What vacuum pump is right for me?

[sestuttle]

I work in an undergraduate lab at a university and need to buy a new vacuum pump. We need to de-air 5 to 10 gallons of water for a TX and Permeability machines. My main concern is how low the ultimate pressure needs to be. Many suggested high vacuum pumps with ultimate pressures of 10^-4 torr, but this pump being in the $2000 dollar it is out of the range of price we would like. I have found cheaper pumps with max pressures around 21 torr, but is suggested for vacuum filtration. Does anyone have any suggestions on my situation?

 

[Ron]

I will assume you are not a student, as student posting is prohibited by forum rules.

Almost any vacuum pump can be used for de-airing water...the only difference is the time it takes to do it. I've used vacuum pumps for this purpose that cost less than $500 and they work fine. Also keep in mind that your container should allow a large exposed surface rather than deeper water.

If your vacuum pump is too powerful, you can collapse your container..

Assuming you are not at a high elevation, 21 torr should be a sufficient differential to "boil" your water.

 

[sestuttle]

You are correct in assuming. I am the lab technician, and am new to geotechnical systems. Thanks for your help!

 

[oldestguy]

Here is a question. What is the test being used for? For instance if one is trying to see if a compacted liner for a landfill is low enough, well will the leachate be de aired? Same goes for lining a detention pond. On that basis I see no need to de air any test,if air content of the job site is different. Might as well run the test with fuel oil, what the heck.

 

[Erdbau]

I don't think 21 torr is enough. I use a vacuum pump that pulls up to 1 bar (750 torr). The water has to be warmed in order to get it to "boil". If the water is heated, then the vacuum doesn't need to be as high. But 21 torr is almost nothing, is that number right?

Oldestguy, standard practice on triaxial and perm tests is to use deaired water.

 

[Ron]

Erdbau...oldestguy has a point. Why de-air water for a triaxial test if the in-situ groundwater condition is not de-aired? If the sample is within an area of groundwater fluctuation, I would agree with him. If the sample is from a much deeper stratum, the likelihood that the groundwater has very little dissolve oxygen is there and the water for the test would need to be de-aired.

This argument for triaxial test procedure can go on and on...as an example, why use backpressure saturation if your specimen is not saturated in-situ. Well...you get wonky pore pressures, but does it really replicate the in-situ condition?