Calibration - Calibration

[Joe90]

Hi, im new to this forum, and i have search for many HOURS how to calibrate a flow bench that i've made.
Its a 2 inch tube and a 4.5 hp shop vac with home made simple pitot, copper tube 5/16. See the pic.
Here's my numbers: 10 in/water at 10 in .

If Flow in CFM = 13.55 x square root of pressure (inches of water) x diameter of orifice in inches squared.

So it pull 10" of suction through a 2 inch tube

CFM = 13.55 x (square root of 10) x ( 2 squared)
CFM = 13.55 x 3.16 x 4
CFM = 171

How do i calibrate? I do a 2 In test oricice to put on top? How do a 2 In orifice Flow? Im confused....Help
???

[Joe90]

Photo of pitot 5/16 tube facing forward

[Mouse]

Hello,

Welcome to the forum.

My best advice to save yourself a lot of time and trouble, is to buy an averaging Pitot tube and a calibration orifice from PTS (this site). Very reasonable prices, and I am sure you will get some excellent advice on how to install and calibrate your system.

John

[2seater]

I must second the recommendation of buying the averaging pitot tubes from Bruce and possibly a calibration plate. The one thing I can see from you photo is the same problem I had. The static pickup is too large and not placed properly for consistant readings. I was seeing consistantly high flow readings and Bruce helped me sort it out. The static tube is connected to the upper end of the inclined and "pulls" the fluid upwards. If the static pickup is not flush with the side wall and very small diameter, the air movement past it will send a stronger signal than actual. I did as Bruce suggested and drilled the static pickup hole only partially through the side wall. I finished the hole through the inner wall with a small number drill, .020" in my case. The static tube was then epoxied in place since it has little support. It should also be placed a small distance upstream, 1/2" or so, from the pitot tube or it may pickup turbulence from the air flow past the pitot. The pitot tube also "sees" the vacuum inside the flow tube, and tries to pull the fluid downwards in the inclined but the signal is reduced by the impact velocity into the pitot orifice(s), so the static signal is greater and the fluid rises. One other trick I found is to establish air flow and connect only the static pickup to the upper end of the inclined. Leave the reservoir open to atmosphere. Be careful you have enough range in the inclined or reduce air flow. Note the reading. Now connect only the hose from the pitot tube to the upper end of the inclined. The fluid will rise but the reading should be lower at the same air flow rate. Rotate the pitot tube back and forth to obtain the lowest reading. That will be the proper position for the pitot opening. To be sure your inclined is calibrated properly, leave the reservoir open to atmosphere, connect only the upper end to a tee connection with the vertical manometer depression pickup in the settling chamber. Gradually block the test opening with air flowing and verify that the reading on the vertical is the same as the reading on the inclined. Try this at several points to be sure the inclined tracks the same as the vertical and you should be all set.

[Joe90]

[color=#000000]Thanks a lot for the advise! Im working on this this week end!
Eventually i will buy a real calibrated pitot, but i like building things and discover... that i should buy a pitot
I dont need to be very precise. Im a mecanic teacher in Qu

[bruce]