Pitot bench - calibration

[wing64k]

Hi guy's,I have recently finished a pitot style bench, but I'm not sure how to calibrate it, anyone have any ideas.This is built like the one that Michael Doty did a story on a few years back.
Thanks

[Mouse]

Using a calibration orifice is best to set your initial trim factor on your Pitot location area.

While you flow test your calibration orifice, you simply adjust the area factor where your Pitot tube is located until you get the proper flow reading for your calibration orifice at the specified test pressure. That's it!

John

[wing64k]

Thanks John,But I'm a little confussed on the area factor?

[Mouse]

The area factor is the area as a cross section of the bounded path that your Pitot tube is located, in square inches or square feet. This determines the volume of flow from your velocity reading.

John

[hollywood63]

Mouse, Could you explain that a little better? The way I set mine up was using a formula for the length of the tube the pitot is in. I believe it was like 2/3 of the way after the last elbow. What I do for calibration is put on a plate that the Bruce made then run 28in of depression and check my incline reading. Using this as 100% then comparing readings from a head. ie if 24" on the incline is 100% (287 cfm through a 2" hole) then 6" would be 50% (143.5cfm) am I correct in this or out in left field.
Thanks Art

[Mouse]

Hi Art,

Sorry, I am not going to get into the incline manometer thing...just not into mechanical manometers. Life is too short (translation...I never took the time to figure out incline manometers).

There are many formulas and techniques for measuring flow volume using Pitot tubes. The short of it is to calculate the velocity, then multiply by the area of the bounded path where the Pitot tube is located to determine the volume of flow. By adjusting the area value, you can effectively calibrate your Pitot tube bench.

John

[wing64k]

Hollywood, Was your test plate flowed on a commercial style bench to get the 287 cfm through your 2" orfice..On my bench I have a Dwyer velocity manometer which measures in velocity and then converted to cfm depending on the tube area where the pitot tubes are located. I will make a 2" test plate and flow it and see what I get,I would think it should be close to your reading..how thick is your test plate so I make it the same.

[Mouse]

One thing I have found with Pitot tube systems is that they usually do not produce a great deal of differential pressure for automotive part flow testing. Also, a mechanical manometer overflow is more troublesome because you may have to dissasemble your Pitot tube from your system and carefully clean the fluid out of it, if possible.

For this reason, you may want to consider electronic pressure sensing with a Pitot tube system. Most Pitot tube systems are like using a single large orifice on an orifice style bench - they produce very little differential pressure, requiring electronic pressure sensing.

Just my opinion, but Pitot tube flow systems using electronic pressure sensing is the best of both worlds when it comes to flow volume measurment.

John

[hollywood63]

Wing,
Sorry for the incorrect number. The plate was made and flowed by Bruce. If memory serves it flowed 274cfm at a calculated 28". I believe Bruce does his testing at 10". I got the 287 number from a formula that is on the forum. It is for Max flow through a orfice.
Art

[wing64k]

OK,Here's what I have when I tested with a calibration plate from SF, it is a sharp edged plate that flows 106.9 @ 15" h2o and 146@ 28" h20,on my bench it flows 107.3@15 and 146.9 @28 when I tested at a lower pressure(10")it flows at 83.4,when calulated it should flow 66.85..Anybody have any ideas why at lower pressure my flows are so far off,could it be that my flow element where my pitots are located needs to be a smaller diameter to keep velocity up,at this time it is a 3" tube approx. 5' long?

Thanks,Dean

[Mouse]

Dean,

What do you have for a Pitot tube? Is it an avaeraging tube?

John

[Mouse]

Dean,

My calculations show your orifice is about 1.43" diameter, and it should flow 87.5 SCFM @ 10". If this is true, good job on your flow bench!!

John

[B. Elliott]

What math is involved with calculating the max cfm thru an orifice? can't seem to find it searching.

I made a 1/8" plate of aluminum with a round 1.004" hole while i was working at a machine shop. couldn't I figure the max cfm thu that hole?

[86rocco]

The air flow through an orifice is proportion to the area and the square root of the pressure difference across the plate.

There are a few other factor involved as well like discharge coefficient which is related to the actual construction of your orifice, air density which relates to the temperature, humidity and atmospheric pressure when testing.

I posted a small excel spreadsheet that does the calculation in

[wing64k]

Let me try this again,inmy last post I gave the wrong info.My sharp edge orfice plate is 1.130",I am using a Dwyer pitot tube gut only using the total pressure port and have a static port in the flow element just flush with the ID of the tube,when I run a calibration test my readings are much higher than the calculated cfm 's.

Calculated Actual
@10"=54.8 83.4
@15"=67.1 104.3
@28"=91.6 145.0

Anyone have any ideas what might be causing this,all monometers are stable,not bouncing around,no leaks. I'm using velocity and converting it to cfm,I was never good at math, so ther could be a problem there,or maybe I just need a correction factor,I just want to believe my bench as we all do.
Thanks,Dean