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Old 11-26-2020, 02:57 AM   #1 (permalink)
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Magnehelic gauge measurements

I am not sure if this or the "instrumentation" thread is more appropriate so mods please excuse me

I just had a morning drive to check if my magnehelic gauge was working and to start I checked if my oil cooler was receiving any air ; being behind the front bumper and only a small corner facing an opening

two 4mm tubes were installed one in front of the oil cooler and one behind facing downwards, then through a very small opening in my passengers window

results: With both high and low ports connected the gauge was showing 0, zero , nada , nothing

it added to my speculation that I had bought a faulty gauge. So I disconnected the low port and behold! 80 pascals
connected it again and dropped to zero again. removed it again and raised my speed from 80 to 100 km/h and was raised to 90 something

then for a final treat I tried opening the window to see any influence. and yes, once I opened it it went from 90 to 100+

so to sum up: does this mean my oil cooler does not receive any fresh air? it does but relative to the inside of the cabin (low pressure area? ) ...uhm....dont know

since this was my first measurements please be kind. I would also like to measure my water radiator, intercooler, and also hood pressures with this method but I am not sure what to expect

read a book of a member in here but I just want to be sure what I actually saw the gauge do

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Last edited by eagle; 11-26-2020 at 03:03 AM..
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Old 11-26-2020, 03:03 AM   #2 (permalink)
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If the comparison was before and after the oil cooler, and the result was zero, then yes there is no airflow through the cooler. (The cabin pressure is not relevant unless a gauge port is open to the cabin - not required in this case.)

It's amazing what you find when you do some measurements!

Now, how to fix the issue. Perhaps some pics of the installation so some suggestions can be made?
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Old 11-26-2020, 03:06 AM   #3 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
If the comparison was before and after the oil cooler, and the result was zero, then yes there is no airflow through the cooler. (The cabin pressure is not relevant unless a gauge port is open to the cabin - not required in this case.)

Now, how to fix the issue. Perhaps some pics of the installation so some suggestions can be made?
yes the low port was open to cabin pressure, thats when I saw the gauge "working". thats why I am asking tbh. why was the gauge working with the low port receiving cabin pressure?


I will try and post some pics in the afternoon
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Old 11-26-2020, 03:17 AM   #4 (permalink)
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If you are measuring flow through a heat exchanger, forget the cabin pressure. The relevant criterion is pressure difference before/after the heat exchanger.

The Magnehelic gauges are very reliable. If you can gently blow on a port and get a needle deflection, it's a 90 per cent certainty the gauge is fine. Best to trust the measuring tool - I've often found I've disbelieved measurements too... but they've been correct.

It's not unusual for heat exchangers to be doing little (eg in my measured experience, the standard intercooler on Peugeot 405 diesel).

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