Author Options:

How to increase relative humidity and temperature in a box ? Answered


I need to increase the relative humidity of a slightly ventilated box up to 60-90% and to set temperature to 20°c (the outside should always be colder than 20°c). The box is closed (only two holes for the fans) and its dimensions are 1000*600*300 mm³. I only need to know which "hardware" to use. They will be switched on and off by a relay which is controlled by a humidity/temp sensor.

Thanks for your answers :)

Best regards,



It is maybe easier than you think to make the inside of a box humid.

At the surface of liquid water, or even a solid surface wetted by water, like a wet towel, there is water vapor.

I mean, if you put a wet towel in a box, 100% relative humidity (RH) is the equilibrium condition, assuming the box is closed, and the amount of water in the towel to start with was larger than the amount of water in that volume of air at 100% RH.

By the way, the box you describe, with a volume 180 liters, or 0.18 cubic meters, of air can only hold about 3.2 grams of water, at 20 C. That is if I did the math right, and I think I did.

The way I calculated this was to start with a table for the vapor pressure of water as a function of temperature.


I assume the pressure of the air was 100 KPa. Then I assume the ratio of partial pressure of water to to total pressure was the same as mole fraction (moles of water to total moles of air), which is about 0.02388.

Also I assume the air is an ideal gas.

Solving P*V=n*R*T, at 20 C, gives n/V = 41.6 mol/ m^3.

Multiplying this by the mole fraction of water to air, then by the molecular mass of water, gives:

(41.6 mol/ m^3)*(0.02388)*(18g/mol) = 17.9 g /m^3 [at 20 C]

Then multiplying by that number, by the volume of your box, gives:

(17.9 g /m^3)*(0.18 m^3) = 3.2 g

My point is it will not be difficult to come up with a reservoir of liquid water, that far exceeds the amount of water in the volume of air in the box, at 100% RH, at 20 C.

Whether the water is in a water is in a test tube, or a wet towel, or a potted plant, the equilibrium condition is the same. Water diffuses into the air until all the air in the box is just as humid as the air at the very surface of the water reservoir.

The main difference between a wet towel and a test tube that is open on the top, is surface area, and thus the rate at which water can diffuse from the reservoir to the air.

Actually, that might be a fun idea for a science project. With a humidity sensor and a clock, watch and wait, and compare wet towel versus open test tube, for to see which can dump water into the air faster.

Add a small fan, to blow air over the surface of the towel, and you've got the recipe for, at least one style of, humidifier appliance.


As Iceng suggests, you can also use heat to boil water, and reading the Wiki article I've linked to for, "Humidifier", reveals that some kinds of humidifier actually work this way. I guess you're going to be adding heat anyway, if the outside of the box is, like you suggest, always less than 20 C.

However I think high RH is going to be inevitable, as long as you have something wet in the box, with large surface area, like a wet towel.

By the way, regarding the situation of a box that is partly open, and thus losing water vapor to the ambient air, I do not think those results will be too different... except for the fact that water is constantly being lost, at some rate.

So you have to refill the reservoir, every now and then.

Maybe this is a problem similar to that of maintaining a potted plant.

I mean, for most indoor potted plants, they dry out and die, if there is not someone, or some thing (like a plant watering robot?), regularly watering them.

Thank you for your answer :)

What you wrote looks (a bit) like what my thermodynamics teacher told me 2 years ago, I should listen a bit more the teachers, even in courses that does not interest me :p

I'm afraid that mold or bacteria remain on the towel.. But according to what you said a water-filled bottom should be OK to increase RH inside :p So, that's what I'm going to do if you validate :)

I actually have no idea how fast evaporation happens, but I am just sort of guessing it is going to be proportional to surface area, so more surface area would be better.


However, you have a relative humidity sensor as part of your setup, so that should tell you how fast the air in the box is getting wetter.

If it seems to be too slow, and the towel idea is not attractive, there are other ways to kind of force water into the air.

For example, if you use an immersion heater,


to do the heating, there could be some local boiling and bubbling, on the surface of that heater, and I would think that would help.

Mechanically messing with the water, like a fountain, or a mister, or something, that might work too.

I forgot to mention: an aquarium bubbler might be a good, cheap way to coax water into the air.


The tiny bubbles have lots of surface area, and there is maybe enough time, as they rise to the surface, for them to get saturated with water vapor, and carry that water vapor into the air when they burst on the surface.

You know what, I will order all these devices : bubbler, mist maker, simply water and experiment them to know which one is the best :)

Yes. First, I wanted to use some ultrasonic mist makers but I do not know how great they are to humidify air.. I will make some experiments to quantify the speed of humidity increase evolution with area. But your intuition seems to be correct, it must increase with area.
Thermal devices are to avoid, I don't want to increase the temperature.


5 months ago

A resistor coverts 100% electricity to heat and can be insulated to heat a pan of water..

The voltage you plan to use and the thermal insulating in your box will determine the power and resistor value...

I would suggest a two stage resistor and relays to diminish large variation in box temperature..

Thank you for your answer :)

Yes, that's kind of what I was going to use but I was not sure how to do it by myself. I saw some heat plates (used for heating up terrarium) but it costs a bit too much.. I will learn about "two stage resistor" :)