Does a flow test need multiple orifice plate changes

[tafeteacher]

I was speaking with a guy who built a PTS bench and he said the one downside was that he needed to change the orifice plate inside the cabinet 4 times to do one test. He's using Performance Trend electronics and I think he said his plates had a bevel on both sides. He mentioned he uses Bruce's depression controller.

I was under the impression that you used the one orifice plate and calibrated it at different points, such as 100cfm, 400cfm and 600cfm to use as examples.
You then just ran the test from lowest lift station to highest.

My understanding is you only needed to acces the orifice plate was to swap from inlet to exhaust.

Is my understanding correct or am I on the wrong track here.

[tafeteacher]

No-one?

[coulterracn]

I have the PTS digital electronics and I do all my test with one plate.

Ray

[86rocco]

The nature of using a single orifice to measure flow is such that resolution improves dramatically as flow increases. Perhaps an example will help clarify that, let's says your differential pressure measurement system tops out at 16"w.c. and the maximum flow you expect to measure is 600cfm so you put in an orifice that generates a 16"w.c. pressure difference at 600cfm, with that orifice, a flow of 150cfm would generate a pressure difference of 1.0"w.c. whereas a pressure difference of 15"w.c. equates to a flow of 581 cfm so the bottom inch of your measurement range represents 150 cfm and the top inch represents 19 cfm.

So it all comes down to a matter of resolution, is whatever system you're using capable of measuring small pressure changes to your satisfaction? If very high accuracy is important to you, you'll use an orifice that puts you in the upper portion of your measurement range and if you want to use only one orifice for best results use the smallest one you can.

[Tony]

As stated above ^^ the measured pressure drop across the orifice rises square law with flow.
At the low flow end, the pressures being measured become very small and much more difficult to measure accurately.

This becomes increasingly difficult with a fluid manometer. What kind of accuracy can you repeatedly measure a one inch total manometer rise by eye, ten percent ? five percent ? The top of the water column is not dead flat, and there are optical effects so there is a bit of judgement involved in assessing the rise against a scale. How good is your eyesight ?

The solution to that is to use a digital electronic manometer that has much greater sensitivity (resolution) and accuracy at the low pressure end of the range. The digits on the display are absolute, no interpretation or estimating required.

The trick is to start out with an orifice plate that has an appropriate maximum flow range for what you plan to measure. With an electronic manometer it will then be entirely possible to cover a useful range of measurements with the same orifice plate fitted.

With a fluid manometer you will need a series of orifice plates that go up in steps of at most x3 or x4 flow, and you will need to change plates during a normal test.

With an electronic manometer you may still need a selection of orifice plates, but just select the one most appropriate for the size and type of engine. And that is all you will need for that particular series of tests.

[Brucepts]

Not much more I could add that hasn't been discussed already . . .

Digital is the way to go, PTS Is now using a 12bit DAQ so the resolution change is much finer which allows it to sense much small changes. Not aware of what Bit count other DM's are using?

[tafeteacher]

Thanks guys.
Basically how I thought. Most of my stuff will be SR20VE heads. I think I'll get away with one plate....yay