Calibration pressure - 28" or much higher?

[SWR]

Hi All,

working on calibration of my new "basement monster",and I was wondering about if I should calibrate it with test orifices made to test at 28",or if doing the calibration at 70" or 80" would be better/more accurate.

Posted this on Speedtalk, Mr Coyle promptly sent me here.

[Tony]

Hi SWR, I noticed your post over there at Speedtalk, welcome to our Forum.

In practice it makes no difference to the measurement accuracy, the numbers all just become very much larger at a higher test depression. But big numbers by themselves, don't always mean a greater precision of accuracy.

Accuracy and repeatability have a lot more to do with being careful and methodical and developing and understanding your own flow bench.

The only real problem with testing at a really high depression, is the huge amount of horsepower, noise, and electrical current required to drive the flowbench. But if that is not a problem for you, then go right ahead.

For accuracy, what matters most is what you are using to measure the flow, not the actual amount of flow produced.

[SWR]

Thanks Tony,

Main reason for testing/calibrating at a higher depression was that I thought I would see any errors in calibration easier,when the largest flow orifice pulls through a (comparatively,quite a bit) smaller,accurate test orifice at a higher depression .

Doesn't this hold true?

[larrycavan]

My experience has been that if the calibration plate values were derived at 28", then calibrate at 28". I have plates that were flowed at 6, 10 & 28". Three different plates.

I calibrated my bench at 28 with the proper plate for 28. It never exactly matches any of the other 2 plates but it's really close. For example I have a quick reference plate flowed on an SF110 on a 3" bore stand @ 6" that flowed 54.6CFM. When I pop it on my test stand it flows within 1 - 1.25 CFM every single time.

If I flow a head at 28" and convert to 10", the converted value is generally a couple of CFM higher than if I actually flow at 10".....I don't worry about it because it so close and very, very repeatable.

[SWR]

[maxracesoftware]

when we were passing those 3 Orifice Plates around
to be Flow Tested amongst ourselves,
one of the plates had a slight bow.

for 2 of the plates, the FlowBench's vacuum depression
was sufficient to "seal" those 2 plates

the other plate i had to push down on the plate
to create a good seal...so depending...i saw a variation
of around 2-3 CFM


with SF-600 FlowBench Manual , there is a secondary set
of Flow Correction Factors as you change Orifice Flow Ranges
and Test Pressures

by using those Seconadry Factors, my SF-600 Bench was
closer to test results others saw with Plates

the above might have a little bearing on your Test Pressure Conversions ??



Also,
i can Port lets say , 4 exhaust ports on 1 Head
and Flow Test and see they all are Flowing around
260 CFM at 28"

then i'll up the Test Pressure to 36" inches,
they might all still flow pretty much the same CFM,
after being converted back to 28"
or maybe 1 port might show higher converted back to 28" ?


then i might up the Test Pressure to 48" inches,
now 1 or 2 of those Ports may flow substantially higher
and 1 of the other ports might not gain a lot more CFM
(after CFM is converted back to 28")

this happens every so often,
thats why i like to double-check Flow gains
at the highest Test Pressure i can achieve on my FlowBench
and convert back down to 28"
and see if my 28" Flow Numbers correlate to my converted
28" from that 48" Test

sometimes especially in Exh Port Flow,
there is a great deal of Flow difference !

On some Heads , an Intake Port will act the same way,
but overall, you hardly see that same effect on the Intake side

[Tony]

[maxracesoftware]

[SWR]

[SWR]

Ok,I now calibrated it. Did so at 10",it was spot on. Problem is... when I use a test orifice and check the calibration at a higher depression (28") it starts to show a 3% too-high flow. Yet higher at 48",it's hovering around showing 4.5% too much...

Could it be the flow orifice bending with pressure due to the higher velocity through it? The test orifices are 2.5mm (0.01") alu,and stiff as h**l. The flow orifices not so.

[larrycavan]

[SWR]

[bruce]

Sharp edge or square edge orifice plates? Might be the Cd is changing on your orifice plate design?

[Tony]

SWR, the problem may be internal bench turbulence.

Here is a thought experiment for you.

Fit two identical orifice plates, one over the test hole on the bench top, and use the other as the flow measurement orifice within the bench.

Run the bench at some low test pressure, (and low flow), you will discover the two measured orifice pressure drops are identical.

Wind up the wick a bit, and increase test pressure to something much higher. Flow will increase, but the two pressures remain exactly the same.

Run the test pressure up really high. You suddenly discover that the pressure drop across the flow measuring orifice is slightly higher than the pressure drop across the test orifice. You swap orifices, the difference is still there. You swap manometers, still the pressures are different.

What this proves is that the flow coefficients of the two identical orifice plates do not remain the same. The test orifice located on the bench top always has undisturbed room air entering. The measurement orifice located deep in the bowels of the bench may be getting some pretty turbulent air. So the test orifice appears to "pick up some flow" at higher air volumes.

It is not the high test pressure that causes this, but the increase in flow volume through the whole bench. At some point the flow paths and internal air volumes of any bench will just become too restrictive for the measurement orifice to work in clean air.

Carry out this test for real, and learn what the limits of your own bench are.

Testing at extreme test pressures and flows is not guaranteed to give any higher accuracy. In fact it can be the source of some significant errors if your bench is not up to the job.

Explore the limits of your bench with the identical orifice test. It will be a VERY revealing experience. Then either do your testing below those limits, or at least know at what sort of flow numbers the bench accuracy begins to fall over.

[larrycavan]