The two objectives in using a vacuum pump for plaster degassing are
1: To get it to boil so as to remove the air, a tap aspirator will
do this quite effectively.
2: To complete the neccessary mixings, vacuumings and pourings
inside (usually) 9 1/4 minutes.
A large tank can take quite a time to empty, this is why you need a
large pump. The crucial factor is the amount of spare space between
the flask(s) and the tank. If you can cut this down you will get to a
hard vacuum much faster. A big flask in a tank only just bigger than
it gets to boil quite fast.
The acid test I learnt was to boil a cup of water at 20 deg C in 45
seconds, if it will do that all is OK. I have a big pump and tank cos
it’s easier to do everything in one system.
You can make quite good vacuum tanks out of HEAVY duty drainage pipe
with a steel plate glued on the bottom. Cylinders are very strong.
Make sure you make the perspex lid thick enough, mine to go on a 12"
tank is 25mm thick, you can still see it dome in slightly. The maths
at 14lb /sq inch are frightening!
I had a customer who was worried about it being strong enough, he
wanted to test it to overpressure, I found it hard to explain that
you couldn’t go past a vacuum.
regards Tim Blades.