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Vac Trappings


#1

Hi Geo, Surfactant is soap! Yes, it’s worth a few bucks to change
the oil in your vac unit. I have two laboratory quality vacs. One
is a dinosaur and the other is a Busch ($2300.00). The old one
draws 27-28" and the Busch draws 29", both depending on changes
in atmospheric pressure. I used a Fast-Vac for years. I still
have it, but I gave it an early retirement. Most good quality
investments have a built in surfactant. Kerr says that you can
take the temp of your investment (leave a F thermometer in it)
subtract your reading from 150F (constant) and the diference
should be the temp of your water. This temp combo plus the active
three min. mix “wakes up” the surfactants present. J.A. Please
leave it at the bottom!


#2
 Most good quality investments have a built in surfactant. Kerr
says that you can take the temp of your investment (leave a F
thermometer in it) subtract your reading from 150F (constant)
and the diference should be the temp of your water. This temp
combo plus the active three min. mix "wakes up" the surfactants
present. J.A. Please leave it at the bottom!

hi j.a.,

are you saying to use water 150F? i’ve always used room temp
water for no other reason than that’s the way i’ve always done
it (and of course fear that the investment will set up before i
pour it). though i’m not sure i understand this part of your
post. i do understand you to say that the investment has to be a
certain temp for the surfactant to become active. i’ll just do
it and it probably will become clear. what temp reading should
it be for the surfactant to become active?

anyway i’ve got the machine back together and getting a solid 27
reading. i’ve installed the gast vac pump. haven’t used it yet
but will let you and everyone else who have been very helpful
the results. i’ll flush and fill the old vast-vac in the
meantime.

ok, your post got me really curious, and i just left the
computer and went into the shop. put the same temp water in the
vac chamber that i usually use. i was shocked/surpised that it
was only 55 degrees! after over 2 min i barely got a beginning
boil at 27". i then put some hotter water (100f) and it was
roiling in less than 30 seconds. boy, do i feel dumb, or should
i say i’m much smarter now thanks to you. would it suffice to
say that the water i’ve been using is too cold? no wonder why it
took sooo long to gloss off, duhhh. boiling as opposed to barely
simmering will most likely take care of the bubbles adhering to
my models, right?

generally speaking, what should the temp of the water be before
investing and still maintain a decent working time (9 min)?

i feel like i’ve just wet my pants in public. i hope you guys
will still talk to me. THANKS to you and every one who’ve helped
me address this problem.

best regards,

geo fox


#3

Geo, No…I mean the temp of the investment plus the temp of the
water should equal 150F. It’s a hypothetical constant. (what does
that mean?) J. A.


#4

George:

I’m certainly no expert, but I can tell you that, using an old
car A/C compressor and a 1/4 horse motor I’m getting about
27-27.8 on my gage on the perf caster. I couldn’t quite boil
water at 68, but going to about 72 made it boil just fine. My
local wholesale caster, Steve Darnell says that he and other
wholesale casters have the same problem every winter if the
water gets cold. They use water about 72-75. Don’t think this
will appreciably cut down your working time. I’ll bet we’ll hear
from skip and John, tho’. Gentlemen?


#5
   My local wholesale caster, says that he and other wholesale
casters have the same problem every winter if the water gets
cold.  They use water about 72-75.  Don't think will
appreciably cut down your working time.  I'll bet we'll hear
from skip and John, tho'.  Gentlemen? 

G’day Jess; You rang milord/milady? Whosomever? I know
absolutely zilch about casting or why one might want water to
boil in the investment. Or is it boiling? Maybe what you need
is not boiling but de-airing? Are the bubbles you seem to need
made of water vapour or air? Have people forgotten that many
gases including air dissolve in water easily? Have you noticed
that fresh tap water if left in a bottle soon develops bubbles as
air leaves? Has anyone tried boiling water, pouring it into a
stopperable vessel and letting it cool with the stopper on, then
using that water - which will be de aired - to make the
investment? I dunno, but you did ring my bell, didn’t you?
Cheers Sir, Modom, whatever.

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

#6

John:

Bravo! I think you have hit it. It is deairing rather than
real boiling that is wanted. I don’t know how you tell if the
action is actual boiling or deairing. I would assume it would
deair before it would boil. As to why it deairs at 72 but not at
68, I am not sure. Could it be that it is boiling, but the
boiling gives the assurance that all the deairing has already
taken place? Is there such a thing as the vapor pressure of
dissolved air in water?

