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Posted:
Fri Dec 09, 2005 11:36 amby blown928
I'd like to build a simple (KISS) pitot bench. The drawing below is an adaptation of Dig's drawing (Thanks once again, Dig!), made to fit my welded steel frame and allow for sucking and blowing. The drawing shows the bench in 'blow' mode. By moving the plug, cover plate, and flow tube to the opposite sides it'll be set up in 'suck' mode. Oh yes, I'll have to turn the averaging pitot tube 180 degrees also. The 90 degree bend at the top is just to keep from blasting straight up onto the ceiling when I'm in 'suck' mode.
Each motor is wired to it's own switch and has a check valve to prevent backflow. One motor is running on a router speed control to fine tune depression.
What do you guys think? Am I on the right track?
Posted:
Fri Dec 09, 2005 12:59 pmby Mouse
Very interesting. At first glance, looks good to me. What does KISS stand for?
John
Posted:
Fri Dec 09, 2005 1:08 pmby 86rocco
John, I'm surprised you've never heard of the KISS principle, it's been put forward in one forms or another by everyone from Albert Einstein to Homer Simpson, it stand for Keep It Simple Stupid.
Posted:
Fri Dec 09, 2005 2:23 pmby Mouse
Yikes! I really bit on that one.
You could have two flow elements and just leave them in place on each side?
John
Posted:
Fri Dec 09, 2005 2:28 pmby blown928
Posted:
Fri Dec 09, 2005 3:41 pmby 86rocco
I opted for the orifice type bench but I'm trying to keep mine as simple as I can too, here's my design, for clarity, I've omitted a few internal baffles from the picture. For access to the stoppers and as doors for the airbox section, I'm using 6' marine deck plates, they're easy to open and close, they provide a positive seal and costs only $6.00 each at a local marine supply house. And control of the airflow will be done electrically with switches and a variac.
Posted:
Fri Dec 09, 2005 3:43 pmby 86rocco
Oops, forgot to attach the picture.
Posted:
Fri Dec 09, 2005 5:36 pmby blown928
Rocco, why did you choose the orifice type? I have not heard a strong argument for one type over the other. I choose pitot because I thought it would be easier to calibrate. Just get a calibration plate with known flow characteristics and make a fudge factor.
Yours does look pretty simple though. Is it built & working yet? Are you running a check valve on the motors? PICS, PLEASE!! 
Posted:
Fri Dec 09, 2005 5:54 pmby Mouse
Second glance, and I just remembered one point: you may have balance problems when everything is not lined up in a straight line. Balance is the ability to measure the same flow rate of a concentrated air stream no matter where it is located over the test hole. So while a head may measure a flow rate, you may not get the same results if you turn the head 180 degrees and remeasure. Not a big problem if you remember to always mount the head in the same way.
John
Posted:
Fri Dec 09, 2005 6:18 pmby 86rocco
I choose the orifice type because I think it's the more versatile design. The nature of the inclined manometer set-up used to measure flow in both orifice and pitot tube type benches is such that they are most accurate near the top of their range and so it's useful to be able to change ranges so that you stay in the upper portion of the manometer's scale and orifice type benches offer the easiest means of doing this. But, if you're using some type of digital read out on your bench this is less important and if the things you are testing consistantly flow within a narrow range, it's not important at all.
I haven't built it so there are no pictures yet but I'll be starting soon and I'll keep everyone posted. The only really significant detail I've yet to work out are the check valves for the motors. Calibration shouldn't be much more difficult than a Pitot tube type except that I've got to do it several times, once for every orifice combination.
Posted:
Fri Dec 09, 2005 11:32 pmby Thomas Vaught
Ed,
I like your design.
I would put a baffle under the cylinder hole to break up the flow toward the orifices though.
Tom V.
Posted:
Sat Dec 10, 2005 12:59 amby 86rocco
Thanks Tom. Baffles are part of the design, I thought they'd clutter up the drawing so I left them out. The upper chamber will be about 16" high,18" deep and 34" , the orifice plate will be more or less centered front to back so I was thinking of a large baffle about 4" below the cylinder hole extending under the whole top to within 2 or 3" of each side. Do you think baffles of some description below the orifice plate would be a good idea too?
Posted:
Sat Dec 10, 2005 3:21 pmby Thomas Vaught
Ed, if the flow is in both directions I would still put the baffle in the lower chamber too.
The blue bench uses the flow valves on either side of the lower chamber so the air on exhaust is not directly in line with the orifice hole. The "bowl" also forces the air to dissipate into the lower chamber in all directions.
You have no valving in your bench (other than the individual motor check valves so you may have an issue without the lower baffle. JMO
Tom V.
Posted:
Sat Dec 10, 2005 4:07 pmby 86rocco
I sort of figured as much, thanks again Tom.
Hey Bill, It seems like I've accidentally ended up hijacking your thread, in the wrong forum even, I hope you're cool that.
Posted:
Sat Dec 10, 2005 6:07 pmby blown928
No problems mate. We're all after the same thing, and learning from each other to get there.
I like your plan for the bench. I was thinking along those lines myself before choosing the pitot style. I have some sketches that resemble your plan, just didn't go that way.