The outer case of my chamber is acrylic not styrofoam.....the only foam in contact with the atmosphere is the edge of the gasket on the lid seal and that is a dense rubberized material.
I've measured gas loss, the air in my system is always at a very slight positive pressure, enough to inflate but not stretch the bag, under those conditions it looses in the vicinity of 1 liter of gas a day.
I understand with Helium that rate will be accelerated, but if I use bottled nitrogen as I plan the rate will be identical to air and quite within acceptable limits.....a 10 liter bottle of nitrogen at 300 bar will, I think, contain 3000 liters of Nitrogen at normal atmospheric pressure.......or over 8 years worth!!!.......actually as the gas mix will be in the region of 80% isobutane and 20% nitrogen the nitrogen will last 5X that or 40 years!!!
I'm more concerned about the loss of isobutane, not because of the cost of replacement, a 960 gram canister would last about 1.5 years and costs under £5.....but because of the safety aspect.
I'd be losing about 800cc per day.....I think that is very minimal and would easily be dispersed by natural ventilation.....but I'm also aware it is denser than air and will tend to pool at low points.....I'll probably put a battery operated gas detector under the floorboards in the vicinity of the unit to monitor that.
I'm not too concerned about the flammable nature of isobutane inside the chamber......it's only flammable in air at concentrations of 2 - 10% so at 80% it would not be flammable even if I just used it and air in the chamber.
But I'm concerned that as it is denser than air it would settle to the bottom of the chamber and the air to the top and at some point as you progress from the bottom to the top you would have a point where concentrations were flammable.
Also if I used air as a background gas then as the isobutane was liquifing as the chamber temp dropped again there may be a point where gaseous isobutane entered the flammable concentration region.
So it is essential that I use an inert background gas which bottled nitrogen (oxygen free) will be.
PS an external rad and direct water cooling of the components would be a good idea in cooler climates for winter use and even in the UK may give temps close to what I'm getting atm with my air cooling.....but you would still have to remove heat from the chamber produced by other non water cooled components and so would need two radiators one inside the chamber and one outside......but if all my plans come to naught and I end up using an internal water loop I could well run a loop to an additional outside rad to allow for that, could just use valves to isolate the external rad when I want to the air con unit.
But ultimately I'm hoping I will get mega low temps using my liquid gas idea...even better than using a water loop.....ideally I'd like to use a gas other than isobutane....a non flammable one, and if it had a lower boiling point, closer to the to the operating temp of the chamber.....perhaps -15c to -20c that would be better again....but I can't find one.....so if anyone has any thoughts on that I'd be grateful.
Edited by technogiant - 10/28/12 at 2:10am