How much pressure

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airrat

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I have no clue if I need to convert this to get the pressure number talked about in other threads.

What would 24 inches mercury at sea level be pressure wise?

What I am trying to find out is this:
I have an old food saver system I dont use anymore. Before throwing it out I was trying to find out how much pressure it created in the canisters. I called the manufacturer and was told the amount above.

Any help would be great.
 
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airrat

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Found this, would this be correct
To convert (in. Hg to psi multiply by 0.4912)
Would give me 11.788 psi at sea level?
 
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Mudder

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Originally posted by airrat
<br />Found this, would this be correct
To convert (in. Hg to psi multiply by 0.4912)
Would give me 11.788 psi at sea level?
Sounds about right;

The total vacuum (which is difficult to achieve) would give you 14.7 PSI.
 

ctEaglesc

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First you are not building pressure, you are "pulling" a vacuum
If you plan on stabilizing don't worry about how much.The solution I use will normall "boil" at first before it maxes out.
I suggest you put a vacuum gage on your set up and see if it will pull a vacuum.
 
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Mudder

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Originally posted by ctEaglesc
<br />First you are not building pressure, you are "pulling" a vacuum
If you plan on stabilizing don't worry about how much.The solution I use will normall "boil" at first before it maxes out.
I suggest you put a vacuum gage on your set up and see if it will pull a vacuum.
You are correct; but when you remove atmosphere from the inside what happens on the outside? Perhaps I'm misunderstanding or reading into the question but I'm thinking that he's trying to calculate the external pressure on a vessel that is under vacuum at 24 inches of mercury?

I think then that it would more correctly be -11.788 PSI internal?


&lt;edit&gt;

Interesting reading:

http://www.npl.co.uk/pressure/faqs/vacuum.html

http://acept.asu.edu/PiN/rdg/vacuum/vacuum.html

too long to post: click me
 

ctEaglesc

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Maybe I misread the question as it has been my experience that the outside pressure has little effect nor can I control the outside <b>pressure.</b>
this is what I tried to answer
Before throwing it out I was trying to find out how much pressure it created in the canisters.
I guessed he meant how much vacuum rather than pressure.
That it why I suggested a vacuum gauge
 
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Mudder

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Quote from National Physical Laboratory UK.:

"Science &gt; Mechanical Metrology &gt; Pressure and Vacuum &gt; FAQs
Where does pressure end and vacuum start?

There is no clear boundary between pressure and vacuum and the word vacuum simply refers to part of the pressure scale. Its definition is not precise but it is commonly taken to mean pressures below, and often considerably below, atmospheric pressure. What is particularly important, however, is to appreciate that a vacuum refers to a pressure measured with respect to zero pressure (that is an absolute pressure) and not with respect to ambient pressure or some other pressure - see pressure modes. Unfortunately some gauges - for example those showing the pressure reduction (from atmospheric pressure) in engine manifolds - sometimes bear numerically reversed scales marked vacuum; they are not measuring vacuum but rather negative gauge pressure."

I suppose it could be argued and interpreted in several ways.

Wouldn’t a total vacuum just be zero pressure? And wouldn’t atmospheric pressure be a certain number of PSI pressure at the particular altitude? Wouldn’t the atmospheric pressure be different in Atlanta than in Denver?
 

hilltopper46

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The maximum pressure on the exterior of the vessel is atmospheric pressure. ~14 psi doesn't sound like much, but when you add up all the square inches of the surface area of the vessel and multiply that times the ~14 psi, you have have quite a bit of force.

Basically, when you pull a vacuum, you are taking away the force that is "pushing out" on the wall of the vessel so the atmopsheric pressure can "push in" with no resistance.

I know what I meant but did you understand what I said? [:)][:D][:D]
 

ctEaglesc

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I don't think you need all that stuff with a food saver or an HF venturi pump.
The only reason I use a gauge is to tell me if I have a leak.
What was it I say about rocket science?
Either you are pulling air out of a blank and replacing it with a liquid/solid or not.
What was it I say about rocket science?
[:D]
 
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Mudder

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Tom;

I guess that in the future I’ll just have to keep my mouth shut for fear of “overcomplicating†things.

