Desiccant really works

I have lost track of the status of cobalt chloride. When I was a kid it was in every toy chemistry set and the Man from UNCLE invisible ink pen.

Sometime around 2000 the blue form got declared a potential carcinogen and there was a bit of a rumpus. A lot of silica gel was thrown away then because it contained a trace of cobalt chloride indicator.

Any suggestions for a safe desiccant that will take RH down to around

uncooked rice?

If you want a constant nonzero RH you need a reservoir of salt solution. See e.g.

Cheers

Phil Hobbs

Phil Hobbs snipped-for-privacy@electrooptical.net wrote in news: snipped-for-privacy@electrooptical.net:

The dual glaze replacement window 'industry' fills the Aluminum spacers between the panes with a granular desiccant. They are less than 1mm each grain. They (the windows) are meant to remain closed and sealed throuout their 'life'.

Good stuff. Are you expecting some slow residual moisture ingress which the desiccant mops up, or is it just to absorb what's in the air at build time?

Either way, as you have on-board diagnostics there should be plenty of warning if the desiccant gets saturated.

Our gear mostly gets used in a much more benign environment, but we have one product where the electronics get regularly cycled between room temperature and -20C. That was a steep learning curve.

Saturated KOH is about right but far too nasty for amateur growers. Lithium chloride might be acceptable. I'm not sure quite how the main seed banks do it but they hold something like -20C and 10% RH. Conditions determined empirically to maximise longevity of most seeds.

I quite like the suggestion of rice made by Lasse. It might just work well enough and is plenty safe enough for amateurs.

The issue is that if you have an O-ring sealed enclosure out in the weather, the pressure differential between the inside and outside can easily be 3 psi. Barometric pressure varies about +-5%, and the temperature can range from -40 to +60C. That's 233 to 333K, a range of

That's enough to work the seals pretty hard, so that they leak a bit and gradually wear out. If the part that leaks is in contact with liquid water, e.g. because of a thunderstorm, the amount of H2O that gets in is thousands of times larger than if it's just air.

Thus the cool kids all equalize the pressure using small filtered vents positioned so that there won't be liquid nearby. You can calculate the expected amount of gas exchange over the lifetime of the unit, and that lets you size the desiccant pouch. Linde 4A can hold its own mass in water before saturating, which is pretty remarkable.

Yup. Packaging and cabling are always a hassle, but this was a new level.

Cheers

Phil Hobbs

Calcium chloride is useful too. It is used in air conditioners, i.e. real conditioners that control the humidity. It is pH-neutral,non-toxic and not more hazard to the environment that table salt.

In quantity CaCl2 may be cheaper. It also can be regenerated to specs if you own a Beam'e meter, either by heating or adding water.

Groetjes Albert

The rice should work fine, pretty much any partially saturated desiccant should do, this is what is done in museum display cases where humidity control is required, humidity at any % can be stabilized this way. A search on "museum case humidity control" or similar will turn up the details.

That sounds like a reasonable idea, but it takes up space that most applications would not have. What I have seen is a small membrane that is a filter, only allowing smaller air molecules to pass while keeping moisture out. That said, I have no idea where to find one other than in an air tight case. Check Pelican boxes to see if they use them.

Air Vent Valve Waterproof nut Exhaust valve

I think water molecules are actually smaller than nitrogen or oxygen molecules, something like 280pm for the former, vs. 380pm for the latter two.

Jeroen Belleman

The density difference can be +-25%, not even counting altitude. It would need a large and very floppy bellows to help much. Plus desiccant is cheap, and with proper design will last 20 years or more.

Cheers

Phil Hobbs

At that size it's not always about molecular size. Quantum mechanics is insane. Water is highly polar and polar attraction might be how the filter works. Dunno, just know that it does. My comment about size was not intended to explain how it works, just that it won't admit larger molecules which also commonly need to be filtered out.

While working on a device pumping air at a relative pressure and a constant flow rate, I found air pressure on the surface of the earth varies HUGELY. I don't recall the exact numbers, but it is around a 2:1 range. So a bellows would need to be fairly large relative to the size of the space involved if the product needs to be shipped to El Alto the pressure drops to 60% of sea level. Then there are high pressure weather conditions that raise the upper end by up to 7%. Goods shipped by air lower the bottom number further. So a bellows needs ample range of operation or at least not suffer damage.