Put your "calibrated" plate on top of a test fixture and see what your bench says it flows compared to flat on top with no adapter. Why does it read different if the plate is "calibrated" for xxx flow at xx"wc? Shouldn't it always flow the same? Because EVERYTHING around an orifice affects the flow thru it. The size of the air box, the diameter and thickness of the hole the plate is positioned over, the distance to any other objects before and after the orifice. Therefore calibrating your bench to another bench of a different construction or design using an orifice mounted on top as is commonly done wont guarantee you anything about how they will measure heads.Carnut1 wrote: ↑Thu Mar 22, 2018 12:52 pmRick, if you have a more accurate method to calibrate a flowbench I would be interested in the procedure. Thanks, CharlieRick360 wrote: ↑Wed Mar 21, 2018 11:11 amThinking that two different flow benches that are "calibrated" using so-called "Calibration plates" will measure flow thru a head the same is absolutely wrong. Calibrating flowbenches in this manner, that are the same exact model, might get you within a few percent of each other. When the flowbenches are different design, you have no idea how close they will truly be.
Calibration plates can't have a Cd or a flow rating unless they are "calibrated" in a system. Then they will only flow that amount in that same system arrangement.
Rick
One way to calibrate it properly is to have a calibrated system mounted on your bench that would normally include 2 straight pipes with a flow measurement device in the middle like an orifice, venturi or LFE. It would need some type of flow control above the adapter to adjust flow thru the system. Keep in mind with the same head that real cfm flow will vary from day-day even though the flowbench reads the same.
Since Superflow are pretty much the standard in the industry I flowed a head on a SF600 at Ron's Porting many years ago and saw the readings and the calibration sticker with my own eyes, then I flowed the head on my bench and adjusted my flow ranges to match. Does that guarantee that my bench matches the stated CFM? nope, but it gives me the best way to compare my numbers because I have no idea how close to actual CFM a SF is. When Speedtalk did the Flow-Around HEADS that my bench came in dead center of the range of benches.
Rick