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While humidifying a room in my home, measuring the Temperature and Relative Humidity (T&RH respectively) every 8 hours, I noticed that RH rises and falls with T when it should be the inverse.

  • I don't understand how this happens.
  • Shouldn't the RH% drop as T rises and inversely so?
  • Shouldn't the avg. RH be much higher than 37% given the context below?
  • I've tested the accuracy of my hygrometer using the moist salt test (it accurately reads 75% RH in a sealed bag over an 8hr period).

For context:

I built four humidifiers each using a 200mm/800RPM cooling fan, a wick, and a bucket. They can each evaporate 64fl.oz. of H20 in 12 hours, roughly 2 gallons/day.

The room is kept at a constant 20°C and I get a pretty consistent 36-37% RH, except for when the heat kicks on and the RH goes up ~4%.

Follow-Up, which is why I actually came here:

I'm trying to estimate how much water is needed to fully saturate the air in my room, and based upon the maths I used, I don't trust the results I have.

Given the following:

  • Temperature = 20°C (68°F)
  • Room Volume = 61.1644m³ (2160 ft³)
  • Grams H2O per 1kg/m³ = 17.3g (10-3 kg/m3)
  • Grams per Cup (8 fl.oz.) = 240g

I conclude that:

  • Total Grams of water the room can hold at 20°C = 1,058g (61.1644m³×17.3g)
  • Cups Needed to Saturate = 4.41 (1,058÷240)

Is this correct? In other words, not compensating for ventilation rates or air changes per hour (ACH), would 4.41 cups of water saturate the air in the room if it were instantly vaporized and mixed evenly throughout the air? That seems like such a small amount of water.


Air - Maximum Moisture Carrying Capacity https://www.engineeringtoolbox.com/amp/maximum-moisture-content-air-d_1403.html

  • $\begingroup$ I assume the rise in RH when the heats turns on is due to the air+water being circulated and mixed, resulting in a slightly more accurate reading. $\endgroup$ Commented Dec 21, 2021 at 3:11
  • $\begingroup$ I do expect a higher humidity level given how much water is being evaporated per day. It's entirely possible that I am grossly underestimating how much/quickly the air changes over in this room. It is winter now, and it's very dry outside, so that could be the entirety of the issue. I also have a staircase connected to the room in question, which means convection could be a much stronger component at play than I imagined. $\endgroup$ Commented Dec 21, 2021 at 3:14

1 Answer 1


Yes, according to this website your 61x17.3= 1058g is correct. If the air is already 37% saturated you could do 63% of that number, so even less.

The reason that the RH goes up with temperature is that moisture is evaporated from the surroundings, furniture etc... and can be held by the air if it's at higher temperature.


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