PTS Flowbench Forum

What's happening when testing head leakage with a single 600cfm orifice. You would think the Δ at close to zero flow would be unperceivable, and therefore not able to be measured accurately, or be repeatable.

What Delta P are you using? Also what is the flow range on the heads you're testing?

Here are the orifices I am using for my bench right now.

John

I'm closer to 28", and will probably use SF600 orifice sizes, but I'm more interested in how people are detecting 1 cfm with a 3.4" orifice and being accurate.

How are you sealing your head to the flowbench that you are needing to look for leakage each time you test?

The FP1 uses a 40" DP sensor on it's CFM range same as the other two channels, the PTS DM is using a 16" sensor so the PTS DM is using the whole sensor's range not a percent of it. In order for you to work with the whole range your DP needs to be 40" across the plate. The PTS DM will work on a larger range but I recommend a 450cfm range before needing a plate change if you plan on looking at the high end of flow and the low end of flow.

Grey,

I am not sure if you really get it or not,

+/- 1 CFM on a 3.490 plate is not that big of deal if you are using the right DM configured to do so.

on a 3.490 plate Bruce’s plate using a 16” DP the max flow through the plate will be 660 CFM.
Now if my DM is reading a DP of 8” the flow will be 467.12 CFM

Now if my DM is a 16” sensor and 10 Bit resolution that means each increment is (16 X 1000) / 1024 = 15.625 or .015625” per increment. (Un Averaged)

Now if I have a 3.490 Orifice and a DP across it of only .015625 it is flowing Mathematically 20.624 CFM (for the first .015625) but the formula is not LINEAR as if it were, based on this example then this same orifice would flow 21118.9 CFM at 16” NOT LIKLEY.

If at 8” “middle of the sensor” we are flowing 467.12 CFM what are we flowing at 8.015625” ? ahhh = 467.57 CFM or roughly .45 CFM. And this is without averaging the data!

I think you need to walk through the math a little for a few orifice plates and then determine the usable range you want. The 40” sensor you are using is roughly a .020” resolution, the issue is you are not most likely going to use the full 40” DP as this is a waste of flow bench vacuum power. If we use the example above and you limit your MENTAL DP to 16” then the resolution difference on a 3.490 plate at your mental mid scale is as follows

Flow at 8” DP = 467.12 and flow at 8.020” = 467.7CFM of .58CFM ???? Without averaging.

I think you are splitting hairs in your thought process. I do not know what the setting are in an FP1 but I am sure I could calibrate it to +/- 1 CFM for across a FAIR given range on each plate.

My question to you is; If this was an SF 600 with monometers just what is the error in reading the monometer between Bruce, Rick, John and Grey each flowing and reading the monometer (meniscus) ? Oh and did I mention John forgot his glasses……

Don’t over think your problem.

Rick

Just ignore what I'm building, I was just wondering how the PTS does such small readings with such a minuscule pressure ∆.

What you are basically saying is that the smaller range of the PTS sensor makes it more sensitive to pressure ∆ - 0-5 volts over 16" is more sensitive than over 40" -, but the added 10 bit resolution divides that range into a smaller 1024 numbers(not factoring noise) making it more sensitive, and then the averaging refines it more.

Does anybody know what this 40 pin IC is, maybe PIC16F877A I/P

Old Grey wrote:Just ignore what I'm building, I was just wondering how the PTS does such small readings with such a minuscule pressure ∆.

There are several reasons for that.

The first, as Bruce has already mentioned is that the whole of the entire range of a 16 inch pressure transducer is utilised, rather than a fraction of the range of a 40 inch range pressure transducer.

These pressure transducers all operate off a five volt dc supply, and the output voltage swing is specified from typically 0.5 volts up to 4.5 volts at full rated pressure. The output voltage from the transducer is unable to swing over the entire five volt supply range.

Typical analog to digital converers have an input voltage measurement range that extends right from zero up to typically five, or sometimes ten volts full scale.

If you hook up one of these pressure transducer directly to an analog to digital converter, (as FP have done) you will only be able to measure at most the four volt output swing that comes directly out of the transducer.

The PTS digital manometer has an amplifier located between the pressure transducer and the analog to digital converter. This amplifier causes a zero to 16 inch pressure measurement range to drive the analog to digital converter over the entire ten bit measurement span, from zero to ten volts.

