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How Does a Hygrometer Work?

How Does a Hygrometer Work

Like many weather instruments, hygrometers - devices used to measure humidity - have taken a wide variety of forms and advanced over the centuries.

In the 1400s, Leonardo da Vinci was among the first to construct an instrument that measures moisture in the air. His clever design centered around a scale with an absorbent material on one side of the scale, and a non-absorbent material on the other. As humidity increased, the absorbent side would become more saturated, making it heavier. The difference in weight was translated into a relative humidity reading.

A few hundred years later, Swiss physicist, geologist, and Alpine explorer Horace Bénédict de Saussure developed an improved hygrometer that measured humidity using a strand of hair suspended under tension. Moisture levels in the air would cause the hair to expand or contract, in turn causing a needle to move on a dial. Where the needle pointed to on the dial was interpreted as a humidity reading.

Early hygrometer designs also borrowed from the thermometer technology of the time. Specifically, the commonly-used "dry and wet-bulb psychrometer" employed two mercury thermometers. As the name implies, one was dry, and the other was wrapped at the base in a wetted fabric sleeve. Relative humidity was calculated based on the difference in temperature readings between the wet and the dry thermometer.


Types of Hygrometers

Today’s hygrometers have come a long way in terms of accuracy, durability, and ease of use. Following are overviews of the types commonly used now.

Analog Hygrometers – Nowadays, analog hygrometers and clocks with humidity generally rely on a composite coil comprising of a thin metal strip bonded to a moisture-absorbent hydroscopic material, typically a paper polymer. As this strip absorbs moisture, it expands while the metal strip remains consistent causing the coil to bend or rotate, in turn moving the needle on a dial indicating relative humidity. In dry conditions, conversely, the moisture will evaporate from the material, causing the coil to contract.

Digital Hygrometers – Most digital humidity monitors use either a resistive or capacitive sensor. While the electrical properties differ between these two types of sensors, the general principle in how they measure humidity is the same. In both designs, electricity flows through a small, moisture-absorbent material that is exposed to the air. As the saturation level of the material changes, so does its electrical properties. Therefore, the more humidity there is in the air and the more water vapor the material will absorb. The measurements and changes of electrical properties collate to specific humidity readings on the hygrometer’s digital display, the weather station’s base station, or in the case of internet-connected hygrometers and weather stations, readings sent to your phone, tablet, or web browser.


What is a Hygrometer Used For?

Hygrometers have two primary applications:

  • Weather Observation – Along with temperature, barometric pressure, wind speed, wind direction, and rainfall, relative humidity is a vital weather measurement. Humidity is a particularly important metric for weather forecasting, since it can help indicate the chance of rainfall, fog, and dew. In addition to that, humidity has a significant effect on how it “feels” outside. It’s no secret that high humidity on a hot day can make us feel hotter, largely because it slows down our ability to cool ourselves with sweat. This is why humidity is a factor when calculating the “heat index.” With regard to cold temperatures, there’s some debate as to humidity’s effect, so it is generally not used when calculating “wind chill.” That said, many agree that moisture can have an effect on the fibers of our clothing, creating a “wicking effect” that can accelerate heat loss.
  • Measuring Indoor Conditions – Relative humidity is an important measurement indoors, too. In addition to affecting comfort, humidity can directly influence mold, allergens, our sinuses, as well as the longevity of our valuable possessions. For this reason, some indoor hygrometers will tell you a recommended range to maintain in your home environment. This is usually between 25 and 55 %RH. Using this guideline, depending on the time of year, you can adjust your air conditioner or heating system, run a humidifier or dehumidifier, and take other steps to keep your home in range.

Why Is It Beneficial to Have a Humidity Meter?

Humidity monitoring benefits, depending on how you use your hygrometer, can include comfort, wellness, care of pets, and protection of your home and personal property.

  • Comfort – As mentioned earlier, knowing the humidity can greatly affect your ability to stay comfortable, whether indoors or out. Indoors, it’s important to keep your home, if possible, between 25 and 55 %RH, preferably between 40 and 50%. Outdoors, knowing the humidity can help you dress more appropriately, know if rain may be on its way, and ultimately better plan your activities.
  • Wellness – Humidity that’s too high can lead to mold and allergens. Humidity that’s too low can lead to sinus issues, itchy skin, and other dry air symptoms. Dry air can also be a contributor to allergens since dust will be more likely to circulate when your heater kicks in. As such, it’s important to keep your indoor environment in range.
  • Care of Pets – Certain types of exotic reptiles and birds, for example, are highly sensitive to humidity. Whereas your dog and cat can be comfortable when you’re comfortable, those conditions could make these animals sick, or worse. Using a humidity monitor to keep tabs on their cage can help you ensure their good health.
  • Protection of Home and Property – The mold caused by high humidity could lead to costly repairs of drywall, sub floors, and more. Dry air can damage wooden trim, furniture, not to mention musical instruments, artwork, etc. Using a hygrometer to help you maintain correct indoor humidity can go a long way toward keeping your valuables in top condition.

  • AcuRite offers a wide variety of humidity monitor products, addressing a wide range of needs.