Creating a vacuum using water

[wittgenst3in]

I was thinking the other day about a problem involving pumps. Many pumps, even industrial ones that cost $40k+ can't prime themselves, meaning if they're full of air they can't suck water up to get started. Also many can't pump from a tank any lower than about a meter below them.

In fact there is a limit as to how high you can suck stuff up to. Since the most suction is acchieved if you have a perfect vacuum in one end of the pipe, and their is going to be one atmosphere of pressure on the water at the other end, you can't suck water up more than about 10 meters.

So I was thinking, if you got a fluid that dosen't vaporise easily, and then filled a bucket with a hose and ran the sealed end of the hose higher than 10m you would end up with a near-perfect vacuum in that end of the hose. Does this sound like a practical way of creating a vacuum to anyone else?

 

[Rolfe]

Er, yes? We did this in school I remember. OK, theoretically for the water, but practically using mercury. Just used a glass tube with a sealed end as I remember.

I haven't died of mercury poisoning yet, but I doubt if they'd allow that to be done these days.

Rolfe.

 

[pupdog]

These pumps are suction pumps, that rely on the pressure of the atmosphere pushing water up a column--so the weight of the water can't exceed the force of the atmospheric pressure.

Submersible pumps don't have that problem--the pump impellers push the water via centrifigal force, and the impellers are placed well below the water surface.

 

[SGT]

Er, yes? We did this in school I remember. OK, theoretically for the water, but practically using mercury. Just used a glass tube with a sealed end as I remember.

I haven't died of mercury poisoning yet, but I doubt if they'd allow that to be done these days.

Rolfe.

This is Torricelli's experiment. And in the upper part of the tube you have only saturated mercury vapor.

 

[wittgenst3in]

These pumps are suction pumps, that rely on the pressure of the atmosphere pushing water up a column--so the weight of the water can't exceed the force of the atmospheric pressure.

I know, I was just referring to pumps located at the top of the system.

Submersible pumps don't have that problem--the pump impellers push the water via centrifigal force, and the impellers are placed well below the water surface.

In the sub pumps I've seen the impellers were all of 3 inches below the water level. They were fitted with a level switch though so maybe that's why.

 

[MRC_Hans]

Another way to get vacuum using water is this:

Get a bottle with a screw-on cap. Put a little water in it and put the cap on, but loose. Now place the bottle in a small pot with some water in it and put the whole thing on the stove.

When the water boils, both in the pot and the bottle, screw the cap on tight (use gloves ), and take it out of the pot and leave to cool. You now have a nice bottle of vacuum.

(I suggest you wear protection goggles during the whole exercise).

Hans

 

[wittgenst3in]

Thanks Hans.

I was thinking of a harder vacuum though. Like for plasma experiments etc. It seems to me that if there's a cheap non-carcinogenic chemical that dosen't vaporize easily, that anyone with a staircase can make good quality vacuum.

 

[SGT]

Thanks Hans.

I was thinking of a harder vacuum though. Like for plasma experiments etc. It seems to me that if there's a cheap non-carcinogenic chemical that dosen't vaporize easily, that anyone with a staircase can make good quality vacuum.

Any liquid will vaporize until the pressure in the would be vacuum camera is equal to its saturated vapor pressure at the temperature of the experiment. For normal temperatures, this pressure is aproximately 10^-6 of atmospheric pressure, so you have a very good vacuum, but I don't know if it is good enough for experiments with plasma.

 

[frenchie]

You had better build or buy a water aspirator, (gets you down to 15 mmHg (15 torr) from any water tap, cost ~3bucks from any chemistry supply company)), and then, if not enough, carry on. A mercury diffusion pump (that's the same thing as the water aspirator, but working with mercury, more expensive and Hg is toxic) goes down to 10^-3 mmHg, then a silicone oil diffusion pump (very expensive)will get you to 10^-6 mmHg.
Another (much better and easier) way to go is looking up Sargent-Welsh or any other rotary multistage vac. pumps at see the prices, (not cheap), down to 10^-2 mmHg easily, then add an oil diFfusion pump.
Staircases are neat, even though water (density 1kg/liter) needs three floors to get down to 15 mmHg, mercury (density 13.6 kg/liter) needs half a floor for hard vacuum, but high vaccum silicone oil would also need two to three floors to go down to 10^-6 mmHg, as its density is no higher than 2 kg/liter and the pressure drop is related to density (Pressure = density of the fluid x g x h)
And then, its going to be very messy, as you would have to start with your whole system full of oil...

 

[wittgenst3in]

Thanks

 

[MRC_Hans]

Thanks Hans.

I was thinking of a harder vacuum though. Like for plasma experiments etc. It seems to me that if there's a cheap non-carcinogenic chemical that dosen't vaporize easily, that anyone with a staircase can make good quality vacuum.

TO get a harder vacuum, use something with a higher boiling-point. Mercury or sodium, for example. But why do plasma experiments need a hard vacuum?

In ye olde vacuum tubes, you needed a VERY hard vaccum in one respect: You needed the partial pressure of oxygen to be practically zero over many years of operation (I have vacuum tube radios that still work after half a century ). To achieve this, a "getter" was installed. A metal loop with some magnesium on it was placed in the tube, and after pumping it out and sealing it, the getter was heated via induction, the magnesium evaporated, burning off whatever oxygen was left. The remaining magnesium was deposited as a thin silvery layer on the inside of the glass and would absorm any oxygen released in years to come.

Hans

 

[Matabiri]

You will get a good vacuum this way, but it's utterly impractical - you can't actually pump air out of anything with it. Unless as frenchie mentioned, you start with your entire system full of oil/mercury/whatever.

 

[scribble]

The idea sucks.

 

[Zep]

The tough one is the other way round: Creating water from a vaccuum.

(Hint: spout "Zero Point Energy" mantras and dance all around! )

 

[MRC_Hans]

You will get a good vacuum this way, but it's utterly impractical - you can't actually pump air out of anything with it. Unless as frenchie mentioned, you start with your entire system full of oil/mercury/whatever.

Vacuum tubes and the like are made by pumping out the air, without any prior fluid filling. It is true that you cannot get a perfect vacuum in that way, but it is no problem to get a sufficient vacuum. The problem about fluid-filling is that it will leave an imperfect vaccum, too. There may not be any air, but there will be vapor of whatever fluid you used.

Hans

 

[Matabiri]

TO get a harder vacuum, use something with a higher boiling-point. Mercury or sodium, for example. But why do plasma experiments need a hard vacuum?

Plasma experiments need a hard vacuum to avoid contaminating the plasma. Any heavy ions start radiating intensely (bremsstrahlung radiation, I think) in magnetic confinement at fusion temperatures, which cools the plasma down very rapidly (and might throw off your results, as you don't have control over what's in the air). Also, the more matter you try and plasmarise, the more energy you use.

But you can still make plasma in your microwave without a vacuum, which is fun.