Test pressure and Incline drop

[varnish]

Hey guys,

I was always under the impression that on an orifice style bench using water manometers, the incline drop had to match the test pressure.
But a guy I know who is building a bench says otherwise.

Example:
Using 10" test pressure, the maximum pressure difference across the orifice plate would be 10", and therefore a 10" incline drop would be used.
That's what the spread sheet tells me anyway.
On a 10" incline drop, 100% on the scale the delta P is 10" (or thereabouts).

If I wanted to test on the same bench at 15"...
I would be able to calculate the flow through the plates at 15", but the manometer would still only be able to show a 10" pressure difference not a 15" pressure difference.

The incline would still show 100% on the scale at 10" pressure difference, irrespective of the test pressure.


Is this right?
(Unless you start playing with the SG of manometer fluid, well sizes and tubing diameters).

Could someone explain this to me in a way I can tell my friend he is either right or wrong?

[coulterracn]

The pressure on either side of the orifice plate is differential pressure. This would be measured on the incline manometer. The orifice plate would be set for 6", 10", 12" or 16" dependent upon how you build your incline manometer or if you use a commercial made manometer like the Dwyer model 246 which has a 6" rise.

Test pressure is pressure of the chamber below the head. This pressure is a separate reading from differential pressure and can be anywhere from 0" - upward of 100" if you have an air flow source capable of doing so.

Ray

[Tony]

Ray is correct.....

The same air passes through two air restrictions placed in series.
One air restriction is whatever you are testing, the other air restriction is your orifice plate.

The test pressure manometer measures the pressure drop across what you are testing, that is why it is called test pressure.

The sloping manometer measures the difference in pressure across your orifice plate.
Both are two quite independent measurements of different things.

[86rocco]

Hey guys,

I was always under the impression that on an orifice style bench using water manometers, the incline drop had to match the test pressure.
But a guy I know who is building a bench says otherwise.

Your friend is correct. It might useful to review how an orifice type flow bench works, it works by comparing the flow through our test piece to the flow of the same amount of air through an object who's flow characteristics are well known i.e. the orifice.

Since flow increases with pressure, a flow measurement always consists of TWO elements, the flow in CFM (or whatever your preferred units are) and the pressure difference across whatever you're measuring. Now, because the flow characteristics of our flowbench orifice are known to us, if we know one element of flow measurement, we can calculate the other. So, in our flowbench, adjust the airflow so that we create a pressure difference across our test piece 10", 15", 28" whatever you like, that's what we call the test pressure and, we measure the pressure difference created across our orifice by that amount of air flow, we call that the differential pressure, very little flow would result in a small differential, more flow in a greater differential now we use the differential to calculate the quantity of airflow.

[varnish]

Cheers guys.

I see it now.
I think I just got hung up on matching the incline to the orifice plates.

If you increase the test pressure, the orifice plates will still be calibrated to the inclined manometer.
You'll just see more flow at a higher test pressure through the same test piece.