Why when I freeze a container does the humidity go down?

I am working on a project to stockpile and freeze photographic film and paper for later use. This is not for academic or commercial purposes, but just for personal use. I've been experimenting and am stumped as t the following:

I place a package of film in a ziplock bag in an environment of about 67F and 35% RH. I included a humidity indicator strip as well as a humidity gauge in the bag as well. The problem is that, when I pace this in the freezer, the humidity drops to 10% as the temperature drops to 0F. THe next day, when I remove the closed bag from the freezer, the temp rises to room temp (67F) and the RH climbs right back up into the 30s within hours.

I am an electrical engineer, but to me this is backwards. If the moisture content stays relatively constant in the bag, it seems the like the RH should go UP when I freeze it instead of down. Can someone please tell me what I'm doing wrong. For this application, lower humidity is better than higher humidity, of course, but I still want to understand what's going on.

Any guidance would be appreciated. I apologize if this is the wrong place to be posting such a question, but I'm running out of options for finding an answer.

Charles Link

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Even though they may might not be visible, when the temperature goes below 32 degrees, you will be getting ice crystals in the solid state inside the bag and much less in the vapor phase.
Ah-ha! That makes sense. And I can test it by putting it in a fridge instead of a freezer. I should see the humidity go up, at least a little, if I don't let it get below 32F.

A very low humidity can cause film emulsion to get brittle, but this sounds more like the moisture isn't really gone, but rather just no longer in vapor form. I've read that freezing film, even at 95% RH will not damage it because there isn't enough moisture for the freezing to cause structural damage to the material (unlike with foods that contain a lot of water). Moderately low humidity will increase the life expectancy of the film, but freezing will extend it much much more.

Thanks for your help, Charles!
After leaving the bag out of the freezer over night, I placed it in a refrigerator. It was at 66.3F and 33% RH when I put it in and 34.2F and 26% RH a couple hours later. Now I'm wondering if the film in the bag is screwing up the results. Some months ago I had placed a desiccant pack in the bag with the film for a few weeks, but then took it out when I saw how low the humidity inside the bag got. I will redo the experiments with some other material that hasn't been tainted from previous experimentation.
It's the paper!

The film has paper packaging which is hydroscopic and it absorbs more water at lower temps. Since the amount of water contained in the paper is greater than the water held by the small amount of vapor in the bag, it influences the RH more than the temperature. This property is apparently used to buffer humidity fluctuations in museums. Putting paper inside a display case can stabilize humidity during fluctuations in temperature.

In the example I found, a temp decrease from 20C to 10 C changes the RH in an empty container from 50% to 95%. Introducing paper to the container results in an RH decrease from 50% to 47%.

I bet if I put the film inside it's own ziplock bag and put that inside the test bag, the RH would behave exactly as expected.

I doubt many people are going to go looking for an answer to this mystery on the internet, but I appreciate your letting me post the question and struggle through it.
Good point, Russ. The humidity strips go from 10% to 60%, but I'm using them in conjunction with a meter. In effect, I am testing the strips as well as the storage method. They aren't readable with much accuracy, but I can easily include one of the strips in each film bag whereas it would be impractical to put a meter in each bag. Once I'm done with the testing phase and actually start storing the film, I figured the strips would only serve to warn of some situation where the humidity got excessive. Once they indicate a high humidity, they can only be reset by heating them with something like a hair dryer.

Here are the meter range specs:
  • Outdoor temperature range: lithium batteries: -40°F to 140°F
  • Indoor/ Outdoor Humidity range 10% to 99%.
I'm using lithium batteries. I can't don't have the accuracy specs in front of me, but it is certainly good enough for my purposes. The meter is connected by radio to the master station, so I can read the temp and humidity without opening the freezer. I have three of them.


Good point, Russ. The humidity strips go from 10% to 60%, but I'm using them in conjunction with a meter.
Sorry, I just saw this when someone "liked" my post: I was actually referring more to the temperature range for the strips. When air is cold, moisture content is low and small changes in moisture content translate into large changes in RH. The real lower limit of the strips may be low absolute humidity (dewpoint) or the freezing point of water. Electronic meters have similar issues at low Temps. Often they come with curves showing how accuracy changes with temperature.
Russ, I hadn't thought of the temp response of the humidity strips. Also, I am intentionally trying to force as much air out of the bags as possible. Even the paper in the test strip may be enough to influence the humidity of that small volume of air. The meter may also have some hygroscopic materials in it.

I just ordered a bunch of metalized poly bags. They are a much better vapor barrier than LDPE freezer bags. As long as the integrity of the bag remains intact, I don't think it will matter if the humidity outside the bag gets gets too high. I've never seen a reading anywhere in the freeze above 65%.

I'm probably over-thinking the whole project, but I really expected more predictable results than this.

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