Thanks for your reply about the "boiling." Why, if the water
retains more dissolved air at lower temps, does the water "boil"
so much more readily at 72 than at 68?
Water will boil at 72 degrees easier than at 68 because it is
hotter and has a higher vapor pressure. There is a trade off,
sometimes it is a good idea to use cold water in making up
investment in order to increase the working time of the
investment, but at the same time this can increase the difficulty
of deairating the investment under vacuum because of the reduced
vapor pressure of the cold investment.
I haven't tried this but to create a little bit better vacuum
within the bell jar which investment is being deairated would be
to have a cold trap. This trap would rapidly condense and remove
water vapor that is in the bell jar that is keeping the pressure
up and delaying the boiling or deairating. Place inside the bell
jar along side the investment flask a beaker of crushed ice and
table salt mixture (approximately 1 part salt to 10 parts ice).
This arrangement is similar to the condenser in a freeze dryer
which assists the vacuum pump in removing water from an item. A
large part of the atmosphere within the bell jar just before the
investment boils is water in the form of a gas. This type of
condenser may be enough to cause boiling with a weak vacuum pump.