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.
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.
Hygrometers have two primary applications:
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.
AcuRite offers a wide variety of humidity monitor products, addressing a wide range of needs.