Vacuum Degassing requirements

We don't sell them anymore because ... as you found ... they are readily available online and simply put we were doing it for our customers that couldn't source them when they were not readily available. But now, they are readily available online and we were not benefiting our customers by offering one that was more expensive as we could not source them cheaper than what people were finding them online. So we simply refer people now as I'm happy to do here.

Silicones will expand 3-4 times the liquid volume you are mixing before collapsing. Therefore if you never intend to mix more than 1/4 gallon at a time ... a one gallon bucket would be sufficient for mixing and degassing in. So a chamber that contains a 1 gallon bucket in it would suffice ... which may end up being a 2 or 3 gallon chamber.

Pump size ... you are exactly right ... elevation does mess with the total available amount of inches of mercury you are able to achieve but more importantly you want to be able to pull down to 29" of Hg as quickly as possible. At 4000 ft above sea level, that may be closer to 27" of Hg however the effect of your 27" to my 29" on the rubber is the same. Ideally you want to achieve near or full vacuum in your chamber in approximately one minute and the rubber rises and collapses within 3 minutes. After the rubber collapses, I let it continue to degas under full vacuum for another minute or 2 before removing it from the chamber and begin pouring.

I personally don't recommend anything less than 6 cfm for any chamber less than 2 gallons. A 5 cfm twin stage pump will work and we sold them for years. But for the extra $20-$30 it now cost to move up to a 6 cfm pump ... I think its well worth it.

Best Value Vacs has popped up recently and has some decent combination units. They look a little more expensive than some of the ones you found and listed but they are also slightly larger (chamber and pump). Might be worth comparing specs prior to pulling the trigger.

www.bestvaluevacs.com/best-value-vacs-3-gallon-stainless-steel-and-mastercool-6cfm-2-stage-pump-kit.html

Just for the record ... we are not affiliated with Best Value Vacs in anyway other than referring you to a company that supplies decent vacuum pumps and chambers at what seems to be a good value. We do not own any and we have not purchased any however many of our customers have and from my understanding have been very satisfied. If I were shopping for a pump and chamber ... I'd probably purchase the combination linked above for degassing Alumilite silicones and resins.

Also ... just to make sure I covered the topic ... yes a 6 cfm pump will pull a smaller chamber down to 29" of Hg faster than it will a larger chamber. How much faster ... depends on the size of the chambers. The smaller the chamber ... the less volume of air it contains and the faster it will pull down as you mentioned in your post. So purchasing a chamber that will fit your needs is important without going crazy big thinking bigger is better. In this case bigger may offer you options down the road if and when you move on to bigger projects but it will be slower every time you use it.

Mike