by larrycavan » Sun May 15, 2005 11:38 pm
JRM
I can understand where you're coming from. Hang in there. I'll bet anything you can solve your calibration issues with a little patience and persistence.
I've just finished calibrating after making changes to my bench. At this point, I've got one problem to resolve but the calibration seems to be coming together. Here are the numbers I got today from calibration tests.
I decided to test at 6 different pressures and compare the results of three ranges at each of the pressure intervals. I felt this would be the most accurate way to compare ranges on the overall.
Also take note. I discovered that my coefficient is higher than what Superflow holes seem to calculate out to. I also found in searching through the archives of the forum that others have found that as well. My flow disk is 1mm thick and the holes are punched from the top side.
MSD calls this 180 CFM
Range % CFM Pressure
161.00 50.0% 80.5 3.00
161.00 58.5% 94.2 4.00
161.00 66.0% 106.3 5.00
161.00 72.5% 116.7 6.00
161.00 84.7% 136.4 8.00
161.00 95.0% 153.0 10.00
MSD calls this 255 CFM
242.00 34.0% 82.28 3.00
242.00 39.5% 95.59 4.00
242.00 44.0% 106.48 5.00
242.00 48.0% 116.16 6.00
242.00 56.0% 135.52 8.00
242.00 63.2% 152.94 10.00
MSD article misprint for 345 CFM [2] 1.5" holes & [1] 2" hole
I blocked off one of the 1.5" holes with duct tape. I calculated the flow at 345. It was higher in real life.
386.00 21.0% 81.06 3.00
386.00 24.3% 93.80 4.00
386.00 27.5% 106.15 5.00
386.00 30.2% 116.57 6.00
386.00 35.0% 135.10 8.00
386.00 39.5% 152.47 10.00
Here are some other things I tested for as well. Hopefully someone else can gain some useful information from it. I put a lot of effort into this and tried things that have raised questions in my mind for a long time.
After installing a new top with a baffel I found that no matter where i located the static pressure pickup, [just not directly under the discharge hole] my readings did not change in terms of % flow. I tested with it in several locations and pointed it in several different directions as well.
I found that if I got it too far away from the flow I could not dial up as much test pressure. When that happened I dialed up what I could reach and recorded the % flow. After settling on a spot that allowed me to read the test pressures I want to be able to test at, I compared the readings and they were identical to the readings of the other location where I ran out of test pressure.
I personally don't feel that putting the test pressure probe in the cylinder adapter is the way to go. Back when I first did that, I could move it back and forth in the cylinder and my pressure would change.
Next I hooked up a piece of 1/8" id clear tubing to the manometer probe in the top plenum. I left it long enough to move it anywhere in the top plenum. I tested every possible location inside the plenum and again found that it really didn't matter where I put the end of the hose. The readings never varied by more that 1/2% on the inclined manometer.
Next came my greatest suspect for flow inconsistency, the flow disk itself. With my new top design I can run the bench with the top off and reach down to the flow disk. [not measuring any test pressure] I found that it leaks air underneath it in the lower ranges. It's not perfectly flat. I could push down on the disk, effectively sealing it on the pie shaped opening and watch the manometer pick up 4% in the 28cfm range. It's leaking air past the holes for the 56cfm range on the bottom side of the flow disk. Fortunately, only the 28 & 56 cfm ranges were affected to any degree by this. My other ranges showed no more than a 1% change so they do seal much better and i trust their readings.
In any case, that situation can be resolved. It would be fairly easy to prevent if starting from scratch.
Larry