help with my averaging pitot tube

[AM91Camaro_RS]

i made an averaging pitot tube just like bruce's. i measured it all out and everything. my problem is that when its hooked up, it sucks the water right out of the top of my inclined manometer. i have the top hooked up as the static pickup and the bottom hooked to the averaging pitot tube. is that right or is it backwards? i haven't had time to try switching them yet. however, if someone could just tell me, that would be great! thanks.

[bruce]

Did you seal the one end of your averaging pitot off? This will give you a false reading if you didn't. One other idea is your inclined manometer might not be inclined enough for the amount of differential pressure you are reading? I always performed my initial tests using a vertical manometer to give me an idea of what was going to happen. Once you know its working then swap over to the inclined manometer.

This saves the cleanup time from sucking the water out, not to worry I'm sure it has happened to all of us sometime or another



Edited By bruce on 1109809933

[Mouse]

What is the range of your inclined manometer?

[AM91Camaro_RS]

my inclined manometer is 36" horizontal and just over 7" vertical. i was going for a 2" rise for every horizontal 10"s. i am using 1/4" OD plastic tubing for my inclined and u-tube manometers. yes, i silver soldered one end of the averaging pitot closed. also, it will suck the water out of the inclined manometer without anything on top of the bench (no differential pressure to speak of). i have 4 large vacuum motors in my vacuum box and copied some plans that i found for all of the piping. i have not done much of a test yet to see how much defferential pressure i can pull with the 4 motors. i pulled about 16" with just two of the motors the other day. that was pulling through all of the piping and a LT1 SBC head with the intake valve opened to about .525" lift.

i am soo new to the flowbench scene...if you can give me any advice, i would really appreciate it! i can get pics of my setup if that will help any.

edit: also, can someone tell me where i can find a formula to figure the air velocity (feet per minute). i've got a formula to convert to CFM from there. thanks a bunch!

[2seater]

What diameter is your flow measuring tube? With the test port open, very little pressure will show up on the vertical manometer but the air flow through the flow tube will be very high. 7" of water on the inclined will be a little less than 250 cfm through a 2" flow tube at standard temperature and barometer. Two of my motors, and likely yours, will pull that much air with the test port wide open. It sounds like the flow tube needs to be larger diameter to get the flow velocity down in range of the inclined or a greater rise to the the inclined meter. My main inclined manometer is a 15" rise and it uses 2/3-3/4 of the total range to measure flows of around 300 cfm through a 2" id pvc flow tube. I use a second inlined with half of that rise for more precise measurements at lower flows. Both inclined manometers are 40 something inches long.

[AM91Camaro_RS]

so, you must have a much steeper angle on your manometer. my flow tube is 2.5" pvc (2.535" ID). wow, its seems like i keep forgetting to mention the important details! any other ones that i'm not posting? um, most of my piping is 3" with 4" at the top (for the two 90*s). anything else you need/want to know?

[2seater]

I'm sure my 15" inclined is steeper than yours but the 7.5" one is probably close to the same. The easiest thing to go would be to either tip the inclined up to a greater angle, not worrying about the correct scale, but just to see how high the fluid is actually going, or alternately, add a section of tubing to the high end of the meter and run it straight up for another foot or so and connect that to the static pickup. I have six cheap motors on my bench , rated @ 127 cfm each, probably at zero lift. With all six running and the test port partially blocked to get 28" of depression on the vertical manometer, it will pull just shy of 9" on the inclined meter through a 3" pvc pipe, just shy of 600 cfm. If you disconnect the static pickup and leave the tubing open to atmosphere, does the fluid pull backwards, or down into the fluid reservoir, when you flow some air through the flow tube with only the averaging pitot tube connected? It should. If the two pickups are not working against each other to leave only the velocity reading, the fluid will go crazy.

[2seater]

Uh, I have to correct myself, what I mentioned as a test is back asswards. If the static probe is disconnected the fluid will rise far more than normal, if the averaging pitot tube is disconnected, the fluid will pull backwards into the reservoir. Sorry for the bad suggestion. ???

[AM91Camaro_RS]

that's what i was thinking about the test but i wasn't sure. i haven't really messed with it too much.

anybody know where i can find calculations/formulas? thank you!

[hollywood63]

I will be watching for the answer to your problem camaro. My bench is doing something similar. :p

[bruce]

All the math is already on the forum in previous posts:

[hollywood63]

Camaro, I figured out what was wrong with my manometers and it just might be something to look at on yours. I feel really dumb about this but live and learn. What my problem was that I had air leaks at top of my incline where my hose was attached to a rigid tube. and also on the top of the resevoir where the hose went in.
I'm not sure how yours is set up but it might be worth a shot
Art

[AM91Camaro_RS]

i don't think i have any air leaks anywhere but i'll double check that.

thanks for the link to the thread about the calculations! i had done a search but i hadn't run accross that thread. thanks, again. it'll probably be a couple of days before i get to mess with it anymore but i'll let you guys know if i get it taken care of or if i'm still haven't problems.

[AM91Camaro_RS]

thanks again for the calculations! i set up a spreadsheet in excel that'll figure it all for me. i only have one question... if i'm not testing at 28" of water, that has to figure in too, right? is that the barometric pressure? or is the barometer pressure just the outside baro. pressure?

according to my calculations...pulling 7 vert. inches with my setup is pulling about 360 cfm. that is with nothing on the bench. pulling 1/4" (which is my lowest marking) on the inclined figures to roughly 68 cfm. that's quite a nice range! well, i like it anyways. do these numbers sound reasonable for a 2.5" test tube? (2.535" ID)

[AM91Camaro_RS]

well, i finally got it working! only problem i have now is that i can't pull 28" water when i go above .200" lift. i have for larger vacuum motor in by box. does this sound like a leak or just not enough motors?

i calculated about 53cfm at .100" lift and about 90cfm at .200" lift. does these numbers sound like they could be right for SBC heads?