What is "Bubble Point" 1

[tc7]

Is anyone familiar with the fluid filter specification regarding "Bubble Point"? More specifically, what is "Bubble Point"? I am guessing it has some relationship to cavitation, but why would filter specs call out bubble point characteristics as well as expected pressure drop for specific flow conditions? I would have assumed that if the filter is correctly designated for a flow situation and pressure drop is monitored, then I should not have to worry about cavitation.
Thanks to any who can reply.
Tom

 

[meintsi]

Bubble Point is a test conducted in isopropyl alcohol (IPA) to determine the single largest pore in filter media. In some cases this value is used as pass/fail criteria for an element.

The bubble point test is based on the fact that for a given fluid and pore size, with constant wetting, the pressure required to force an air bubble through the pore is inversely proportional to the size of pore diameter. In practice, this infers that the pore size of a filter can be established by wetting the element with a fluid and measuring the pressure at which the first bubble stream is emitted from the upper surface of the element when air is applied from the underside.

The point at which the first stream of bubbles emerges is the largest pore. Therefore, the bubble point value can be used to obtain a relative measure of the size of the single largest pore in a filter element.




Remember...
"If you don't use your head,
your going to have to use your feet."

 

[tc7]

Wow! that was a beautifully conise and quick reply, thanks meintsi.
If I may further ask then, if we conduct other multi-pass filtration tests to determine Beta ratios or if we conduct other "Maximum Particle Passed" tests as some Mil-Spec Filters require, what value does the Bubble Point Test serve?

Tom

P.S. Any suggested introductory reading on fluid filter engineering? I know there is a lot of stuff available on the Web, but would like your suggestion on the basics.

 

[meintsi]

The bubble point value of a given filter, when coordinated with largest particle passed values is a reliable indication of the largest opening in the filter. The bubble point test is frequently used to determine fabrication integrity since it is a nondestructive test.

I can't really point to any specific material about the basics. The knowledge I have comes from dealing with the engineers form the companies we buy our filters from, college texts (MET with a fluid power design specialty) and 5 years of being involved with the design and manufacture of precision high-pressure hydraulic pumps and motors with several very knowledgeable engineers who have been doing this for 20+ years.

We deal with a lot of low lubricity fluids, and proper filtration is a must in our lab testing and out in the field, given the higher than average cost of our units when balanced against the customer's expected "life" of our units.





Remember...
"If you don't use your head,
your going to have to use your feet."

 

[aspearin1]

Meintsi pretty much hit it right on the nose. I've had the pleasure of running a few (a few too many) bubble point tests myself. Bubble point testing is probably the most valuable non-destructive quality release criteria used to test membrane cartridges. The bubble point curve is correlated to data obtained most often by particle size or bacterial retention. Many pharmacuetical companies use a bubble point tester to integrity test their cartridges prior to use, just as an added insurance. It gets real expensive when a filter fails.
To answer tc7 question, if you find the beta ratio and you are comfortable that you have a repeatable product coming in your door, a bubble point test will not serve you necessarily well. It really depends on your application and the sensitivity of your application. Neither a bubble point test nor any standardized test will model your application.

Sorry to say there are not a lot of easy access filtration resources out there. There are a few rather expensive filtration text books on amazon.com. Since working in the filtration market, I've learned that too much about filtration is unknown to filter users. Because of this, I'm actually in the process of pitching a filtration technologies curriculum to a local college.

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee