According to dictionary.com, a hygrometer [hahy-grom-i-ter], is any instrument for measuring the water-vapor content of the atmosphere.

But in common terms a hygrometer is a device used to measure the humidity of the atmosphere; detecting the percentage of the moisture in the air.

Modern hygrometer uses changes in electrical capacitance, the temperature of condensation or resistance to measure humidity differences. Moisture-absorbing, material such as phosphorus pentoxide is widely used. The older versions of the hygrometer use materials that attract moisture, like human hair because it changes in length depending on how much moisture it absorbs.

Other materials can also be used to determine relative changes in the humidity since they can change color when they absorb moisture. The most common hygrometer is a psychrometer which consists of two traditional dry and wet thermometer bulbs. From these two thermometers, one is kept dry, and the other is covered with a wet cotton or linen wick around its bulb to help determine the humidity levels.

Leonardo da Vinci was the one who invented the first crude hygrometer in the year 1480, but in 1755, polymath Johann Heinrich Lambert created the modern version.

Types of Hygrometer

Traditional Hygrometers

  • Hair tension hygrometers – It is called Hair tension hygrometer because human or animal hair is used. The hair is hygroscopic, it stretches in length with humidity which may be magnified by a mechanism and indicated on a scale or dial.
  • Psychrometer – A psychrometer is the most common type of hygrometer. It consists of two identical thermometers, one dry and the other one is kept moist with distilled water on a wick. Adolph Richard Assmann invented one the most accurate types of wet-dry bulb psychrometer in the late 19th
  • Metal-paper coil type – In Metal-Paper Coil Hygrometer, water vapor is absorbed by a salt-impregnated paper strip which is attached to a metal coil thereby causing the coil to change shape. The change is what causes an indication on a dial.
  • Chilled mirror dew point hygrometer – This device uses a chilled mirror and optoelectronic mechanism to detect condensation on the mirror’s surface. 

Modern Hygrometers

  • Thermal Hygrometer– In thermal hygrometer, the change in thermal conductivity of air due to humidity is measured. This device is accurate in measuring absolute humidity rather than relative humidity.
  • Capacitive Hygrometers– Capacitive hygrometers measure the effect of humidity on the dielectric constant of a polymer or metal oxide material.
  • Gravimetric – A Gravimetric hygrometer measures the mass of an air sample compared to an equal volume of dry In countries like US, UK, EU and Japan, the National standards for this type of measurement have been developed.
  • Resistive Hygrometers – In resistive hygrometers, the change in electrical resistance of a material due to humidity is measured. In most cases, conductive polymers and salt are materials used.

Uses of Hygrometer

  • Hygrometer is used in greenhouses and industrial spaces
  • It is used in some humidors, museums, incubators and saunas.
  • It is used to take care of wooden musical instruments such as guitars, violins, harps and pianos to avoid being damaged by abnormal humidity conditions.
  • A hygrometer is also used in assisting humidity control.
  • It is used in coating industry to help in measuring the humidity and dew point due to the application of paint and other coating materials.
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