Pitot Measurement Reversal

Pitot Style Bench discussions
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KiwiFlow
Posts: 1
Joined: Sat Feb 27, 2021 3:24 am

Pitot Measurement Reversal

Post by KiwiFlow »

Hi All
I have a homemade flow bench setup using a couple of vacuum cleaners in parallel sucking through 2" PVC pipe, that expands to 4" and bolts to the cylinder side of the head. I set the test pressure once I have set the valve lift via a bypass valve near the vacuum source. I am consistently using 10" H2O. I measure the total pressure via a small tube (copper brake line) inserted into the 2" pipe with a 90deg bend into the flow, and the static pressure with similar insert onto the edge of the 2" pipe at about the same point in the tube as the inlet to the bent/total pressure tube. Both these tubes are then connected to a manometer. The differential pressure then records the velocity pressure. (Total pressure - Static Pressure = Velocity Pressure)
At low flows (under 45CFM & 2000ft/min) everything works well. Total pressure is higher than static. When I get over this flow the differential reverses, with the static side seeming to have higher pressure than the total pressure (which of course cannot happen). I thought maybe the pipe was too small for flow rate, so I changed pipe(s) to 2" (from 1.5"), but same problem at about same flow rate.
Wondering if its the insertion tubes themselves (too small?). Thinking maybe resistance/disturbance/interference at the inlet to the bent total pressure tube relative to its size might be causing issues. But before I pull it all apart again, thought I'd see if anyone else has had similar problems, or knows why this could be happening. Any suggestions gratefully accepted.
Thanks
KiwiFlow
"if it ain't broke, don't fix it"
"carpe diem"
Brucepts
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Re: Pitot Measurement Reversal

Post by Brucepts »

How big is the hole in your static pickup?

My thought is if it's to large you are getting impact pressure on the rear wall of the hole, found that when I played with pitot style flowbenches years ago. The hole should also be flush with the pipe inner surface.
Bruce

Who . . . me? I stayed at a Holiday in Express . . .
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