Calculate humidity raise/drop when opening a window

[SentinelAeon]

Hello,

I am making a very simple device. It will measure temperature and humidity outside the window and also measure temperature and humidity inside the room. And depending on the collected data, the device will calculate whether u will drop humidity in the room if you open the window or will you raise it.

I will give you an example. Let's say that sensor inside the room reads 20C and 50% and sensor outside reads 5C and 50%. Now while both inside and outside its the same humidity, when we open the window, the air that comes inside will warm up and this will drop its humidity - in effect we will be dropping humidity in the room. I need an equation that will calculate this.

Bonus: As a bonus ... how would i calculate the potential humidity change if we exchanged all the air in the room with air from outside and obviously warmed that air to the room temperature. I stress potential, because obviously we will not have the window opened that long. But it would be good to know the potential of outside air to change the inside air humidity. For instance, opening the window has a potential to drop the humidity of the room for 10%.

I need to finish this device in a matter of days, it will be a present for a family member and i would be very happy if you could help me.

 

[jrmichler]

Everything you want to know is shown in a psychrometric chart (search the term). The important numbers are the outside and inside dew point temperatures.

If you exchange all of the air in the room with cool outside air, then heat the cool air to the same temperature as before, the inside dew point will change from the previous inside dew point to the dew point of the outside air. You just read the new inside relative humidity from the psychrometric chart. Lots of simplifications and assumptions here, but this will give you an idea.

 

[Lnewqban]

Consider that those percentages refer to “relative humidity” or the capability of the surrounding air of evaporating water from a “wet bulb”.
That concept does not give you an exact idea of the actual amount of water that the surrounding air contains, which greatly depends on the temperature of the air.

Please, see:
https://en.m.wikipedia.org/wiki/Psychrometrics

As @jrmichler has advised above, a psychometric analysis is needed to take into consideration the many factors that affect air humidity.