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Dave,
Interesting post, as are some of your other posts.
When you refer to a "sharp edged" orifcice are you really referring to a "square edged" orifice? To be a sharp edged orifice shouldn't the inside edge of the orifice be machined at an angle down to a "sharp" point?
Do your calculations for "flow" convert to ACFM or SCFM?
Are we really concerned with how much air "flows" thru a head on a flowbench? It will be different depending on the atmospheric conditions from day to day, room temperature, baro, etc etc. Isn't what we really want to know how easily air flows thru a head? I don't believe that a SF (or any other typical flowbench) can accurately measure airflow. They do however measure the resistance to flow very well and will give a repeatable number in a broad range of conditions. A SF style (internal orifice) bench will compare the resistance to flow to the internal range orifice.
When you measure your accuracy and indicate they are less than 1% you are using a testing method that also has a measurement error of 1%. It is my understanding that you can never be better than 1% when using an orifice. After they are calibrated to some much better standard (such as LFE or nozzle in a lab environment) and placed into service, you are never certain it is better than +/- 1%.
Your comments about pressure recovery inside a bench (in another thread) was interesting and I appreciate the enlightenment, it was something I hadn't thought of.
I built a orifice style bench many years ago and used a head that had been flowed on a SF600 to "calibrate" it. How much the "actual" flow is doesn't matter. It will give similar results to most SF benches I've compared to, which is really only for comparing to others, which always allows for a wide margin for error. I use a "CFM" number to talk to others about airflow, even though I know it may not actually be that many CFM, it is just used for comparison of resistance to flow. It has always repeated very well.
Rick
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