Vacuum

[spuffock]

Given a tired old rotary pump, anyone any ideas how I can guess the sort of vacuum it will pull?

 

[JimB]

Given that the pump is old and tired, it could be any where between zero and the original pump rating.

Was the pump designed as a vacuum pump or as a supply pump for gasses or liquids?

JimB

 

[spuffock]

It is a vacuum pump, Leybold D12. I get the idea that a new one ought to pull 8 millitorr or so. I reckon I might be getting about 20 torr.

 

[Hero999]

Given that the pump is old and tired, it could be any where between zero and the original pump rating.

No it won't be better than it's origional rating.

Don't worry I know what you meant.

 

[JimB]

It is a vacuum pump, Leybold D12. I get the idea that a new one ought to pull 8 millitorr or so. I reckon I might be getting about 20 torr.

I suppose at this point one could say "wow, that sucks", but I won't.

How do you reckon on 20 Torr ?

I guess the only way is to connect a suitable pressure gauge.
20 Torr OK, but 8 milliTorr is way out of my range of experience.

JimB

 

[tcmtech]

Say what now? My brain only works on PSI, Inches of HG or H2O, and ATM's when dealing with vacuum.

 

[spuffock]

I'm guessing by 1 ) what the arc from a neon sign transformer looks like, not much different from atmosphere, but with small bright glow areas on the electrodes, and 2) what I would expect if the pump was full of water, which it could well be.
I have a thermocouple head, but the pressure is too high to get anything out of it, 2 torr reads as atmosphere. If only I could find a few centimetres of glass tubing, I have mercury.....

 

[crutschow]

I'm guessing by 1 ) what the arc from a neon sign transformer looks like, not much different from atmosphere, but with small bright glow areas on the electrodes, and 2) what I would expect if the pump was full of water, which it could well be.
I have a thermocouple head, but the pressure is too high to get anything out of it, 2 torr reads as atmosphere. If only I could find a few centimetres of glass tubing, I have mercury.....

Most hardware stores (at least in the U.S.) have clear plastic tubing. Connect one end to the pump and place the other end in the mercury (with the tube vertical of course). Crude, but it should give you a good indication of the pump's operation.

 

[tcmtech]

You could just spend a few dollars at the auto parts store and buy a vacuum gauge. Its accurate enough to give you a fair idea what your working with. Unless it a very low vacuum but with fair volume capacity behind it.

 

[jpanhalt]

See if it boils water at room temperature. 20 Torr is horrible. Be sure the oil has been changed.

John

BTW, I mean boiling, not just degassing. A decent pump should actually be able to cause water to freeze.

 

[Hero999]

A decent pump should actually be able to cause water to freeze.

I don't see how it could cause water to freeze at room temperature. Deceasing the pressure, both decreases the boiling point and increases the freezing point but the maximum freezing point of water is around 4°C where it has no liquid phase at low pressures.

 

[jpanhalt]

Obviously, the frozen water is not at room temperature. As I said, though, the water in a flask (say a 50 to 100 mL round bottom flask with 10 to 20 mL of water) initially at room temperature should freeze with a decent pump. That means that the pressure and volume of gas removed is sufficient to maintain a few Torr. Boiling at RT only shows a pressure of less than 15 to 20 Torr, depending on what RT is.

The OP suggested the ultimate pressure may be as high as 20 Torr. Rotary vane pumps in decent condition should pull less than a Torr, which will cause water to freeze. I have done that experiment. I suggest, of course, that the vent on the pump be open, as one does not want the water vapor to condense in the oil.

John

 

[sheldonstv]

my only dealings with atms is when i try and force money out of them ?????..............

 

[AllVol]

I don't see how it could cause water to freeze at room temperature. Deceasing the pressure, both decreases the boiling point and increases the freezing point but the maximum freezing point of water is around 4°C where it has no liquid phase at low pressures.

Prof. Clint Sprott demonstrates water boiling, then freezing under vaccuum in virtually all his "Wonders of Physics" lectures at http:sprott.physics.wisc.edu/wop.htp

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