PTS Flowbench Forum

Tony wrote: Check out a section of this Forum "Useful programs and spread sheets"
All the "top secret" orifice plate magic and formulas will be revealed.

I've got a spreadsheet there under the heading "Flowbench calculations" that'll give you just about everything you need.

I'm a bit of a math geek so I'm a big proponent of learning and understanding the underlying math behind things. It's a great BS detecter and keeps you from going too far down the wrong road when you're trying new stuff.

Ed

Got my table done today.Not sure on the size of the surge tank I will need.Vizard's book says not less than 35 gallons, not sure what others use.I took a speakerless 40 liter "Blaupunkt" sub woofer bass tube and closed the speaker opening.Made a cradle to mount it in the table.It's a far cry from 35 gallons (10.5),but my reasoning is closely matches my shop vac displacement.We'll see.

Since I mostly goof around with 350 chevys,I will be using a 4 inch inlet ,dropping straight down into the the speaker/surge tank.Not sure on how tall the inlet it should be.I was thinking equal to the stroke,if it even matters.

Tinkered around on a water manometer to read 28 inches.Even at 20 up and 20 down ,to stay well out of the bend, this thing is to big for my taste.I looked at some digital manometers for measuring differential pressure.Not sure if they register below 2psi though.Anybody ever tried one?

I'll check out the spreadsheets.
This thing with measuring differential pressures across the orifice plate strikes me as similar to measuring voltage drop across a resistor.

Alan

You are definitely on the right track Alan.

An e-bay digital manometer usually measures pressure in multiple units, including inches of water.
The problem with the cheaper Chinese ones is that they are sometimes not too accurate.
Especially after they have been dropped or over pressured a few times.
Always good to have a water manometer to check it against to make sure, even if you never connect the water manometer direct to the bench.

Anyhow, a low cost digital manometer is rather like a ten dollar multimeter. It will probably work fine when brand new, and it will get you started, and that is the main thing.

Yep.
A low cost digital vacuum meter will probably need to be re-calibrated from time to time,as with any thing of this sort.Could possibly be checked with an old fashioned water manometer.

I looked at the excel spread sheet for the orifices.Diameters vs cfm are clear but I don't see what thicknesses are being used.

Alan

The simple answer is "negligible" thickness compared to hole diameter.
A piece of thin sheet metal will do that easily.

With thicker material the hole can be chamfered, so the sharp edge is on the up stream side.
This then effectively becomes a thin orifice plate, but made from thicker stronger material.

Thanks,
I was looking at the Helgesen calibration plate in one of my books.Is thicker, but chamfered.
I'll just make a couple of orifice plates from some 2mm aluminum plate and see where it takes me.
Gonna spend the rest or the day working on a decent sized Manometer.

Alan

One interesting test you can try is to make two identical orifice plates.
Place one over the test hole, and the other inside your settling chamber to measure flow.

The one on the bench top will be feeding from undisturbed room air.
The one inside will inevitably see some turbulent flow.
If your bench is perfect (?) both should have identical differential pressures.
Swap the two plates over to make sure.

This test is easy with a small orifice diameter and low flow.
As the orifice diameters both become larger, flow and turbulence through the settling chamber both increase.
It gets really difficult at maximum bench flow, and that is where fitting and experimenting with baffles in the settling chamber can show some worthwhile improvement.

What I have so far:

Started with an old table on wheels.


Found something quick to use as a settling tank.Took an old bass tube,and pulled the carpet off.Also closed up the speaker opening.Looks kinda crappy now with the glue residue.Will see what I can do to make it look better.


Made 2 cradles to hold the Tank.




Tank in the table




Table top from a sturdy piece of kitchen counter top.As it is now,I'm not sure if my test cylinder is going to be tall enough.At four inches height,it will be just in the tank with about an inch and a half sticking out of the table.I kinda wanted to use a piece of clear acrylic so I could look at swirl properties,but I don't think I will be able to see anything.How important is the length /height of the cylinder anyway?


And the backboards.Not too shabby for a cost of 2 bucks for the measuring tapes.Everything else was just sitting around.



Alan

Got both manometers constructed now.I'll try to get some pictures up this evening.
My U manometer uses a simple 20" up 20" down scale.I haven't decided on if I want an inch scale on my inclined manometer,or if I should go the "percent of flow" route.I'm not really clear on how to even read the inclined manometer,or how to figure the CFM flow from it.I know it has to do with the pressure differential across the orifice plate at a preset/constant test pressure, but could someone explain the math behind this witchery?

Alan

A pretty good explanation is available here :
http://www.flowbenchtech.com/forum/view ... hp?f=3&t=5