In my day to day job I have to focus on the technical aspect of many different processes and procedures; one of them being vacuum impregnating sintered bronze bearings. Therefore I tend to take things to the “n-th degree†and I think I might have missed the point of your post.

If you are asking if an old food saver system could be used to draw a vacuum for stabilizing the answer is Yes. Before I bought my vacuum pump I used an old “seal-a meal†with good results. I would suggest as others have that you get a good vacuum gage to see the actual “draw†and have some type of valve so that you can turn off the vacuum pump while keeping vacuum in the vessel. The gage will also show you if you are losing vacuum and need to draw again.

One little side note or “tip†that has worked for me is to release the vacuum every hour or so and then redraw it. I did an experiment and found that most blanks will actually pick up a few more grams of the stabilizing agent than it I drew vacuum once and left it.


&lt;edited for spelling&gt;
 

airrat

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Ok all, I guess I didn't ask it correctly or with proper terminology. In another threads its says:
Originally posted by MesquiteMan
<br />Russb,

I have never done a snakeskin so I can not say how it will work. A couple of things, though. Alumilite clear absolutely must be pressure cast or you will end up with tiny champaigne bubbles. The Alumilite people recommend 60 psi.
The reason I used the term pressure is the psi from the quote. Now am I confusing this with vaccuum? Is this type of procedure done with "applying pressure" or "taking away pressure" with a vaccuum for instance?

I am wanting to remove bubbles from a clear cast, for example from the prickly pear cactus blanks I wish to make. Will that be enough pressue? too much? too little?

Eagle unless you know of a guage you can put inside the cannister to measure the pressure, I dont have a way to hook one up to a line. I know it creates a vaccuum. That is how a food saver works. It removes the air from the cannister.

Someone enlighten me.
 

jodoidg

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This thread was too good to leave alone. I have worked and taught instrumentation at nuclear plants for the past 25 yrs. The biggest problem people have is the terminology and you are all so close and pretty much correct. Keeping it simple vacuum is normally read in (in.Hg). Most vacuum gauges read 0-30 in.Hg. with 30 being a perfect vacuum. So the canner pulls a 24 inHg. vacuum which is a pretty good vacuum. Vacuum is relative to sea level only because atmospheric pressure changes with elevation and any “pressure†less than atmospheric is considered a vacuum. The only way to read this combination pressure/ vacuum is with a psia gauge (“a†being absolute) . These gauges are not very common. Kind of an apple and orange thing.
 

jodoidg

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The casting of blanks is done with pressure (PSI).
You could use your vacuum pump to stabilize blanks.
To throw a twist in I believe Virgle pulls a vacuum on his “resin†and then pours the molds which he then pressurizes (PSI). This is a very cool idea because he is de-aerating the resin with vacuum and then compresses any air left in the resin.
 

airrat

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yeah, no hose eagle. I will post a picture of it in a bit.

Thanks John, I knew I had seen somewhere someone posted about using a vacuum on their resin. I forgot they then put pressure on it.

I am glad to know I can use it for stabilizing.
 

MesquiteMan

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Tom,

You want to use a pressure pot and ADD air (pressure) rather than a vacuum pump to remove air (pressure) when casting the cactus blanks.
 

airrat

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Thanks Curtis, I was confused on it. Guess I wait a little longer to cast those blanks.

Eagle is that a "pressure pot" your referring too?
 

its_virgil

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Tom,
I think this is what Eagle was speaking of...a venturi type vacuum pump. Works off of an air compressor.

http://tinyurl.com/y24jpt

Do a good turn daily!
Don
Originally posted by airrat
<br />Thanks Curtis, I was confused on it. Guess I wait a little longer to cast those blanks.

Eagle is that a "pressure pot" your referring too?
 
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