Jess (Roy, a boy)


#7
   Geo, No...I mean the temp of the investment plus the temp
of the water should equal 150F. It's a hypothetical constant.
(what does that mean?) J. A.

hi J.A. John B. would definintely know. i can only guess that it
means that in a hypothetical sense (if it could be), it is, or
should be a constant factor .

just a note to you (again) and others who’ve helped me and this
forum in general:THANKS! i’ve learned quite a bit about things i
didn’t know, things i’ve known and forgotten and things that
should be left forgotten.

you are absolutely right about changing only one thing at a
time.

best regards,

geo fox


#8
    I don't know how you tell if the action is actual boiling
or deairing.  I would assume it would de-air before it would
boil. 

Probably right, but whether it is spectacular or not would
depend upon the amount of air dissolved in the water.

    As to why it de-airs at 72 but not at 68, I am not sure.
Could it be that it is boiling, but the boiling gives the
assurance that all the de-airing has already taken place? 

Not necessarily. If by “boiling” in the above context you
perhaps mean bubbling, you should also bear in mind that the
appearance of bubbling would also depend upon the amount of solid
material present which would act as sites for the first tiny
bubbles to form upon. (termed ‘nucleation sites’) F’rinstance, if
you take some very pure, micro-filtered distilled water and heat
it with a scrupulously clean vessel and thermometer, you could
well see the temperature rise above 100C - and then large bubbles
of steam form with a series of small thumps. This, in science is
known as ‘bumping’ and some liquids are quite prone to this
phenomenon. Chemistry students are taught to add a few chips of
brick - called ‘bumping stones’ - making nucleation sites to
avoid this and a possible sudden boiling over of the heated
liquid. This is very common when distilling liquids under
reduced pressure. The same bubbling/bumping could occur at the
de-airing stage well before boiling temperatures are reached -
for any given external pressure. ‘Bumping’ is of course, caused
by superheating the water.

Cold water dissolves much more air - or any other gas - than
warm water, by the way - which is why, when you use one of those
home soda pop machines for making carbonated drinks, cold water
produces far more ‘fizz’ than warm would. It is also why you
might boil water to de-air it - but you’d have to keep it
stoppered to stop air dissolving as the water cooled.

    Is there such a thing as the vapor pressure of dissolved
air in water?

Let’s imagine a bottle of a liquid - yes, even mercury - with
all the air or gas removed from above the surface, then sealed.
The thin gas above the liquid would be at the natural vapour
pressure of that liquid for any given temperature. Thus, the
vapour pressure of mercury would be very low, whilst the vapour
pressure of - say - acetone, would be relatively high. Remember
the glass tube filled with mercury and inverted in a bowl of
mercury to make a barometer? Well; the space above the mercury
wouldn’t be a true vacuum, but would contain lots of atoms of
mercury vapour all jostling and banging against each other -
giving a slight but real pressure. If you put a little acetone
in the tube as well as the mercury you’d see a difference in
mercury level when you inverted it - the vapour pressure of
acetone! And I’d better stop here, before I get caught up with
Brownian motion and all sorts of other erotica!!

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

#9
 Geo, No...I mean the temp of the investment plus the
temp of the water should equal 150F. It's a hypothetical
constant. (what does that mean?) J. A.
 hi J.A. John B. would definintely know. i can only guess that
it means that in a hypothetical sense (if it could be), it is,
or should be a constant factor .

Grrrr! & G’day: you rattled my cage again, and this time I
offer an unequivical DUNNO! I’ve no idea. Now can I go back to
sleep?

Please?
Cheers

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

#10

The vacuum that is applied to investment material when it is
done correctly is actually causing the water to boil. Boiling
occurs when the partial pressure of a liquid is greater than the
pressure surrounding the liquid. When excess vacuum is applied to
investment you can easily double the volume of the investment in
the flask by boiling, forming large bubbles as in boiling water
on a stove causing it to splatter somewhat violently and over
flowing if unattended. Air bubbles and dissolved air in the
investment does come out first but the additional boiling of the
water if the vacuum is monitored and controlled will help to
vibrate and sweep any trapped air bubbles that do not find their
way out with gravity. The temperature of the investment does
actually cool by this vacuum operation. This indicates the phase
change of a liquid to a gas. If left under vacuum long enough
with these typical setups water will approach the freezing point
with a new vacuum pump.


#11
       The temperature of the investment does actually cool by
this vacuum operation. This indicates the phase change of a
liquid to a gas. If left under vacuum long enough with these
typical setups water will approach the freezing point with a
new vacuum pump.

G’day: Mike, I agree completely with your comment above. As I
said earlier I have never done any casting; my remarks about
de-airing were simply intended to create thought as to whether
the water really needed to de-air or to boil at the low pressure.
You say it does matter, and of course I accept that. I learn.
Cheers,

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