Measuring pressure drop across an inline filter

A few people who have bought the Watts or other manufacturer’s coalescing filter have asked me how they can determinine whether the borosilicate glass fiber filter element is clogged enough so that it is both detrimental to performance of the PCV airline and subsequently, needs to be replaced. I mentioned previously that a seriously-clogged filter is unlikely for quite a while unless the engine oil is extra dirty due to negelect (ie: proper oil change intervals were not followed). However, eventially, any filter will require changing and after 1000 miles of hard driving, I decided to check my filter to see what the pressure drop was across it.

You can do this, yourself, with the following materials:

(1) Dwyer Minihelic II differential pressure gauge, model 5205 Cost = $50
(2) Nylon hose tees for 3/8-inch ID hose
(2) Nylon 3/8-to-1/4-inch ID hose adapters
(2) 6-inch long, ¼-inch ID tubing or hose
(2) 2-inch long, 3/8-inch ID hose
(2) 8-inch long, 3/8-inch ID hose



After connecting everything as shown and starting the engine, here’s what I saw:



The gauge read 0.60 PSI. What does this mean? Well, not much unless you know what the gauge reads without the filter plumbed across it. I then bypassed the filter and looked at the gauge. This time, it read 0.50 PSI. You might ask, “Shouldn’t the gauge read 0 PSI if there are no restrictions?” Yes, and in this case, the reason why the gauge didn’t read “0” is that I had negelected to set the pointer on zero before I started. (bang head on wall)
Regardless, the important number in this case is the differential pressure difference between having the filter connected to the PCV and intake hose, and that of not having any filter in place. Essentially, the difference in pressure before and after the filter is a very miniscule 0.1 PSI. Folks, that is really insignificant and means my Watts coalescing filter is flowing very freely.
Most Coalescing filter manufacturers recommend replacement of the filter element when the differential pressure between the outlet and inlet ports of the filter reaches 10 PSI (assuming 100 psi line pressure). Because the Watts filter “sees”only 10 psi (equivalent to the actual 20 inches of mercury vacuum pressure), I may have to see a 1 psi differential pressure before even thinking about changing the filter element. Remember too, that a serious flow restriction in the PCV line will cause engine idle to drop noticably.

By the way, here is a zoomed-out shot of my differential pressure setup:


For those of you who have missed the first part of my article, click on the PDF link (PCV Line Oil Removal 101) that is listed here:

http://www.conceptualpolymer.com/corvette_c5_corner.htm

Dave

 

Thanks for this info - I'm learning a lot

 

Many people look at coalescing filters and wonder how they can flow so well with such densely-packed glass fibers. This technology has been around for many, many years and has been perfected to a point where both excellent filtration and very good flow are both possible. I just wanted to show how little pressure drop an excellent filter is capable of - amazing, to say the least!

 

Dave

Took me a while to find this....kept looking for "Watts or Catch Can"

I dumped a LOT of oil thru that little filter......so they do flow !!!!!

More good work by Professor Dave (Bet you watched Mr. Wizard when you were a kid)


DH

 

Thanks, Howie.

From what I understand about your setup, the Watts filter was plumbed AFTER the expensive catch can. If this is true, then it goes to show you that oil aerosols do get past the low-tech cans fairly easily.

Dave

 

Yes Dave, I already new oil was getting by my AMW can thats why the interest in your project

In all fairness I did not install my Elite CC but have not reason to expect different results.........oil vapors will get by a metal mesh based CC.


DH