Is there a formula to calculate flowbench plenum size?

[Flash]

Well......watch out, because here i go again!
Will be following much close to bruce's plan........but not quite!

[storm]

Quick post, on topic, no fluffing.

I have had some replies to this question from other sources and what I have learned is, to me at least, interesting.
First thing, as far as I can find, by doing my own "homework", there is no set formula but .....
second thing, the plenum size does need to be carefully thought out depending on the type of "tool" used to create the vacuum or pressure. This is because no matter what is used they all create pulses in the air flow. If the person building the bench uses vacuum motors the pulses are only slight and the more vacuum motors the smoother the air flow becomes. If the person building the bench uses something like a roots blower the pulses, especially at low revs, are quite pronounced. The size of the plenum chamber can be instrumental in smoothing out the pulses.

Think of it this way, you have a cup of water and you throw the water into a bath tub you get the ripple waves and they last for a while going back and forth because they hit the opposite wall and come back again. You do the same thing to a backyard swimming pool and the ripples disappear after a short while because very few hit the opposite wall and come back again. It is the same basic thing with a plenum chamber.

[Brucepts]

Pulsing can also be addressed in other areas besides the plenum but that gets beyond your question here . . .

[Tony]

All I asked was "is there a formula to calculate flowbench plenum size?" .

No known formula.

Its basically a tradeoff between achieving an acceptably stable flow ahead of the measurement orifice, and sheer physical size of the bench.
Final bench dimensions have been reached empirically from what has been found to work well in practice by forum members over the years.
The design of the baffle is also an important part of that.

So what has happened is that through trial and error a simple to build, efficient, proven design has evolved over time.

I realize that for teaching purposes that is not a very helpful answer, but that is how it is...