TidePool Geek said:
Of course, acrylic is far more scratch prone. One idea I had for a planned cold water tank was to build it with a glass viewing window spec'd according to tank dimensions and with another sheet of thinner glass on the outside enclosing a dead space that would be filled with helium. Filling the void with gas should eliminate the moisture from which condensation arises. I'm planning on using helium because it's easily available compared to other inert gasses like argon.
Why is it useful for the gas to be chemically inert? Wouldn't dry air be just as good? My gut reaction is that it'd be easier to put air in with a little dessicant, like those (silica?) crystals they often put in packed products that shouldn't be too exposed to moisture. Also, if it's just air, it doesn't matter if there are small leaks, beause if a bit of water vapor leaks in, the dessicant will take care of it-- you can just make it "mostly" sealed as opposed to hermetically, so probably just some caulk is sufficient...
If making it chemically inert is useful, plain nitrogen is worth considering, too-- it's not a noble gas, but it's pretty darned unreactive, and it's not nearly as prone to leaking out... I guess either He or N2 is dirt cheap, though, so there's not much of a tradeoff. I'm used to thinking of He as prone to leakage since it's such a small molecule (atom, really), but I guess it's only at atmospheric pressure, so there's not much gradient across the seals.