Hygrometer facts for kids
A hygrometer is a cool tool that measures how much humidity is in the air. Humidity is just a fancy word for how much water vapor (water in gas form) is floating around.
Hygrometers work by checking other things like temperature, pressure, or how much something weighs. They can also look for changes in how a material acts when it soaks up moisture. By doing some math and setting them up correctly, these measurements tell us the humidity.
Newer electronic hygrometers can find the temperature where water vapor turns into liquid (this is called the dew point). Or, they can sense changes in how electricity flows through materials (capacitance or resistance) to figure out humidity levels.
Did you know that a simple hygrometer was first thought up by Leonardo da Vinci way back in 1480? Then, in the 1600s, Francesco Folli made a more useful one. Robert Hooke also made improvements to weather tools, including the hygrometer. A more modern version was created by Johann Heinrich Lambert in 1755. Later, in 1783, a Swiss scientist named Horace Bénédict de Saussure invented the first hygrometer that used human hair to measure humidity!
The amount of water vapor air can hold changes a lot with temperature. Cold air can't hold as much water as warm air. So, temperature can really affect humidity.
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How Hygrometers Work
Hygrometers have changed a lot over time. Let's look at some of the different ways they measure humidity.
Early Hygrometers
Some of the first hygrometers were made in Ancient China during the Shang dynasty. They used them to study the weather. The Chinese would take a piece of charcoal and a lump of earth. They weighed them when dry, then weighed them again after they had been in the air for a while. The difference in weight told them how humid it was.
If the air was dry, the charcoal would be light. If the air was humid, it would get heavier. They even made a simple device by hanging charcoal and earth on a stick to see which side dropped!
Metal-Paper Coil Hygrometers
Metal-paper coil hygrometers are often found in cheaper devices. They show humidity changes on a dial. They might not be super accurate, sometimes being off by 10% or more.
These hygrometers work because a paper strip, soaked in salt, is attached to a metal coil. When the paper absorbs water vapor, it changes shape. This change makes the metal coil move, which then turns a needle on a dial to show the humidity.
Hair Tension Hygrometers
These devices use a human or animal hair that is stretched tight. Hair is "hygroscopic," meaning it likes to soak up moisture. When it absorbs water, its length changes. This tiny change in length is then made bigger by a special mechanism and shown on a dial or scale.
In the late 1600s, some scientists called these tools "hygroscopes." That word isn't used much anymore, but "hygroscopic" is still used to describe things that absorb moisture. The traditional weather house (a small house with figures that come out depending on the weather) works using this idea! Sometimes, whale bone or other materials can be used instead of hair.
As mentioned, Horace Bénédict de Saussure built the first hair-tension hygrometer in 1783. He used a human hair about eight or ten inches [20 or 25 cm] long. One end of the hair was fixed, and the other went over a small wheel, pulled tight by a silk thread and a small weight. The wheel was connected to a pointer that moved across a scale. To make it even more sensitive, you could clean the oils from the hair by soaking it in a chemical called diethyl ether.
Psychrometers (Wet-and-Dry-Bulb Thermometers)
A psychrometer uses two special thermometers. One is kept dry, and the other has a wet cloth or "sock" around its bulb. This wet bulb is kept moist with distilled water.
When the air temperature is above freezing, water evaporates from the wet cloth. This evaporation cools the wet-bulb thermometer, so it shows a lower temperature than the dry-bulb thermometer. If the air temperature is below freezing, the wet bulb needs to be covered with a thin layer of ice to be accurate.
The relative humidity (RH) is figured out using the temperature from the dry thermometer and the difference between the wet and dry temperatures. You can also find the relative humidity on a special chart called a psychrometric chart. If the air is completely full of moisture, both thermometers will show the same temperature. The bigger the difference between the two temperatures, the drier the air is.
Psychrometers are often used in meteorology (the study of weather) and in the heating, ventilation, and air conditioning (HVAC) industry.
Sling Psychrometer
A sling psychrometer has two thermometers attached to a handle. You spin it around in the air until both temperatures stop changing. This is handy for quick measurements outside. However, newer electronic sensors are becoming more common because they are easier to use.
Chilled Mirror Dew Point Hygrometer
The dew point is the temperature where air, at a certain pressure, becomes completely saturated with water vapor. If it gets any colder, the water vapor will turn into liquid water (condense).
Chilled mirror dew point hygrometers are some of the most accurate tools for measuring humidity. They have a mirror that is cooled down. A special light and sensor system watches the mirror to see when tiny drops of water (condensation) form on its surface. The temperature of the mirror is carefully controlled to keep a balance between water forming and evaporating. This way, it measures the exact dew point temperature. These devices can be very precise, often within 0.2 °C. This means they are very accurate for relative humidity too, usually within about ±1.2%.
Older versions needed cleaning, but newer ones use very smooth surfaces that stay clean. Some even use light to tell exactly what kind of condensation is forming. Chilled mirror hygrometers are often used as a standard to check if other hygrometers are working correctly. This is because they measure a basic physical property: temperature at which condensation occurs.
