Another new guy,that's been lurking for a good while but is now stepping out, trying to gain a much better understanding of airflow. I built a static/dynamic "J" bend pitot tube and did a little probing and now have more questions than answers. I am seeing pressures from negative to positive as I move around in the port. I have read that the static pressures, of each tube, cancel each other out so that would seem to indicate that any air moving toward the valve should be positive. Is this right ? I also probed with a straight piece of tubing and find variation but nothing showing positive pressure. I feel a bit like the kid with the blindfold being spun around to pin the tail on the donkey ! I think I need someone to point me toward the donkey.
I will see if I can get a photo posted up. Its been a long time since I've tried to do that so it may take a day or two.
It does have one tube impact and the other static but I am sure there is more to a successful design that. I couldn't seem to come up with anything solid on dimensions so I went with the "looks about right" method. Made of brass tubing of 1/16th" O.D. for inner (impact) , 1/8th" O.D. for outer (static) with six .026 holes approximately 5 diameters from the tip.
Here is the probe in question. I seem to only be able to put one picture in the post.
The outer or static tube is on top and the inner tube was inserted through a drilled hole in the outer tube, at the bend, and an 1/8" piece over it to facilitate the use of same size tubing to the manometer (u-tube). The overall length is 17 1/2" and 13" from "J" bend to the divergence point.
It appears you have it all correct with two separate circuits for the airflow. So you are seeing static and impact and your manometer is reading the differential pressure between the two.
Your numbers are not going to be steady I can assure you on that
Bruce
Who . . . me? I stayed at a Holiday in Express . . .
I wasn't expecting steady numbers but I thought the range would all be on the negative side. I did the probing at 10" water and the range was from about 8" negative to about the same positive. The positive pressures were more towards the roof areas of the port. This is what started me thinking that maybe the impact pressure should be positive if the static has basically been removed/offset. This is also a very low angle port. Do any of you guys see pressures ranging a good ways above and below the "0" mark ?
I guess the question is if the probe is reading correctly is this type of probe usefull in determining where to start grinding or filling to even out the velocity ? Just seems like a pretty large variance.
Your design has all the elements for testing air speed in a linear environment, but i think your J leg is to long and your static pickup to far back thus in certain positions in the port the static holes are actually picking up impact pressure. You may be able to prove this by using a piece of pipe and testing then changing the angle of the probe exposing the static ports to impact air.
I have a document some place that state how to build a pitot. It states to have the static hole within 300% of the tip. Yours seem to be much further away. I feel Rick is onto the issue. btw I have a piece of PVC pipe I test in. Make sure you check the id with inside mics as the PVC is not close to their claim. If you want to have an accurate calibration you need to check this.
I think I need someone to point me toward the donkey.
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