Problem With Bench Calibration Using A Superflow Type Plate

[stef-1]

Hi All,

I've had my bench for over a year now and not really bothered much about comparing it to Superflow figures until a little while ago. I have to develop a new head against a "benchmark" set of figures obtained on a Superflow bench.

I decided to make a copy of the Superflow calibration plate. After a little while I have mangaged to get the full dimensions of one and had an exact copy made.

Now, Superflow state in their SF600 manual that the calibration plate with both of the holes open should flow approximately 238cfm at 25" of water at standard conditions. They also state that if the bench is within + or - 3% its ok.

I tested the plate on my bench and it flowed 265.1cfm at 25" (at 20*C). Corrected, it's 248.1cfm. My bench seems to be over reading by ~4.2%.

I don't know why? There were no leaks. The orifice plates in my bench are sharp edged made to ASME/British Standards and the calculations for the flowrate are also done to the standards as well as the calculation for the discharge coefficient. I double checked the calcs when I did the spreadsheet ages ago. I guess I will have to go over them again. I double checked the diameters of the calibration plate and that was correct.

I know that I could apply a correction factor to my test results to match the Superflow numbers but I would like to know why it's over reading. Or, should I not worry about it?

[Brucepts]

My take on this; The SF plate is probably a production stamped orifice hole and not as precise as everyone thinks.

I know this to be fact on their internal plate on the SF600, the hole "quality" is not that great the holes are laser or water jet cut.

Did you compare your machined "SF" plate to the actual SF plate?

[200cfm]

"should flow approximately 238cfm at 25" of water at standard conditions. They also state that if the bench is within + or - 3% its ok."

If you assume a 3% plus superflow error swing the value becomes 245 cfm and your % swing is not that much.

[stef-1]

My take on this; The SF plate is probably a production stamped orifice hole and not as precise as everyone thinks.

I know this to be fact on their internal plate on the SF600, the hole "quality" is not that great the holes are laser or water jet cut.

Did you compare your machined "SF" plate to the actual SF plate?

The Superflow plate is laser cut. Mine has machined holes. The plate was also lapped on both sides to make sure the edges were sharp and no burrs, etc.

I am going to retest at the weekend as I also need to check it on exhaust mode.

I wonder what standard that Superflow use for calibration and what formulas they use to calculate the flowrates of their internal orifice plates?

[Brucepts]

I wonder what standard that Superflow use for calibration and what formulas they use to calculate the flowrates of their internal orifice plates?

Contact SF and ask what they say??? I doubt they will tell you what they use, imagine that?

[1960FL]

I guess i will ask a dumb question, DO you want to calibrate to SF standard or to what is scientifically correct? Just because they are the biggest does not make them correct.

stef you appear to have access to one so flow your calibration plate on a "Calibrated" SF bench and then you will have the real answer.


Rick

[stef-1]

I guess i will ask a dumb question, DO you want to calibrate to SF standard or to what is scientifically correct? Just because they are the biggest does not make them correct.

stef you appear to have access to one so flow your calibration plate on a "Calibrated" SF bench and then you will have the real answer.


Rick

Rick, I had always aimed my bench to be scientifically correct. However, I re-tested the bench again on Saturday. This time around I refitted the calibration plate to the bench with some fresh sealer and shorter bolts. The bolts I used last time were a little too long and I had to use a few washers under the head to get the correct grip length.
The results were;

238.3
236.2
238.7
241.1
238.7

Avg: 238.6 cfm

All corrected to standard temp, etc. I was damm sure I had no leaks last time (I checked with smoke) but I must of somewhere. Shows you cannot be too carefull with setting up.

Exhaust testing showed an interesting trait with using a large diameter orifice on top of the mounting board. The combination of diameters, board thickness changed the orifice Cd to something like 0.745. I need to figure a way to fit the orifice on the underside of the mounting board or something else. I noticed the big PTS bench has a metal mounting plate for the orifice. Perhaps I need to figure a way to do the same...

[Brucepts]

Exhaust testing showed an interesting trait with using a large diameter orifice on top of the mounting board. The combination of diameters, board thickness changed the orifice Cd to something like 0.745. I need to figure a way to fit the orifice on the underside of the mounting board or something else. I noticed the big PTS bench has a metal mounting plate for the orifice. Perhaps I need to figure a way to do the same...

Just for reference on the small PTS flowbench the Cd changes to somewhere around 0.695 in exhaust mode. Not sure what happens on the large style as I have not tested it yet in my shop. I plan on building a PTS large flowbench at some point maybe this Winter. I first have to build myself a second small PTS flowbench.