expectations

[Vicoor]

First off I would like to thank everyone involved with this board for the great info that is made available to anyone seeking it.

I'm new to all this and have just "completed" my first flowbench. (pics to follow in a couple of days)

I am using 4 of the vacuum motors I got from surplus center pulling through 4" pvc with an orifice to measure pressure drop across. I have diy manometers with way oversized wells. Right now I have no settling "box" but will probably be working on that this week.

I think I finally have a grip on the scale being a square root of the distance along the scale. If I understand correctly this will allow me to calculate percentage differences in flow volume.
My initial objective is to use this bench to help me to evaluate differences in flow through a toyota 22r head I'm going to port. I am going on the assumption that actual cfm values aren't as important to me as just being able to distinguish the difference I make on the ports.

I would like opinions on whether I'm on the right track by trying to keep this as simple as I can, Or Is there a reason I've overlooked for me to endeavor to get precise cfm measurements?

[bruce]

[Vicoor]

Will a settling chamber overcome the "orifice in a pipe" problem?

I'm thinking of using a short piece of 6" pvc with baffles to break up the flow. I'm thinking that the baffles will dissipate the energy of the air velocity and allow the orifice to just "suck the air in".

[115-1172523331]

From the pics I remember of Bruce's bench, there is an orifice on top of a round "settling chamber" with the flow element just below that. Is that right, Bruce? Perhaps you can easily reconfigure your bench to match that type of configuration, i.e., the orifice at the end of the pipe. It seems to be a tested design. Just a thought. Good Luck! -- Doug

[bruce]

[Tony]

Agree with Bruce, orifice in a pipe is not a good idea, because of the many problems and uncertainties associated with it. An orifice ideally needs completely stagnant undisturbed air on the upstream side, otherwise the flow and calibration coefficient of the orifice could be anywhere.

Best bet is to make your settling chamber as large as possible, and mount your measurement orifice in the center of one wall. A sufficiently large pipe down stream of your orifice will be fine, provided you can overcome the problem of accurately measuring static air pressure in a fast moving turbulent airstream.

Best design approach is to have the largest possible settling chamber upstream of the orifice, and another settling chamber down stream. If you plan on reversing the flow through the measurement orifice, then the two settling chamber design with the orifice mounted flush in a thin dividing wall is the way to go.

[Vicoor]

alright already...

I've included a settling chamber

Pics can be seen on the readers flowbench projects forum

[115-1172523331]

Just a question on the problems with "orifice in a pipe". I've been using a Audie FlowQuik which has a calibration device which I would describe as an "orifice in a pipe". Does anyone have any experience with these, or is it not what you are talking about? Thanks, Doug (slracer)

[Vicoor]

It looks to me that those setups are pitot style benches and require flow through a pipe to be measured.

I think I understand that the problem with an Orifice plate mounted in a pipe is that the kinetic energy of the air velocity creates problems with modeling the flow characteristics through the orifice creating sketchy results.

Do I have that right?

[Tony]

An orifice in a pipe is possible, and will certainly work, but the problem is you cannot just build one yourself at home, and then "assume" that it has some sort of known flow calibration constant.

But you could probably mass produce something in plastic, test it, and sell it, and it might work reasonably well. But I doubt if it could be successfully measured up and copied.

A home made orifice in a pipe would need to be individually calibrated against known accurate airflow standards, and then you have to trust it blindly without having any means to verify accuracy. So this idea is practical for a mass produced commercial flow meter, but not practical as a home fabricated flow measurement element.

On the other hand a sharp edged orifice with an infinitely large up stream settling chamber, will always behave in a known and predictable manner. If great care is taken to ensure the upstream air is disturbed as little as possible, the flow through the orifice will be very predictable.

[Vicoor]

The only problem with an infinitely large settling chamber is that the vaccuum source would never pull a stable test pressure.

the infamous warpspeed would know that

[Tony]

All you need is an infinitely large vacuum source, and a bit of patience.