Basic orifice queries.

[jsmith]

Hi,

I am unsure about a bit of very basic theory here.

I understand that you test at a pressure of your choice (based on bench capactity etc.) - lets say 10" in this case.

Without any obstruction, you know that a given orifice will flow a certain amount of air due to the 10" of pressure and this will be read as 100% on the inclined manometer.

Surely this will 100% will be 10" vertically up on the inclinded manometer. Therefore, how come some people use inclined manometers which aren't as high vertically as the test pressure?

Regards.

[larrycavan]

That's not how it works exactly...well..sort of..here's how it works.

Your inclined manometer is nothing more than a verticle manometer that has been tilted on an angle and scaled in percentage for easier viewing.

Test Pressure and the Inclined manometer readings go hand in had but are two entirely different readings. The test pressure manometer has absolutely nothing to do with the 100% number on the inclined except to be used as a reference value to standardize testing.

The inclined is measuring vertical inches of water column. So lets say you have a simple U-tube manometer scaled 0 - 6" that is connected on each side of the orifice and replaces the inclined. When the airflow across the orifice has reached the point where the U-tube manometer reads 6", you have reached 100% of 6" pressure drop across the orifice.

The next question would be "How much force did it take to create the 6" pressure drop across the orifice. That would be measured on a second manometer and would be your test pressure.

A Dwyer 246 manometer with the red .826sg fluid is actually measuring 6" of vertical water column.

Your inclined manometer does not have to be at a height equal to the vertical column of water is measures. You can alter the manometer with heavier or lighter fluids.

If you choose a lighter fluid [lower specific gravity] you can effectively extend the percent scale to make reading the manometer easier. The graduations get spread out.

Go the other way and you can extend the readings beyond the 6" of vertical water column [use heavier fluid]

In any event, the s.g. of the fluid impacts the measurable inches of water column measurement.

If you use 1.0 s.g fluid then the inclined manometer raised height on the high end would be equal to your intended vertical water column number..ie. for 6" you would raise the end 6".



Edited By larrycavan on 1159911263

[jsmith]

Ok, So I know (from efunda calculator) that when there's a 6" drop across my 2" orifice, I have a flow of 128CFM and this would read 100% on my 6" inclined manometer.

But how would I know that this occured at 15" test pressure?

It may turn out that when the bench is built, I needed 12" of test pressure to create a 6" drop across the orifice and then what?


I'm confused (an probably being a bit thick too.)

THanks..

[86rocco]

An orifice flowbench works by comparing the flow through a known element, your 2" orifice, to that of an unknown element, whatever you're testing. The pressure difference measured by the inclined manometer is a measure of how much air is flowing through your orifice and by extension how much is flowing through your test piece. In your case whenever you have 6" differential pressure on your inclined manometer you will be flowing 128 cfm, what the depression turns out to be at that rate of flow, is stictly a function of how much air your test piece is capable of flowing, a free flowing piece may generate 12" of depression whereas a a more resistive piece may generate 15". In this example, a piece that flows 128cfm @12" is eqivalent to flowing 143cfm@15". If you wanted to do all of your testing at 15" of depression, you would either need a taller inclined manometer or a larger orifice. The ability to test at a depression of our own choosing is one of the reasons why orifice type flowbenchs usually have multiple orifices.

[larrycavan]

Let's drop back a bit and examine what each manometer is measuring.

The inclined - measures the pressure drop across the orifice. The pressure differential between the top and bottom chambers inside the bench. It yields the flow capability.

The verticle - measures the pressure differential between the atmoshpheric pressure and the top chamber. It yields the force you are using to achieve the flow you are measureing with the inclined.

You have 2 distinctly different pressure differentials being measured.

As you increase the test pressure [force] your flow capability will increase [cfm].

You don't base the inclined manometers verticle rise value based on anything to do with your test pressure manometer.

You have liberties that you can take with the inclined manometer that are related to the orifice sizes as I explained in my first reply to you on this.

You can achieve the same CFM flow potential using two different size orifices and two different inclined manometer configurations.

While you may continue to apply test pressure to an orifice, you will eventually reach the measurable limits of your inclined manometer.

The measurable limit of the inclined manometer will effect the range selections you choose. You should choose your ranges with two things in mind.

1. Number of motors you have to pull the depression you want to test at.

2. Calculated measurable range of the inclined manometer in terms of verticle water column measured.

Superflow could have just put a huge hole in the top of the 110 and used the same manometers. They didn't because water guages are accurate within a range that is much narrower than good electronic equipment like the FP1. That guage will allow you to use a single 2" orifice and you will get good low flow values that you can trust.



Edited By larrycavan on 1160001953

[86rocco]

As I said earlier, an orifice flowbench works by comparing the flow through a known element, to that of an unknown element. The flow through each element generates a pressure difference, the inclined manometer measures the difference across our known element, the orifice and the vertical manometer measures the pressure difference across the unknown element, our test piece.

Typically, you adjust your airflow until the vertical manometer reads your desired test pressure then read the resulting pressure difference on the inclined manometer to give you your flow measurement. So if your inclined manometer get to 100% before you get to your desired test pressure you need to use a larger orifice.

When designing a flowbench, we can use whatever inclined manometer set-up and orifice sizes are most suitable for the type of testing you intend to do. If for instance you have an commercially made inclined manometer that has a 6" range, you would select orifice sizes that would give you close to a 6" dp across the orifice at the test pressures and flow rates you expect to test at.

[laser3kw]

So then the ps2 port on the FP1= the inclined and the ps1 port is the verticle?
When we "set our test pressure" on our test part, we are actually creating an ambiguous pressure differance across the part, from atmosphere to the test plenum. Then we use the inclined to measure the pressure drop across a known orifice. If the two pressure drops are eaqual, the the test part has the same flow as the known orifice. If the inclined reads half, the the test part flows half the amount the the known orifice does.
Am I understanding that right?

[86rocco]

[larrycavan]

[jsmith]

[larrycavan]

The range calculations look correct for 6" and .62Cd.

You are headed for a headache with 8 ranges to calibrate.

You will need multiple calibration plates to verify the ranges.

Is it possible? - Yes.

Is it worth the aggrivation? - IMO...No

[jsmith]

especially when I don't know how to calibrate 1 range!!

[larrycavan]

[jsmith]

Ok, I'm building a small bench due to space constraints and power. Probably only 3 vacuum motors which I hope will give me 200CFM at 10" (don't know how I can tell - this is just a guess really). If 200CFM is my max - what other 3 ranges would you opt for? 200 140 80 and 20? or similar...

[larrycavan]

200 CFM @10" with only 3 motors is not realistic unless you have really good [expensive] motors.

The link I gave you previously will get you some pretty good motors for under 20$ each. Buy at least 4.

I'd go with 25, 75, 100 for the ranges. and use rubber stoppers with the port hole door setup. Running all 3 open gives you 200.

Contact Bruce, get 3 plates made up for calibrating if you want the best accuracy.

Price the manometers and compare the price to and FP1. Weigh out the difference using your own terms. Remember to calculate in the cost of manometer fluid when you cost the manometers.

If money is tight, build your manometers and save up for the digital stuff. You can build manometers for very little cost. The spreadsheet for that is downloadable and works great.

Every math formula you need is here in the spreadsheet section. You can make the scale yourself. There is no need to pay for one.



Edited By larrycavan on 1160272019