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  #1  
Old 11-28-2011, 09:39 PM
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Default Help with measuring cowl pressure differential

I didn't give this much thought before I purchased a vacuum/pressure gauge. It's one of those single port devices that measures negative or positive pressure from 0 to 10 psi or 0 to 30 in/Hg. Exact model is here.

My plan was to place tubes at various locations under the cowl to read the pressure at that specific spot. I could run several lines, label them, then swap the gauge to a different hose in the cockpit while flying. But, I'm not sure if this instrument is going to work. Maybe it will....it just seems to need a pretty good amount of suction to move the dial, and I'm not convinced I'll be able to read that much variation under the cowl.

What do you guys think? I really don't want to build a manometer for inside the cockpit, so the dial idea was ideal. Any help is greatly appreciated.
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  #2  
Old 11-28-2011, 09:59 PM
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Not going to work because thats an absolute pressure gauge.
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Bob Japundza CFI A&PIA
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  #3  
Old 11-28-2011, 10:23 PM
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Nor is it sensitive enough....1 inch Hg is 13.596 inches of water. Differential pressures under the cowl are often just a few inches of water. Heck, 100% dynamic pressure at 200 knots and sea level isn't quite 2" Hg and less than 1 psi.
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  #4  
Old 11-28-2011, 10:47 PM
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Thanks guys...once I got it I sort of realized it wouldn't be sensitive enough. But, Bob, what do you mean about it being "absolute"? Couldn't I use a (more sensitive) gauge to measure the absolute pressure at various locations under the cowl, then compare the results?
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Old 11-28-2011, 10:53 PM
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Actually its gauge pressure, not absolute pressure. Gauge pressure means one side of the gauge is vented to ambient. And unless the gauge (if it were sensitive enough) would have to be in the area you are comparing for it to have a meaningful reading.

What you need is a differential pressure gauge that is sensitive enough. Something like a Dwyer Magnahelic.
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N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
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Last edited by rocketbob : 11-28-2011 at 10:56 PM.
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  #6  
Old 11-28-2011, 10:54 PM
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I'm guessing something like this is the right thing???

http://www.transcat.com/Catalog/prod...tm_medium=base

I think the difference is measuring relative (AKA: differential pressures) vs absolute pressures (not relative differentials). I don't know, but I'm just guessing that's the idea since this device measures differential.

I might snag one if it's the right tool. It sounds like something that could be worthwhile and save a whole lot of frustration.

Phil

Last edited by Phil : 11-28-2011 at 11:00 PM.
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  #7  
Old 11-28-2011, 11:15 PM
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Quote:
Originally Posted by rocketbob View Post
Actually its gauge pressure, not absolute pressure. Gauge pressure means one side of the gauge is vented to ambient. And unless the gauge (if it were sensitive enough) would have to be in the area you are comparing for it to have a meaningful reading.

What you need is a differential pressure gauge that is sensitive enough. Something like a Dwyer Magnahelic.
Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.
Differential pressure is the difference in pressure between two points.

So, why can't I use Absolute or Gauge pressure and read either at various locations under the cowl? That would provide the data I need. I really don't care about the values as much as I care about point A being higher/lower than point b, c, d, and e. If I'm just getting a number at each location, wouldn't I know how air is flowing in and out of my cowl?

Sorry for my ignorance...just trying to find an easy solution that I don't have to "re-rig" for several different test flights. And, I don't want to pay $200+ for an instrument (sorry Phil ).
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Old 11-28-2011, 11:22 PM
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Yeah, I hear ya.

I just wrote the check for a new IO-540. So from the new IO-540 perspective, $200 is a really cheap way to get some really helpful data.

I agree it's painful, but I don't want to sink any more money into engine repairs when I could just take care of the really expensive one I own today. So I don't mind paying an extra $200.

On a side note:
$200 is pretty easy to make up in fuel when comparing a trial and error approach with using instruments to collect exact data. It's probably close to a wash.

Last edited by Phil : 11-28-2011 at 11:27 PM.
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  #9  
Old 11-29-2011, 12:07 AM
CMW CMW is offline
 
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Default Digital manometer on Ebay

There are a bunch of new digital manometers on Ebay for $37. +/-2psi range. Here's a link.

http://www.ebay.com/itm/Digital-Mano...ht_4657wt_1010

-Chris
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  #10  
Old 11-29-2011, 12:18 AM
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Very cool. For 37 bucks, I can wait for it to come from Hong Kong!

Thanks!
Phil
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