Raising humidity - how much water is needed?

[SentinelAeon]

Hello, my first post here so nice to meet you all :)

I have a question. Let's take a small room, about about 16 m^3 (172 feet^3). This room has a very low humidity, especially in the winter. It is around 35%, i would like to raise it to 55% with a custom made swamp cooler. I am trying to figure out how much water would be needed for that in order to figure out how big of a container with water i need to have, to only add water every few months. So i need 2 things:

1) How much water it takes to raise humidity in the room from 35% to 55% if we assume there is no fresh dry air coming in
2) A good way to calculate how much fresh air is actualy coming into the room if we assume that during the winter windows are mostly shut (cold air from outside will lower the humidity fast, so we want to avoid opening windows except maybe few minutes a day)

I know this is really difficult to answer but i am looking really for an aproximation. to know if i need 10L container, 20L, 30L and so on.

 

[Nugatory]

One important factor that you haven't mentioned is the temperature - that affects the quantity of water the air will hold at any given relative humidity. But with that information, you'll find tables online that let you calculate the amount of water you'll need to add.

How do you plan to control your swamp cooler so it won't push the humidity above your target 55%?
Is there any possibility of losing moisture to condensation on the windows or other surfaces that will be cooler than the room?

 

[SentinelAeon]

I plan to control the humidity with switching the cooler on/off automaticly with my simple esp8266 board. The temperature, its hard to say, i would say about 21C during winter and around 26C in the sumer. As for condensation i am not sure how much is lost.

 

[jrmichler]

Search for a psychrometric chart. The psychrometric chart shows the amount of water per cubic meter of air at any combination of temperature and humidity. Find the amounts of water at the two humidities, subtract, multiply by the volume of the room, and the result is how much water you need to add.

You will find that adding that much water will raise the humidity by the desired amount, after which the humidity will start to decrease. All rooms have air leakage. The amount of water that you need to continuously add in order to maintain the humidity at your desired level is proportional to the amount of air leakage and the humidity of the outside air.

Indoor humidity in cold weather should be low enough that you do not get condensation. Condensation causes rot and mold, both of which are bad.