Absolute beginner

[Ravoll]

Hi everybody,
Not my first post here.My one and only post was a few years back about other possibility's of measuring flow,but I had to put my flow bench project on hold for a while,so it never went anywhere.
Of lately my sons have taken interest,and we've decided to build a simple type flow bench with 2 water manometers, and a high power shop vac. Have tons of questions ,but I'll post em one at a time.



I've been reading a lot on the subject and my first hurdle is trying to understand the operations.When I set my test pressure to 28 inches with the valve closed,it changes when the valve is opened.Should pressure be readjusted back to 28 inches? I'm not sure if 28 inches is supposed to be a constant or if it's only a starting pressure that's is supposed to change with increasing valve lift.

Alan

[Tony]

Hi Ravoli, welcome back.

Yes, you keep adjusting the test pressure back to 28 inches for each measurement.
As flow increases, with a greater valve opening, the test pressure will drop, and you need to get it back up to 28 inches for the next flow measurement.

[Ravoll]

Thanks,
My next question concerns the water manometers:

Does the inside diameter of the tubing ,or the volume of water, have any effect on the precision?

For some reason I want to think that if I was using an 8mm inside diameter tube,it would be easier to pull the water up one side, as the atmosphere pushes the other side down,than if I were using ,say, a fire hose ,or some gigantic type of tubing.
Reason I ask is I have, lots of tubing of various ID.I would like to keep it as compact as I can.

Alan

[Tony]

Tube diameter has no effect on manometer rise, except very small bore tube tends to trap air bubbles more readily.

If you have a lager bore tube, the weight of water to lift will be higher, but the air pressure also has a larger surface area to act against, and the two effects exactly cancel out.

[Ravoll]

Thanks Tony,
That's what I thought.Just wanted to here it from someone else.

Got some material rounded up today.Going to start building an 8 cubic ft. box to act as a surge tank.

My next question pertains to placement of the inlet and vacuum motor/connection.
The inlet will be on top naturally ,but for best performance,where should the downstream be? 180 degrees straight down or 90 on the side?

Alan

[Tony]

Your best bet will be to get a set of (low cost) flow bench plans from Bruce, and just build the half that contains the settling chamber and measurement orifice, and connect your shop vac to that.

Its a very well proven design that can be "adapted" in many ways. But its an excellent starting point.

[Ravoll]

I'm sorry,but this is only supposed to be an experiment to see how these things work.

Alan

[Tony]

An experiment is fine, we have all done that to start with.
Eventually you will overcome many problems to get something up and running that works.
My first attempt at airflow testing was with a leaf blower.

[Ravoll]

I've been all over the net looking at various flow bench designs.What I'm seeing in some constructions are many incorporate baffle systems to get the airflow more linear. The one I'm using as my base line is from a book I have ,David Vizard's "How to Build and Modify Small Block Chevy Cylinder Heads" (fig6-5 page 68). In his sketch he has the vacuum flowing from the cylinder port through a 35 gallon drum to act as a surge tank.As flow exits the drum it is confronted with a metering orifice.A pressure adjustment is last in line right before the shop vac.He doesn't go into detail about the interior of the drum itself,if its baffled to route the air to a more streamlined form or not.I'm sure that there will be some turbulence in the drum,and if I'm using a cubed tank I'm positive there will be some need of baffles.Guess I'll just have to play with it a see what works best.

Have also searched the web for plans to cut my own orifice plates,with no avail.Is there a formula I can use to determine what hole dimensions will flow what cfm @ 28'' vacuum? I've also looked at this Helgesen plate.I could make it,but probably never have it verified for less than a couple hundred bucks.


Alan

[Tony]

Have also searched the web for plans to cut my own orifice plates,with no avail.Is there a formula

Check out a section of this Forum "Useful programs and spread sheets"
All the "top secret" orifice plate magic and formulas will be revealed.

Bruce that runs the Forum here, can also supply orifice plates of known high accuracy at a very low cost.
But there is no problem in making your own.

And you are definitely on the right track with a large settling chamber with central orifice plate, that is able to flow air equally well in either direction. A large drum or a box fabricated from particle board or MDF is what we here all use.
It will need an inspection window so that the orifice plate can be quickly changed for different flow ranges.