Modern Hygrometers
Capacitive Hygrometers
When you need a smaller, cheaper, or less fragile humidity sensor, capacitive hygrometers are often used. They might not be as super accurate as chilled mirror ones. These sensors measure how humidity affects the electrical properties (dielectric constant) of a special polymer or metal oxide. When they are properly set up, they can be accurate to about ±2% RH for most humidity levels. If they aren't set up perfectly, they can be two or three times less accurate.
Capacitive sensors are tough and can handle things like water drops or short periods of high temperatures. However, they can be affected by dirt, changes over time, or just getting old. Still, they are good for many different uses.
Resistive Hygrometers
Resistive hygrometers measure how the electrical resistance of a material changes because of humidity. Materials like salts or special conductive polymers are often used. These sensors are not as sensitive as capacitive ones, meaning the material changes less, so they need more complex electronic parts.
Also, the properties of these materials can change with both humidity and temperature. This means they usually need to be paired with a temperature sensor. How accurate and tough they are depends on the material used. Some strong, condensation-resistant sensors can be accurate up to ±3% RH.
Thermal Hygrometers
Thermal hygrometers measure how the thermal conductivity (how well heat moves through something) of air changes with humidity. These sensors measure "absolute humidity" (the actual amount of water vapor) rather than "relative humidity" (how much water vapor there is compared to how much the air can hold).
Gravimetric Hygrometers
A gravimetric hygrometer works by taking all the water out of the air (or other gas) and then weighing that water separately. For example, it might weigh a special material called a desiccant before and after it has soaked up the water. The temperature, pressure, and volume of the dry air are also measured. With all this information, you can figure out how much water was in the air.
This is thought to be the most accurate way to measure absolute humidity. Many countries use this method as their main standard. However, these devices are not easy to use, so they are mostly used to check and calibrate other, less accurate humidity sensors.
Optical Hygrometers
An optical hygrometer measures how much light is absorbed by water in the air. It has a light source and a light detector with air in between them. The amount of light that reaches the detector tells you the humidity. This is based on a scientific rule called the Beer–Lambert law.
Some types use special light from hydrogen (Lyman-alpha hygrometer) or krypton (krypton hygrometer). Another type, the differential absorption hygrometer, uses two lasers. One laser's light is absorbed by humidity, and the other's is not. By comparing them, it can figure out the humidity.
What Are Hygrometers Used For?
Hygrometers are used in many places! Besides greenhouses and factories, you can find them in:
- Some incubators (for hatching eggs)
- Saunas
- Humidors (to keep cigars fresh)
- Museums (to protect old artifacts)
They are also super important for taking care of wooden musical instruments like pianos, guitars, violins, and harps. If the humidity isn't right, these instruments can get damaged.
Hygrometers even help firefighters! The lower the relative humidity, the more easily and strongly fires can burn.
At home, hygrometers help people control humidity. If the air is too dry, it can hurt your skin. If it's too humid, it can lead to mildew and dust mites.
They are also used in the paint and coating industry. Applying paint and other coatings can be very sensitive to humidity and the dew point.
Why Is Humidity Hard to Measure Accurately?
Measuring humidity accurately is one of the trickiest things in science. Even with good instruments, it's hard to get perfect results.
Imagine comparing two thermometers: you can put them both in a bucket of water and stir it well to make sure they are at the same temperature. A good thermometer can stay accurate for years. But hygrometers have to be calibrated in air, which doesn't transfer heat as well as water. Many types of hygrometers also tend to "drift" over time, meaning they need to be checked and reset regularly.
Another challenge is that most hygrometers measure relative humidity. Relative humidity depends on both temperature and the amount of water in the air. So, even small temperature changes in a testing room can make the relative humidity readings jump around.
In very cold and humid places, ice can form on the sensor of a hygrometer. When this happens, the sensor measures the humidity based on ice, not liquid water. Regular hygrometers can't measure correctly below the "frost point" (the temperature where frost forms). One way around this is to use a heated humidity sensor.
How Hygrometers Are Checked (Calibrated)
Saturated Salt Calibration
Scientists have found a cool way to check hygrometers using saturated salt solutions. If you mix certain pure salts with distilled water until no more salt can dissolve, these slushy mixtures will keep a nearly constant humidity inside a closed container.
For example, a saturated table salt (sodium chloride) mixture will eventually give a reading of about 75% humidity. Other salts give different humidity levels:
- Lithium chloride gives about 11%
- Magnesium chloride gives about 33%
- Potassium carbonate gives about 43%
- Potassium sulfate gives about 97%
Salt solutions can change a little with temperature and take time to settle. But they are easy to use, which makes them good for checking simpler mechanical and electronic hygrometers.
Images for kids
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Psychrometer probably made in Switzerland circa 1850 by Kappeller (MHS Geneva)
See also
In Spanish: Higrómetro para niños
- Automated airport weather station
- Dewcell
- Humidistat
- Moisture analysis
- Friar of the weather
- Soil moisture sensor
