Sciencemadness Discussion Board - Suitable vacuum usage - safe to take it to its limit?

Suitable vacuum usage - safe to take it to its limit?

JefferyH - 14-8-2014 at 10:47

I recently picked up a vacuum, which according to the online rating claims Ultimate Pressure Torr: 0.02

Does this mean I can safely take my vacuum down to 0.02 Torr without damaging it?

The vacuum also has ratings like "End Vacuum: 29.9" Hg", to which I have no idea what that means.

hissingnoise - 14-8-2014 at 11:08

Quote:

The vacuum also has ratings like "End Vacuum: 29.9" Hg", to which I have no idea what that means.

It means that the partial vacuum achieved will support 29.9 mm of Hg ─ 1 atm will support ~760 mm!
As to the pump, how good it is depends on the make and model . . .

Dr.Bob - 14-8-2014 at 11:17

Vacuum pumps generally work best at the best vacuum that they can get down to. They only do much work when working at conditions other than a good vacuum. So the answer is that they last longest when put under no load, thus kept at a maximum vacuum possible. The worst thing you can do with a vacuum pump is to run it with an open system for long, as then it will work much harder and generate much more heat, which is bad for it, if excessive (many oil pumps do better once they are warmed up some, as the oil be thinner then.)

I think that they mean it will pull down to only 0.01" Hg left from the 30 inches of 1 atm. That would be a normal range for a oil based vacuum pump. Once you get down that area, the quality of the tubing and connections used often matters more than the pump, as any leaks will have a huge effect on the ultimate vacuum obtained. But that is general fine for most work.

Marvin - 14-8-2014 at 11:18

That's inches, it's the glass half full i.e. calling a full atmosphere '30 inches' and vacuum '0 inches' is the same as calling a full atmosphere '0 inches of vacuum' and a full vacuum '30 inches of vacuum'.

JefferyH - 14-8-2014 at 11:27

So given the proper tubing and equipment the vacuum should be able to get down to its "ultimate pressure"?

Dr.Bob, how long would be considered too long for an open system? A mate of mine suggested a big reason for getting a vacuum is for rapid vacuum filtration/drying of solids. So how long is too long exactly? 10 minutes? 60 minutes? 24 hours?

[Edited on 14-8-2014 by JefferyH]

Magpie - 14-8-2014 at 12:07

I might as well throw in my $0.02:

Quote: Originally posted by JefferyH

I recently picked up a vacuum, which according to the online rating claims Ultimate Pressure Torr: 0.02

Does this mean I can safely take my vacuum down to 0.02 Torr without damaging it?

It means that the pump will provide an ultimate absolute pressure of 0.02 Torr.

0.00Torr would be a full vacuum.

Yes, the pump can handle it.

Quote: Originally posted by JefferyH

The vacuum also has ratings like "End Vacuum: 29.9" Hg", to which I have no idea what that means.

29.9"Hg, "Hg being inches of mercury, is just another way of saying very near a full vacuum. It's just English units for vacuum ("Hg) vs metric units for absolute pressure (Torr).

30.0"Hg is a full vacuum.

JefferyH - 14-8-2014 at 14:47

thank you everyone

Dr.Bob - 14-8-2014 at 18:43

A heavy duty oil based rotary vane vacuum pump could likely handle a major air flow for 10-30 minutes fine, but will eventually start overheating depending on the quality of the pump. If it is a diaphram pump or one designed for heavy flow, then it might be fine for hours or days. But you don't really need a big vacuum pump for filtrations and air drying, a simple aspirator or diaphram pump will do fine for that type of operation. You need a pump with a large CFPM rating for high flows (cubic feet per minute), rather than one with a high ultimate vacuum rating. There are different designs for large flows verses high ultimate vacuum.

Kinda like a balance, you can get one with lots of capacity or precision, but getting both in one scale is tough and expensive.

hissingnoise - 15-8-2014 at 03:31

Quote: Originally posted by Marvin

That's inches, it's the glass half full i.e. calling a full atmosphere '30 inches' and vacuum '0 inches' is the same as calling a full atmosphere '0 inches of vacuum' and a full vacuum '30 inches of vacuum'.

Ooops! Inches, centimetres ─ Iran, Iraq ─ stalagmites, stalagtites ─ yeah, SNL . . .

JefferyH - 16-8-2014 at 03:31

Now... I'm a little confused on one thing....

Org. Chem guide says I should always use a gas inlet tube partially clamped, and never completely close the system off, or else it will implode. Doesn't this mean that to have a consistent vacuum that the vac pump will need to be 'on' constantly?

Keeping a water aspirator on isn't much of a problem, but vacuum pumps, for much lower pressures, get very hot, so how can I keep the pressure low and consistent, without keeping the pump on, while using a gas inlet tube?

Magpie - 16-8-2014 at 11:13

Quote: Originally posted by JefferyH

Now... I'm a little confused on one thing....

