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Hi Hugh. Hmmm. I'm surprised that such a low temperature is needed to get you a 30%RH. With refrigeration dehumidification you would have to cool the air to 18.85 F and then reheat to 47 F and at such a low dew point temperature you could only do it with a chilled glycol system. However you would always have the problem of ice build-up on the heat exchanger for refrigeration no matter what you used to chill the air. For a working temperature of 68 F, your dew point temperature would be 35.45 F close enough to freezing that I would probably use chilled brine or glycol. Desiccant dehumidification would certainly be a better way to go. 

Depending on how low your humidity gets during the winter indoors (assuming that you normally would use a little heating), you should be able to accomplish 30% in a can reasonably easily (or close to it.) My colleague measured the 90% equilibration time for a roll of 35 mm film lying on its side (so one exposed face) at 12 days (15 days in a cardboard box.) at 20 C (68 F). So if your film initially had the same water content as it would if it was at equilibrium with say a 70% RH environment and you put it into a 30% RH room for 15 days, then it should have the equivalent water content as it would at equilibrium with a 34% RH environment. He also found that taping a metal can (in good condition) like the motion  picture film people do, could produce a pretty good (though not perfect) vapor barrier with a humidity half-life of about 2730 days. He used a polyester tape with a vacuum deposition of aluminum on it. So put the film in a place with only dry heat during the winter for about two weeks and then put them in metal cans in good condition (dents around the rim can ruin the seal, of course) and then tape them. If you put them in an environment at say 75% RH, then  they'll have a 52.5% RH equivalent water content in 2730 days.

Even better, we looked at hermetically sealed bags for storage of microfilm in southeast Asia for the Ford Foundation and bagging the film in a metal foil laminate bag would work very well. This would be something like MarvelSeal. The material has a layer of low-density polyethylene that allows it to be heat-sealed. Motion picture archivists may be familiar with the material as FICA bags. It takes a little bit of practice to make a good seal, but it has a humidity half-life of decades ( the data is very small so the relative error is pretty big, but it looks like about 50 to 90 years).  (So unless someone wants access to the film, then dealing with the next conditioning and rebagging will  be someone else's job.)

The downside is that it's more labor intensive than using desiccant bed dehumidification and the bags are relatively expensive (we purchase premade bags in quantities of a thousand or more). They're also single use unless you're careful about the placement of the seal and where you cut the bag to get the film. I have seen bags of this material made with zippers (like the zippered bags that you can get at the grocery store), but I don't know how good the zipper seal is versus a heat seal.

Often institutions with cellulose acetate microfilm are looking for cold and dry storage. Surprisingly, this can be easier to achieve than room temperature dry storage.

Good luck.

-Doug
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We did figure it out, we used "Desiccant Dehumidifiers" which can hold any humidity level; and, do not require the cooling/drying system to run to achieve these low humidities.

Microfilm is not as prevalent as it once was but we used this same technology for protecting computer media so the technology lives on.  I did call Dr. Edelstein back and he was only mildly interested that we had found a way to provide in reality the temperature and humidity that would be required to deliver Image Permanence.

Hugh Smith
FIRELOCK Fireproof Modular Vaults

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