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Vac Trappings--- boiling at 68/72


#1

John:

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?


#2

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?

Cold water pipes freeze less easily than hot water pipes too…
Boiling has to do with vapor pressure.

Rick Hamilton
Richard D. Hamilton

Fabricated 14k, 18k, and platinum Jewelry
wax carving, modelmaking, jewelry photography

http://www.rick-hamilton.com
@rick_hamilton


#3

G’day, Roy;

 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?

I’m now guessing, but suspect that the air has more closely
reached saturation point at the higher temperature, and thus is
’pulled out’ more easily. But like I say, it’s a guess.
Cheers,

        /\
       / /    John Burgess, 
      / /
     / //\    @John_Burgess2
    / / \ \
   / (___) \
  (_________)

#4

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.

Mike McKim


#5
     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.

G’day; My own belief (wot, theism here?) is that a trap inline
with the pump would be better. The ice/salt mix would be good,
so long as the trap is so arranged that the speed of the
evacuating gas/es isn’t too rapid for the trap to work properly.
It wouldn’t hurt to back it up with a tell-tale silica gel trap
too. and this would help to keep your valuable pump from getting
rust inside the works. But I wrote about this earlier.
Did’n’I? Cheers

        /\
       / /    John Burgess, 
      / /
     / //\    @John_Burgess2
    / / \ \
   / (___) \
  (_________)