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Posted:
Mon Apr 25, 2005 7:45 pmby 84-1074663779
Some interesting ideas there Larry.
A completely silent bench is an impossibility, there will always be air "roar" when any decent sort of flow is established. But low frequency pulsing, surging, or flutter can easily be introduced at sub audible frequencies. You cannot "hear" anything below about 20Hz, but you sure can sometimes "feel it" as a rumble or low frequency vibration that goes through the whole bench.
Posted:
Tue Apr 26, 2005 12:17 amby JRM
Posted:
Tue Apr 26, 2005 12:34 pmby Mouse
Posted:
Tue Apr 26, 2005 5:51 pmby larrycavan
Posted:
Tue Apr 26, 2005 6:03 pmby RRBD
I have read that an orifice machined in a plate no more than .125 thick is considered a "sharp edged orifice", I just so happened to have made three of them today. Mouse, send me a couple of dimensions you have had problems with and I'll make a couple and mail you to check, maybe to rule out thickness issues. I been making them out of some 3.5 x 4" aluminum I have laying around. I just interpolate them on my CNC, any size you need preferably over 5/8" diameter.
Scott
Posted:
Wed Apr 27, 2005 9:11 amby larrycavan
Posted:
Wed Apr 27, 2005 11:37 amby RRBD
In my opinion the disc thing is too limiting. I use individual plates, they are easier to seal, and way easier to put in an optimum position. Also, if you are not happy with a certain "range" it is much easier to get a new plate made than to disassemble your bench to modify the orifice disc.
BTW, Larry, in your post about calibrating the orifice's to your inclined manometer, I have been struggling with the same thoughts, any new info? I think you have to calibrate this way, I think SF sizes their orifices so there is a 6" pressure drop across the orifice at 25" of depression. What do you think?
Posted:
Wed Apr 27, 2005 12:51 pmby Mouse
Scott, Thanks for the offer. I don't have any problems with a particular size. Sometimes one of them will just flow a bit off. It really isn't a problem since it takes about 60 seconds to make an orifice, and the rejects can just be cut to a larger size. But if you happen to need some orifices flowed just email or PM me.
Larry, no, I cannot cut orifices on a disk. I cut them on 6" x 6" plexi squares, and it has to be a fairly thick non-metalic material.. sorry.
John
Posted:
Wed Apr 27, 2005 6:45 pmby larrycavan
Posted:
Wed Apr 27, 2005 8:05 pmby Rick360
Another thing which has to be considered is the pressure or depression that the range is calibrated for. A SF-300 has ranges that are calibrated for 25" test pressure/depression and a 600 is made for 28" (i think). To change from 25" to 28" requires multiplying the range by 0.997. This would change a 606.8cfm range to 604.98cfm. Test at other pressures would require different range conversion factors. There is a conversion from whatever the orifice/bench is calibrated to, to any other test pressure. These are listed in the SF300 manual, and I would assume in the SF600 manual.
These conversions would apply to home made benchs as well. This is why the exhaust range is always higher than the intake range, usually by 5-8%?.
Rick
Posted:
Wed Apr 27, 2005 10:46 pmby RRBD
Hey guys, I'm having a little struggle with a small averaging pitot tube based bench and calibrating it using a percent of flow scale. Please check out the Pitot Bench section, I'll post specifics there. Thanks..
Scott