when needed.
Different types of hygrometers use different technologies to measure humidity:
- The tensile property of hair (which stretches when wet)
- The dew point of water (condensation on a chilled surface)
- The difference in rate of evaporation (from a dry vs. wet surface)
These technologies, buried within the hygrometer, work better for different types of uses than others. First we'll cover the different classes of humidity meters, then we'll give you basic principles by which they are chosen.
Where Hygrometers are Used
Hygrometers are used in any situation where the air needs to be especially moist or especially dry - where the level of moisture in the air will threaten human activity or safety, either with too much or too little humidity. For example, one of my nephews was born prematurely and had to be kept in an Intensive Care Unit (ICU) for several days, where the humidity could be kept constant and he could be closely monitored.
In addition, a technician taking measurements in several locations will want a portable device, whereas fixed locations (like manufacturing facilities) use wall-mounted or plug-in devices. The lists below show different types of humidity meters and their uses.
Weather Hygrometers & Uses
Weather hygrometers are used to predict weather-related events that could prove dangerous or inconvenient for humans. The purpose is to take preventive action ahead of time, so any disaster can be minimized:
- Farmers anticipating a rainstorm can protect their cut grain and animals.
- Drivers anticipating fog can make sure their vehicle's low beams work and the tires have treads.
- Villages anticipating floods can move temporarily or build up their dams.
- People with curly hair can get a haircut.
You know it's humid when the first thing you do every day is throw your pillow in the dryer.
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, e.g. predicting icy conditions along an aircraft's flight path or alerting fire 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 & Uses
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, which is 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 (NOAA) 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
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.
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 Devices & Uses
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, controlling 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.
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.
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.
High quality humidistats carefully regulate the air where extra-sensitive warm/hot humidity is required, such as egg incubators for chickens and ducks and ICU units for premature babies, sick animals or people.
Wall Hygrometers - These are often, but not always, built into a wall unit with a thermometer (thermohygrometer), showing both temperature and humidity readings on the same dial. Wall clocks with humidity and temperature dials are also available. These can be used for a home, office, food storage area, or small greenhouse.
Hardware stores and online stores like Amazon.com also sell 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). For outside a building, there are nautical wall units that include temperature and humidity, plus a barometer that measures air pressure. This is especially good for locations prone to tornadoes or hurricanes.
Manual Hygrometers & Uses
Manual hygrometers 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 below).
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.
All 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.
Reminders for Choosing a Hygrometer
More complex meters measure more accurately, so the one you choose for a particular situation will depend on how sensitive the reading needs to be. In life and death situations, like an ICU unit in a hospital, the hygrometer used will be more complex than will an office wall unit where the humidity is not critical to life.
Complex hygrometers can be expensive to install, but simpler ones need greater maintenance, so a compromise is needed where money is an issue. In addition, certain atmospheric conditions can distort a measurement's accuracy. In all of the uses above, the following characteristics of hygrometers need to be kept in mind:
Humidity is dependent upon temperature, which is why many hygrometers come accompanied by a thermometer. Whereas humidity increases at high temperatures, water vapor condenses more quickly at lower temperatures, which especially affects the dew point type of meter. Most hygrometers measure "relative humidity" or RH.
Humidity readings can be contaminated by existing dust. This is a special consideration for the coating industry, where coatings are sprayed onto surfaces that need to stay smooth (like car bodies).
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.
- Most handheld hygrometers will need regular calibration and cleaning in order to provide accurate measurements. See the following video for a simple calibration process.