Pitot tubes

[What]

The bench I'm building is going to based on advice from a friend who has had a lifetime of advice in airflow research. It's going on simplicity and cheapness (for my sake) while still trying to keep some of his recommended 'principles' intact. Small diameter pipe, straight pipe after bends with length 10 times diameter of pipe, and then take, somewhat painstakingly, 9 point measurements accross diameter of pipe with pitot tube (we'll see how much of the 9n measurements I do in practice I suppose). He was less confident with going on the surface of the pipe and figured I should use a 1/8" stainless tube, 90 degree bend 1" into the stream... 10" out of the out of stream before then running off to manometers. The idea behind the principles of the system is to try and get some kind of regard for laminar flow using principles apparently accepted by mechanical engineering with-out spending the money on the LFE stuff and huge plenum chambers and so on...

Has anybody access to such tubing and done this before? I can get just 1/8" stainless tubing, but only in 20ft. lengths... then I'd have to bend a perfect 90 and so on. Just wondering if anything is available out there amongst everyone for a good cost, or if I shall attempt a few hack version of making the tubes he suggests.

[bnelson]

You can buy many pre-made pitot tubes from Dwyer Instruments........and they are pretty affordable.

On the web see Dwyer Instruments at:

www.dwyer-inst.com

9pts?? Why not just use an averaging device?? say either a super trick pitot array or better yet an anubar.

[What]

I wouldn't mind trying both ways actually.... just wondering if there will be changes that you will be able to see only in different points that you might not see having it pre-averaged.

Don't suppose you have one of your averaging types available for something as small as 2" ID pipe?

[86rocco]

[98-1074649673]

My annubars are drilled to the industry standard dimensions for "duct traversing" in the airflow industry. If you want to machine them yourself look up duct traversing on the net for the dimensions. I have found them to be as accurate as they need to be for our application (well my application for my flowbench)

As you can see I make them to fit in various standard PVC pipe diameters they are 1/8" brass tubes. I'm not at all trying to sell u on my annubars. Trust me on this I sure don't make alot of money on them. The holes are machined on my milling machine and not a drill press. Someday I might actually make a fixture to drill them instead of each one individually. I'm simply offering them to others who want to build a simple accurate flowbench. I also want to stress I only setup to machine them when I have an amount worthwhile and the time to do them. And I stress the "do-it-yourself" part . . . I'm not here to go into production making annubars*

* Unless of course I get a BIG order and I can retire making them

[What]

[98-1074649673]

Annubars can be made to any inside dimension you need, I just have to do the math to know where the holes go. Since I do not use a fixture to drill the holes I can change them to any location you need them in.

I do not find any adverse airflow due to the joints in the PVC connections. My bench flows exactly to the calibrated sharp edge orifice I use for calibration. As long as its repeatable, thats what u need to be concerned with I have found.

[What]

I can understand the repeatability and that is something I'm after. The steel piping I already have from various other projects, so no sense in buying new stuff. Besides, I don't have much in the way of bends to worry about so there will only be a few minutes of welding anyway. 1- 90 degree in the entire system. I've made the vacuum box moveable which means I don't need to compensate for any bends to change the direction of flow... I'll just move the seperate vacuum box forward or back a few inches to match up the inlet or exhaust to the pipe going to the rest of the system. The smaller 2" pipe lets me keep my head off the ceiling and still do the 10x diameter rule amongst the restrictions and placing of the pitot tubes.

As for diameters, I can't seem to find anything right now... posted ODs of the thin wall and can't find either of my good calipers to properly measure. The pair I did find measured 1.85" ID which I suspect is close. If you are ok with setting it up differently... let me know where to send the funds and how.