I’ll be real interested to see how low you can drive the humidity with your desiccant wheel dehumidifier.
However, I can think of one other way that for not much more than the cost of the i10, and some “relatively straight forward” hacking, would probably guarantee you the -40c dewpoint air that you said you wanted. Consider:
or for a little more, the next size up:
That gets you to, allegedly, -40F, which just happens to also be -40C, air temperature.
So, first consider it with its factory configuration straight out of the box. You plug it in, set the temp to -40F, and close the lid with whatever ambient air inside. On the way to 32F water condenses on the walls and runs down the side. Then it starts to freeze to the sides. When it finally hits -40C, I thought that would mean the dewpoint was -40C, but it turns out to be a little better than that. There’s a distinction that I don’t completely understand, but probably @mzip understands it, where the “frost point” is indeed -40C, which turns out to be equivalent to a dewpoint of -43.6C (so, even better than your stated target).
By the way, apologies in advance if you know all this stuff already, or it’s too glarinly obvious, or whatever. I don’t know our background, so I’m just going to assume your starting point is similar to my own. I’m not pretending to be a know-it-all. Far from it. This distinction is something I learned only just this evening, and so I’m sharing it in such a way that hopefully anyone reading this can follow along.
So, then, what is the RH% inside the freezer at that point? Answer: 100%, which is the opposite of what we want. But suppose we pump out that air at a slow rate while backfilling with ambient air by drilling a couple of holes into the side of the container and filling with with bulkhead connectors and some thin tubing going in and out. As the air moves along the tubing, its temperature rises Maybe you run it through a length of metal tubing or a heat exchanger to encourge that. When it gets to ambient temperature, or close enough, you dump it into a drybox as a purge gas. What is the RH of the purge gas at 20C? 0.6-0.7%RH. Mission accomplished. Assuming an empty drybox, the air inside the drybox should asymptote to that RH% as it dilutes and purges out the air inside it.
So, what’s the hacking I alluded to? Well, there’s no auto-defrost on these cheap freezers. Instead, you have to “manually” defrost them by,. at minimum, cutting power and waiting for it to heat up. Hopefully there’s a drain somewhere at the bottom already, or you punch your own drain, pump it out, or whatever. Straightforward. Not rocket science. Probably easily automated.
I did find some on amazon with auto-defrost and even energy star rated, for a no-hack alternative, but it comes at a price (I found one for $$712 and huge capacity, but I didn’t look hard and so maybe/probably there are better deals than that):
