How many - is enough? - For ranges..

[Ken73]

After careful consideration of my current flowbench design, I'm really wondering - how many tubes do I need? Originally, I planned on 4 tubes; 1.5", 2", 2.5", and 3" .. only to discover that I don't think anyone makes 2.5" PVC (well, at least my Lowe's doesn't carry it if someone does.) So I figured I'd leave 2.5" out and have a 3 tube design.

Then it hit me - is it possible to pull through two pitot tubes at once, have two seperate manometers for each one, and simply add the two together? i.e., only have two tubes - say 1.5" and 2" - then for the larger flow range, run both tubes? Similar to how a 3-way bulb works.. it has two seperate bulbs of differing wattage that can be turned on seperately, or both to obtain a maximum.

Are there any ramifications to this..?

[2seater]

This sounds like the bypass idea to extend the range, but since both would measured, it should be possible to make the readings additive without a problem. Maybe a little complex, but the theory shoud be solid. Twin or multiple tubes on a single scale would look like a set of carb stiks but would avoid the possibility of multiple scales skewing the results. Interesting idea. 2.5" pvc is an odd size and would probably have to be ordered. I lucked out and found a scrap piece at work. Adapters are almost impossible to find too. I had to make a homemade bushing on the ends to fit a 3" fernco.

[Ken73]

Then I guess that begs the idea - how about two 2" tubes? That way you know you're getting the exact same measurement from tube to tube, you just multiply the reading by two if you're running both. (Or three or four 1.5" tubes?) Run knife gates on all of them....?

[2seater]

I guess I would never assume the flow is equal through one or more tubes, at least not without testing. It sounds appealing, and would be convenient if it worked, but air flow can be a funny thing that sometimes defies logic. I would certainly verify the theory with an independant flow tube to gather he total in or outflowto see if it agrees with the projected results.

[trader]

According to Andy Orlowski (tech) at Dwyer, and in reference to their DS 300 series flow sensors, I quote:

"To achieve laminar flow, we normally recommend about 8 pipe diameters upstream and 5 pipe diameters downstream of the sensor. Depending on the obstacles before and after the flow sensor, you might need a little longer, but that should give you a good idea."

If you have two 2" tubes, 26" long, and one has a pitot in it and one doesn't, do you think they would both flow the same cfm?

[Ken73]

If I put a dummy pitot tube in the other to duplicate the "obstacles" in the other tube? I would think so, yea.. in your scenario though, where one does and the other doesn't, probably not (exactly.)

[jsa]