Do pressure drop meters really work?

[kanling]

There has been talk of pressure drop meters installed across auto air filters. It is a good idea in theory, but I question the accuracy of them in practice.

I haven't used one in an auto application, but I recently installed a Magnehelic across the air filter in my home furnace/AC.

Due to the direction changes in the return duct nearby the filter and fan (turbulence in that area), the slightest position changes of the probes will give me wildly different pressure drop readings. Some spots will give me pressure gains across the filter! I don't think there is any place in the duct that has stable enough air for a good measurement.

So, the numerical reading of my meter is meaningless, I am only able to use it as a relative measurement based on my chosen probe position.

I would think that auto applications would be just as bad, if not worse, for turbulent airstreams across the filter media.

 

[kenw]

Your observations are correct. the placement of the air pressure probes are quite a science unto themselves. There are many requirements for spacing, distance, etc in order to get accurate values.

In a nutshell, the space inside a typical automotive airbox is nowhere near the size that it realy needs to be to get a decent measurement, and flow control cones and such would also have to be added to ensure consistent readings under all variables.

However, you aren't trying to get absolute measurements, you are only looking for change. so if you chose to go this "change from normal" route, it will probably give you the basic results you need.

Minor fluctuations should be expected (since loading isn't always perfectly even) and considered as within the normal range.

 

[kanling]

"Change from Normal" isn't perfectly useful either because "normal" starts out with a brand-new perfectly clean filter.

You almost have to swap old and new filters as the pressure reading goes up. Determine if the performance increases with a new filter. If not, stick the old one back in and let the pressure reading build some more.

After several years, you may find your ideal pressure drop point to change filters. But, by then, the media of the new filters will be different so the result will be meaningless.

It almost makes you just want to mindlessly change your filters every 3 yr/36000 mi and hope for the best!

But, seriously, you are right Kenw... "change from normal" isn't a bad method. As the air filter begins to get really dirty, the pressure drops should increase at a much faster rate than before.

I learned my lesson from my furnace, though. My car didn't come with any kind of air filter monitor. I'm not going to bother putting one in.

 

[gudmund]

if you like to see one now in auto use, check out the 2005 Ford Focus. A friend bought one of the base models for under $10grand and when we were checking it out, there it was a air filter monitor attached directly to the air filter casing. I've seen them on big trucks but this blow me away to see one on a car and on top of that a CHEAP car by todays standards.

 

[XS650]

quote:

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Originally posted by kenw:
Your observations are correct. the placement of the air pressure probes are quite a science unto themselves. There are many requirements for spacing, distance, etc in order to get accurate values.

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A good start would be to hook the hose to a piece of closed on one end tubing that runs across the airbox. Drill a few very small (compared to the tube diameter) holes along the length of the tube. Every inch or so. Something like 0.375 diameter tubing with five or six 0.062 holes

That's not real scientific, but it helps average out the errors.

 

[jason07-1zzfe]

Static pressure measurements should be perpendicular to flow.
That tubing method may cause you to measure total pressure as well.
There are static pressure tips available at dwyer-inst.com if it's not working for you measuring at the duct wall.

 

[jason07-1zzfe]

Just re-read, looks like you are using probes. You may not have them parrallel to flow which will cause total pressure measurement. I would try just measuring right at the duct wall. Should be good enough for DP estimates.