With the FP, 16 inches input pressure is only 40% of the 40 inch transducer measurement span.
Not only that, the 0.5 to 4.5 volt output swing from the transducer is only 80% of the analog to digital converter's input measurement span.

The result is that the analog to digital converter can only actually read 80% of the 40% of the specified transducer pressure range.
A ten bit converter will have 1024 steps of resolution.
The FP will only be able to access 32% of those 1024 steps, or 328 steps of resolutionfor for 0 to 16 inches of input pressure measurement.

This makes the resolution of the PTS digital manometer which uses the entire 1024 measurement steps for 0 to 16 inches, THREE TIMES HIGHER, even before you start applying any averaging software to the readings.

The real power of very fine pressure resolution, especially at the low pressure end is in the averaging software.
But if you cripple the pressure measurement system resolution right at the measurement point, as FP appear to have done, you can never regain that loss with software.

I am just wondering if there is some confusion on the 16" delta p and the 28" test pressure? The 16" delta P we are talking about is for the "internal" orifice in the bench and is not your test pressure. It is basically the difference between what is above and what's below the inside orifice. This is also what determines the range of the inside orifice only.

Now your test pressure can be something entirely different. Let's say 28" H2O. If you have a 2.800" internal orifice with a cd of .6 then at 16" H2O it will flow 422.7 cfm. That would be the 100% reading. If you have an inclined manometer and your scale reads 0% to 100% then at 100% reading no matter what the test pressure, be it 10", 18", 25" or 28" the reading will be 422.7 cfm. Just at a different test pressure. So if you throw your head on and test it at 28" and the manometer hits 50% then your head is flowing 211.35 cfm at that test point.

16" is just what you are using to calibrate the range of the orifice inside you flow bench. I actually used 12" delta p for awhile but found the orifice would get more range with 16".

John

jfholm wrote:I am just wondering if there is some confusion on the 16" delta p and the 28" test pressure? The 16" delta P we are talking about is for the "internal" orifice in the bench and is not your test pressure. It is basically the difference between what is above and what's below the inside orifice. This is also what determines the range of the inside orifice only.

Now your test pressure can be something entirely different. Let's say 28" H2O. If you have a 2.800" internal orifice with a cd of .6 then at 16" H2O it will flow 422.7 cfm. That would be the 100% reading. If you have an inclined manometer and your scale reads 0% to 100% then at 100% reading no matter what the test pressure, be it 10", 18", 25" or 28" the reading will be 422.7 cfm. Just at a different test pressure. So if you throw your head on and test it at 28" and the manometer hits 50% then your head is flowing 211.35 cfm at that test point.

16" is just what you are using to calibrate the range of the orifice inside you flow bench. I actually used 12" delta p for awhile but found the orifice would get more range with 16".

John

Sorry my mistake, yes it will probably be 16" water across the orifice.

Brucepts wrote:How are you sealing your head to the flowbench that you are needing to look for leakage each time you test?

I thought it was pretty much mandatory to check leakage every time a head is removed and replaced.

We have done thousands of flow tests, which can be attested to by burning out 4 x SF110s and 1 x SF-600 - the guy running it now has a SF110, SF-600, and a SF-1020 -, and even I get surprised at how you can get caught out. It could be something simple like forgetting a head gasket, something caught under, valves hooking up, replaced radius holding the head up, different length dowels accidentally swapped and bottoming out and holding the head up, non located heads lined up by eye and exposing water holes, misplaced masking tape, o-ring popping out, etc. The classic was the young guy forgetting the spark plug and finding 30 cfm, he only tested high lifts as a quick back-to-back and didn't see the low lifts change drastically.

We get so many weird heads it's not worth making proper fixtures for something that may only be flowed once, plus we use a less versatile system that is more ridged and has to be fabricated for anything new. Also we don't charge that much, so you cut corners to make it viable.

Old Grey wrote:We get so many weird heads it's not worth making proper fixtures for something that may only be flowed once, plus we use a less versatile system that is more ridged and has to be fabricated for anything new. Also we don't charge that much, so you cut corners to make it viable.

You are correct I wasn't thinking about various setups my mistake . . .