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Posted:
Thu Apr 28, 2005 12:07 amby RRBD
Over the past few months I've constructed a small averaging pitot tube bench to be used soley for Briggs Animal cylinder heads. I'm having trouble getting it calibrated accurately. I'm using PVC that has an average I.D. of 1.360", my averaging pitot tube is .150 in diameter and has holes drilled as per duct traversing standards. I have a flow straightener in the end of the tube. Why is it when I calculate flow using the 4004.4 x sqrt of velocity pressure(6") divided by duct area in sq. ft.(minus averaging pitot tube area) I get numbers very different than when I punch in a circle with the same total area into rocco's spreadsheet?The inclined manometer has a 6" rise from 0-100% and the percent of flow scale is right from rocco's other spreadsheet which takes the different well diameters into consideration. Should I be sensing static pressure for my depression and my inclined in the same spot? I have my static for the inclined in the PVC(1/2" upstream and 90 degrees from averaging pitot tube) and the static for depression right under the test hole near a corner. What gives? Is there a difference in discharge coef's? The bench is almost exactly like the one Dig proposed.
Posted:
Thu Apr 28, 2005 8:01 amby larrycavan
Am I understanding this correctly Scott?
Are you saying that the two formulas don't calculate to the same cfm value?
Or is it that you perform a calibration test, plug in the pressure drop you actually read on the manometer into the formula and the cfm values don't come out to what they should be?
How far off are they?
Larry
Posted:
Thu Apr 28, 2005 9:55 amby RRBD
V= 4004.4 x (sqrt of Hv)
where: V= velocity in feet per minute
Hv= Velocity pressure in inches H20
Q=A*V
where: Q= Quantity of flow in cfm
A= cross sectional area of duct in square feet
V= Average velocity in feet per minute
These are the formula's straight out of Dwyer catalog
that I determined the flow of averaging pitot tube set up.
Now, I made an orifice with the same area as my flow tube and my readings are way off(around 60%) and rocco's spreadsheet says around 51 cfm and formula above says my flow tube is flowing about 86 cfm. Then I made another orifice that is flowing 86 cfm on rocco's spreadsheet and now I cant pull enough depression across it to get a reading (i'm using a 6.5 hp shop vac on this little bench)
I have faith in Rocco's stuff as well as Dwyer's and think it's me. Both formulas are not giving me the same results, I would like to find out where I went wrong.
Scott
Posted:
Thu Apr 28, 2005 4:20 pmby dougk
Scott,
You seem to be having the same issues I am(see following post "calibration question"). Hopefully, someone can shed some light.
Jocke,Bruce,Rocco?
Posted:
Thu Apr 28, 2005 9:30 pmby larrycavan
Posted:
Thu Apr 28, 2005 10:06 pmby bruce
I'm going to take a quick stab at this one, your flow tube will be flowing closer to 1.0 Cd. The orifice will be flowing at a lower Cd would be my "guess". Setup your inclined manometer to be reading your max velocity at the test press you plan on testing at. So 100% would be the max amount of air the flow element will handle at "X" static pressure. I would calibrate your inclined manometer to your bench and not what the spreadsheet says it should be reading. Are you using any correction factors?
I personally would not work with a percent scale on the manometer though, I would set it up to read in inchs and make up a spreadsheet to calculate exact cfm's from those numbers. But thats my personal preference.
Posted:
Mon May 09, 2005 4:28 pmby gaz
RRBD: I think you would get reasonable results if you use the Cd in the last formula.
Q=A*V*Cd where Cd would be 0.63...0.64
try it !?