How equilibrium relative humidity (ERH) works is obvious in one way but quit stable when trying to apply the concepts to real life situation. To begin with, you will have to imagine invisible water molecules moving around in the air.
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The important fact:
Most materials absorb water to a greater or lesser extent
The next step in the logic is that he water molecules you are imagining moving around in the air come into contact with materials. We will think about two things that can happen:

  1. The water sites on the surface of the material.
  2. The water at the surface penetrates into the material.

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 Assume that there is insufficient water at the surface to cause condensation and we are still thinking about invisible water molecules. The water on the surface can evaporate back into the air or it can move into the material. If it penetrates into the material, this is known as ‘absorption’. Water that has already penetrated into the material can stay there, or move back to the surface and evaporate into the air. Where the water is lost into the surrounding water, this known as ‘desorption’.
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(water molecules)

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(water vapour)

See this process of absorption as continually active (or dynamic) with water molecules entering and leaving the material. At some point, the amount of water entering and leaving the material will be same (balanced) and we have an equilibrium.
As you know, relative humidity is a way of measuring the amount of water vapour in the air. Imaging we have a container that is filled with air containing moisture. We can measure the level of moisture in this container and we call it the %RH. Imagine that we now place a very dry material in the container, for example, a desiccant such as silica gel. You know that the little silica gel sachets you get with some things that have “do not eat” on them. Assuming that you haven’t eaten them and you pace them in the container and tightly close the lid, what happens next?
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Naturally, as you would expect the silica gel starts to take the water out of the air by the process of absorption. Because the air now hold less water vapour, the %RH will fall and we have dehumidified the air, which of course is the function of the desiccant. However, this absorption will not continue to zero %RH as you expected but will stop at particular point. The %RH at this point occurs when we have absorption and desorption at equilibrium. The %RH value achieved is equilibrium relative humidity, which for silica gel usually around 10 to 25 %RH. This means the desiccated air in the container is been held at 10% to 25% RH.
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Definition of the (ERH): it is the relative humidity value measured for the air surrounding a material that is interacting with water vapour. By necessity, this has to be done in a closed container to prevent the air surrounding the material from mixing with the atmospheric air, which would add to or reduce the available moisture.
 
 

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