Machined Flow % Scale - Where can I find one?

[84-1074663779]

I think some important factors have been overlooked here.

The pressure drop across your measurement orifice follows a square law. Pressure increases to the square of flow. It does not matter what size your orifice is, or what flow or pressure you use, it always follows a square law.

Your flow scale will have zero at one end and 100% flow at the other. The 50% flow mark will always be one quarter way along the scale up from zero. ALWAYS.

This will be true because when you double the flow, pressure goes up four times. When flow goes from 50% to 100% (double) the fluid goes four times further up the tube, from one quarter the way up, to right up to the top.

Any flow scale on any manometer will be like that, no matter what the length or angle. This is a very important concept to grasp.

You can make your scale any length that is convenient.

Having then made your scale and marked it off in flow percentages from 50% to 100%, the next step is to decide at what pressure drop will correspond to 100% flow for the particular orifice size you plan to use.

The formula to use that most people here seem to agree on is:

Flow in CFM = 13.55 x square root of P x diameter squared

Where P is the orifice drop in inches of water, and the diameter of the orifice is in inches.

For example a two and a half inch orifice giving a sixteen inch water pressure drop would be :

CFM = 13.55 x 4 x 6.25
CFM = 338.75 (approximately)

So you take your thirty inch long, or forty inch long, or one metre long manometer and tilt it so the 100% flow mark is sixteen inches higher than the zero mark.

100% flow then corresponds to 338.75 CFM with that orifice.

If the water only goes up to the 37 % mark, flow would be 37% of 338.75 CFM

It is a RATIO, the actual length and angle of the sloping manometer make absolutely no difference.

If you wanted to make a sixteen inch vertical manometer you could, but the short length would make the scale cramped and difficult to read. But the fluid would still go up sixteen inches at 338.75 CFM with that particular orifice.

Hope that helps.

[larrycavan]

86rocco,

Now we're looking at the same thing with similar perspectives. As for the scale angle making the scale wrong or the bench needing recalibration, that's really 6 in one, half dozen in the other. The point is, the readings are affected and something has to be compensated.

Best Regards,

Larry

[larrycavan]

[Psi_man]

[larrycavan]

Psi_man,

Yes, the formula will work for any pressure you choose. The test pressure refers to the verticle manometer readings you get during a flow test. It's a measurment of static pressure in a moving air stream. The greater it is, the greater the flow drop across the orifice disk. It does not figure into the calculations when making the inclined manometer scale however. During flow tests it is used as a constant so you can compare readings. There are conversion formulas to calculate readings from one test pressure to another.

Best Regards,
Larry

[Psi_man]

thanks Larry for the answer.
matt

[Mouse]

[84-1074663779]

Mouse is spot on, it is a theoretical ideal, that you may not be able to achieve. But it is a pretty good starting point for most of us.

It assumes the upstream air is undisturbed and non turbulent. Fitting a thin sharp edged orifice flush with the top of your bench over the test hole comes pretty close to the ideal.

The same identical orifice fitted INSIDE the bench used as the measurement orifice can behave quite differently. Turbulent air will not want to go through that hole quite as readily, and the flow coefficient will be less.

If you pointed a high pressure air hose straight at that orifice, a lot more air will go through than otherwise would.

Flow conditions upstream of the measurement orifice will effect the results significantly, and they can change in either direction. That is the problem.

An excellent test is to make two identical orifices, place one flat on top of the bench, and use the other as the measurement orifice located exactly as it will be in the final configuration.

In a perfect world the differential pressure drops should be identical. How much different they are will give you a few clues as to the internal flow conditions inside your bench.

If the largest measurement orifice works properly at maximum flow, the smaller orifices will always work properly too. It is very convenient to have predictable pressure versus flow, you can then plan a range of orifice sizes to give convenient flow ranges.

If flow through the measurement orifice is "wild" you can still calibrate your bench, but all the sizes will be all over the place, and if flow conditions change slightly, so will the calibration. To get stable consistent results "taming" the flow through the measurement orifice is a very important design goal.

Others have different ideas, but I like to have a very large plenum space upstream of the measurement orifice, place the orifice in the centre of a broad flat area well away from the walls, and keep the up stream air velocities as low as possible. This plenum space can simply not be too large.

Another way is to place the orifice in a long strait pipe, but the flow will then be "wild". Flow and pressure drop will not correspond to the very simple orifice formula above. It can still work, but the results are far less predictable.

[84-1074663779]

One other thought about sloping manometers.

I have heard people ask what angle should a sloping manometer be made?

The actual angle is not relevant and need not be even known or measured. There are only two things that matter, the length of the scale and the height of the high end above the low end.

It is like resting a ladder against your roof. The length of the ladder and height of the roof matter, the actual angle of the ladder does not.

So don't start trying to figure it all out from a fixed angle and then try to work out the lengths. Make the two lengths anything convenient, and forget about the actual angle, it is not important.

If you have say twelve inches of pressure it will raise the fluid twelve inches, no matter how long or at what angle the manometer tube is mounted.

All of this is pretty simple once it is pointed out, but for me at least, it took quite a while to figure out initially.

[larrycavan]

[Thomas Vaught]

Quote:

"While I"ve read in another post that Superflow intentionally introduces turbulence into their bench design through the addition of a baffel, I would tend to think that the baffel is more so required to keep the air stream from blasting directly through the flowed mechanism [head, carb] directly onto the flow orifice."

I was the one who posted about the baffle.

I feel it serves both purposes as mentioned. It removes energy
efficiently and it causes the air to be turbulent.

On the inclined deal, I agree with the 0 percent and the 100 percent deal.

If you decided to use a meter stick for example (39 inches+ long),
you could read ANY value on the meter scale, put that value into a computer program that would calculate where that point was in relation to the 100 percent point and spit out the given
"percentage of flow" just like if you read it off of a created scale.

The length can be anything as long as it isn't too low of an angle
or too short to read the divisions properly.

Some manometers have the ability to vary the scale height so that
you could correct for atmospheric changes. It depends on it you
really care about that exact accuracy.

I originally had some orifice plates made like the Super Flow 1200
plate with multiple holes that were opened up depending on the flow value you wanted. The Super Flow 1200 and the Super Flow
100/110 both worked that way.

The 1200 bench has about a 2 foot cube of air on each side of the
orifice plate.

One gentleman's bench after being calibrated can read within 1/2 percent all day long.

I would be interested in having some longer scales made if the price was affordable. Probably in the 39.37" or 40 inch length.

Tom V.

[Mouse]

I have been experimenting with orifices to find out their "environmental limits", since I am planning to build an orifice style bench and ran across an "orifice odity"! I was pushing air through an orifice located on the discharge hole of my bench, and lowering a board toward the orifice to see what height would begin effecting the orifice flow. While the discharging air through the orifice would push against the board, at about 2" from the orifice plate, the board gets sucked down to about 3/8" over the orifice where it seems to want to stay. Imagine that, the discharging air sucks the board toward the orifice. I understand what is happining but it did surprise me because I was not expecting it. Omnidirectional ground effects!

John

[Mouse]

[larrycavan]

[Thomas Vaught]

I have noticed that as the hole size gets larger the discharge co-efficient changes too.

I will try and run your number on my spreadsheet at work.

Tom V.