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I noticed that food gets dry in the refrigerator, so I would like to use this effect to dry some objects otherwise difficult to dry (more specifically: nylon filament for 3D printers, which would requires many hours of heating in an oven).

How can I calculate the humidity inside the refrigerator?

My refrigerator can be set down to -18°C (255 K) so I think the air very close to the cooling pipes reaches, during active operation of the compressor, maybe -20°C (253 K). My assumption is that, at that temperature, air is completely saturated and every excess gets converted into frost/ice.

Since I set the temperature to -12°C (261 K), and looking at the psychrometric chart at http://www.uigi.com/UIGI_SI.PDF, my idea is to move horizontally to the right starting from -20°C, 100% rel. humidity. It looks like air at the end is about 50% rel. humid air -12°C.

Is that reasonable? I would say that at 50% humidity the food should not get that much dry, since it's about the humidity of the external atmosphere (maybe it's 60% outside), so maybe my assumption about -20°C of the cooling element and air 100% saturated around it, or the way I calculated the rel. humidity, is wrong.

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  • $\begingroup$ Hello, and welcome to Physics SE. Please look around, and take the tour. You are on the right track - you can look up what the saturated water vapor content vs temperature is for air (using, e.g., steam tables). You will see that the maximum water content is indeed strongly affected by temperature. $\endgroup$ – Jon Custer Sep 26 '15 at 17:18
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I think that your reasoning is correct. You're right in using the temperature of the cooling pipes at -20˚C rather than the freezer temperature of -18˚C to determine how much moisture was squeezed out of the air at the lower temperature setting. Also, as far as I can tell your interpretation of the humidity chart is correct at the air humidity should be around 50% at -12˚C.

As for food in the freezer getting dry or getting freezer burn, although 50% humidity can be considered high for many purposes such as the relative humidity of your living room, food will still dry out at 50% humidity. You need to reach 100% humidity to stop all drying action.

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  • $\begingroup$ But you fail to include time requirements in your advice :-( $\endgroup$ – Carl Witthoft Sep 26 '15 at 20:14
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I doubt this will work, for two reasons:

  1. The moisture content of a solid does NOT always decrease when placed in non-saturated air. There's a balance between the moisture content of the solid and the moisture content of the gas. As an example, wood flooring in a house expands and contracts with changes in air humidity, even when the air never reaches saturation. I don't know if it's as simple as "if the air is 50% of saturation, the solid will tend towards 50% of saturation", but in 50% saturated air the solid will certainly never completely dry out.

  2. Water's migration through solids becomes slower and slower as the temperature goes down. You're talking well below freezing here, so it will be extremely slow. Think about an ice cube sitting on the freezer shelf; it will take weeks or even months to evaporate, and that's an ideal situation for evaporation of water (no non-water solid to get in the way).

It would be far easier, quicker, and more effective to dry your nylon filament in an oven. The consensus treatment seems to be 4 hours in a 150°C oven, which seems pretty hassle-free to me.

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  • $\begingroup$ You provided useful information, but not actually on the real question, so I only upvoted you. Thanks for mentioning the speed of water migration and of moisture equilibrium. I had thought about the latter, but the first one didn't come to my mind. About the oven: the spool holder unfortunately won't hold that temperature and I'm limited to 100°C, that are not really enough. I haven't written it in the question, but right now I'm using cat litter (silica, but not as effective) in a box with the spool. Keeps about 45% r.h. $\endgroup$ – FarO Sep 26 '15 at 21:39
  • $\begingroup$ Well, if you start with room temperature 50% RH air and heat it to 100°C; then the RH will be around 2%. Given more time (ten hours?) should still dry pretty well. $\endgroup$ – Daniel Griscom Sep 27 '15 at 1:21
  • $\begingroup$ And, you're right: I didn't answer what you asked, but what I thought you wanted to know. $\endgroup$ – Daniel Griscom Sep 27 '15 at 1:23
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Cinnamon sticks I attempted drying in my fridge.

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