Relative Humidity (RH)

Definition - What does Relative Humidity (RH) mean?

Relative humidity (RH) is the amount of moisture in the air compared to what the air can hold at that temperature. When the air can't hold all the moisture, then it condenses as dew. Relative humidity indicates how much the air is moist.

Corrosion can be accelerated by high relative humidity. Corrosion is slowed down significantly when relative humidity is below 50%. Relative humidity is useful only when measured at the surface.

Relative humidity is useful in climatic control for:

  • Comfort, health and safety
  • Technical requirements of machines and processes
  • Buildings, vehicles and other enclosed spaces

Corrosionpedia explains Relative Humidity (RH)

Relative humidity may be defined as the ratio of the water vapor density (mass per unit volume) to the saturation water vapor density at a given temperature and usually expressed in percentage. The relative humidity of air depends on temperature and the pressure of the system of interest.

It describes exactly how much moisture is present in the air. The more moisture there is, the faster iron will rust. For example, it is difficult for any rust or corrosion to arise in clean air with a relative humidity of 45% or less. If the RH increases to 60%, corrosion increases as well. As RH increases, the rate of rust formation and corrosion increases exponentially.

A device employed to measure humidity is a hygrometer, while one used to regulate it is called a humidistat or sometimes a hygrostat. These are comparable to a thermometer and thermostat for temperature, respectively.

Marine environments normally have higher percent relative humidity (%RH), as well as salt-rich aerosols. Studies have shown that the thickness of the adsorbed layer of water on a zinc surface increases with %RH and that corrosion rates enhance with the thickness of the adsorbed layer.

Some production and technical processes and treatments in factories, laboratories, hospitals, and other facilities needs specific relative humidity levels to be maintained using:

  • Humidifiers
  • Dehumidifiers
  • Associated control systems
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