Org. Chem guide says I should always use a gas inlet tube partially clamped, and never completely close the system off, or else it will implode. Doesn't this mean that to have a consistent vacuum that the vac pump will need to be 'on' constantly?

This is BS. You can pull a complete vacuum and run the pump for hours with a decent pump. If you are not moving any air the pump is not doing any work. The pump will reach a steady-state safe temperature due to internal friction, however.

Quote: Originally posted by JefferyH

Keeping a water aspirator on isn't much of a problem, but vacuum pumps, for much lower pressures, get very hot, so how can I keep the pressure low and consistent, without keeping the pump on, while using a gas inlet tube?

Use an aspirator. (I don't know what a "gas inlet tube" is.)

[Edited on 16-8-2014 by Magpie]

unionised - 17-8-2014 at 00:41

If you are filtering stuff under vacuum with a mechanical pump (rather than a water jet pump) you should arrange for some air to leak into the system, so it's not BS.

When the system reaches equilibrium all the liquid will have been vacuum distilled through the pump.
That's not what you want.

Once the pump has sucked most of the air out of the system, it starts pulling the solvent vapour through.
That vapour won't do much harm, but the pump works by compressing it and, at that point, the vapour condenses and screws up the oil.
With water vapour you end up with pump "oil" that looks like butter.
It's not a good coolant or lubricant and so it doesn't do the job properly.
That's why pumps have "gas ballast" valves. They leak some air into the pump to sweep out the vapours.

Incidentally, if you plan to announce your ignorance of the subject by saying "(I don't know what a "gas inlet tube" is.) " you might want to think twice about commenting on it.

It is BS to say the system will implode, but the air inlet does serve a purpose.

When it comes down to it, mechanical- oil filled- pumps are not good for filtering liquids. Use a water jet pump.

Magpie - 17-8-2014 at 07:50

Hey, your corrections were very educational. Why didn't you pipe in earlier? You had plenty of opportunity.

unionised - 17-8-2014 at 08:26

Strange as it may seem, posting here isn't the only call on my time.

JefferyH - 17-8-2014 at 11:38

Maybe you guys can point me in the right direction. I have been searching for a suitable vacuum gauge, since there's no point using my vacuum if some of the distillates vaporize at negative celcius at maximum vacuum of the pump. Controlling the pressure would be ideal.

Problem is all of the "vacuum gauges" online I encountered start at 0 and go down to -30 mmhg, which I don't understand this unit of measurement seeing as 0 mmhg = 0 torr, and is a total vacuum.

The gauges I did encounter that go from 760 torr down to 0, are all $500-700+ high precision gauges...! Why does it seem so impossible to find a suitable gauge that isn't so "high precision" and pricy, and still in torr? Does anyone know how to explain usage of these gauges that go into negative pressures?

I actually did find one cheap gauge that goes from 760 to 0 Torr, might be checking it out after some more research.

[Edited on 17-8-2014 by JefferyH]

Texium - 17-8-2014 at 11:46

Quote: Originally posted by JefferyH

Problem is all of the "vacuum gauges" online I encountered start at 0 and go down to -30 mmhg, which I don't understand this unit of measurement seeing as 0 mmhg = 0 torr, and is a total vacuum.

Does anyone know how to explain usage of these gauges that go into negative pressures?

That's exactly what you need actually. The gauge is referring to the amount of pressure relative to the ambient pressure, so -30in Hg on the gauge would be ambient pressure - 30in Hg. If the ambient pressure was 31in Hg, then the pressure in the vessel would be 1in Hg.

Edit: I'm assuming that the ones that you are seeing reading 0 to -30 are in inches rather than millimeters, as 30in is approximately 760mm. The one that I have shows a double scale, with mm Hg and in Hg. It goes from 0 to -30in/-760mm. (mm Hg and torr are the same thing)

[Edited on 8-17-2014 by zts16]

JefferyH - 17-8-2014 at 12:08

Quote: Originally posted by zts16

Quote: Originally posted by JefferyH

I did actually find a one that goes from 760 to 0 torr, but displays in.hg as well. It has a low rating on amazon, but it only costs $30. I think it will serve my purposes fairly well. I just need approximations of the vacuum for distilling various things.

Magpie - 17-8-2014 at 13:57

Here's one:

http://www.amazon.com/Interstate-Pneumatics-G2024-030V-Inch-...

It reads in inches of Hg. 30 inches of Hg is a full vacuum.

elementcollector1 - 17-8-2014 at 14:17

I bought a similar vacuum gauge for $5, and it is now perpetually stuck at a reading of 5-7 in. Hg, but an applied vacuum will still pull it down to "30". When it wasn't broken, I got around 27" or so, which was the specified maximum for the aspirator I was using.

[Edited on 8-17-2014 by elementcollector1]

Magpie - 17-8-2014 at 14:51

I agree that is a very cheap gage. I have one with a 2" diameter face that looks similar but it is an Ashcroft, a reputable brand. I paid $10-15 for it about 10 years ago at Ace hardware. It still works well.