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I'm building a curing chamber in an old fridge, and I've started by capturing some data on the fridge's performance:

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I'm really unclear on what's happening when the compressor changes state. When the compressor turns on, immediately the relative humidity drops by 20%, before beginning a slow climb. When the compressor turns off, the immediately the relative humidity rises by 20%, before beginning a slow fall.

The slow climb and slow fall make sense to me: I believe this is consistent with the fact that air at higher temperatures can accommodate a higher quantity of water per unit volume -- so assuming we have a fixed total amount of water in the air is constant, relative humidity will fall slowly as temperature rises, and rise slowly as temperature falls.

What I don't understand are the dramatic 20% jumps that occur right when the compressor changes state. Can someone help me understand what's going on? (The behavior is consistent with a substantial quantity of water being immediately sequestered from the air when the compressor turns on, and released back into the air when the compressor turns off, but I'm unclear on whether this is what's happening, or why.)

closed as off-topic by Daniel Griscom, ThreePhaseEel, Machavity, Tyson, Michael Karas Jan 14 at 7:37

  • This question does not appear to be about home improvement within the scope defined in the help center.
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  • Hello, and welcome to Home Improvement. Unfortunately, this has nothing to do with home improvement. – Daniel Griscom Jan 11 at 1:30
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    I'm voting to close this question as off-topic because it has nothing to do with home improvement. – Daniel Griscom Jan 11 at 1:30
  • why don't you google relation between temperature and humidity – jsotola Jan 11 at 2:04
  • I'm building a curing chamber in an old fridge, and these jumps in humidity are unacceptable. What can I do about that? – Mazura Jan 11 at 9:10
  • @jsotola this relationship was clear; it was the impact of the compressor I was having trouble understanding. Thanks! – Alice Jan 11 at 17:08
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When the compressor turns on, the temperature of the evaporator coil plummets, and moisture starts freezing to the cold metal (i.e. forms frost). This decreases humidity of the air. When the compressor turns off again, the coil quickly warms back and the frost will start evaporating to re-establish a more-or-less equilibrium value of humidity at a higher level.

  • Thanks -- this was super clear, and this was the part I missing! – Alice Jan 11 at 17:08
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The frost free feature of the fridge requires the cooling coils to defrost, while the moisture on the coils is changing from solid ice to water and draining into a tray under the compartment the humidity increases, there are several methods employed to do this but that is the reason for the high humidity. The water usually drains into a pan that sits on top of the compressor, the compressor is quite warm when running and the moisture from inside the fridge is evaporated and the next time you open the door the humid air enters and the cycle starts over. I hope this helps on the why but that's what is going on.

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