Flowbench Digital Manometer Sensitivity at Low Flows

[user-23911]

http://www.tlv.com/global/TI/calculator ... ifice.html

That doesn't work either.

Here's someone claiming that flowbenches are highly accurate .....A MAF isn't...lol.
There's no adjustment for the weather either.
Not that it matters but.......how accurate is it really?

As per normal, you never get told the whole truth.

[RL]

I will repeat it; The PTS Products are NOT FOR YOU! It will NOT work with what you are asking Period!

It was YOU who said the FP1 couldn't do single orifice. And it was YOU that said that only the PTS DM could do a single orifice - unfortunately YOU didn't mention that it was on the proviso that you didn't do a leak test -.

I was merely trying to ascertain if the unit could actually do the leak test if purchased, by asking you to prove that it could do the whole thing with a single orifice. Unfortunately the answer never came so I was forced to go to the orifice disk again.

I don't care if you do a leak test, as it is YOU that is risking inaccuracies, I just want to know if I need a small orifice so that I can do leak test.

[RL]

http://www.tlv.com/global/TI/calculator ... ifice.html

That doesn't work either.

Here's someone claiming that flowbenches are highly accurate .....A MAF isn't...lol.
There's no adjustment for the weather either.
Not that it matters but.......how accurate is it really?

As per normal, you never get told the whole truth.

In a orifice flowbench the test head sees the same weather conditions as the internal orifice - air flows straight through both with no external added influence (ratiometric) -, so the weather conditions are cancelled out and doesn't have to be accounted for.

MAF benches have been built, and are adequate or the backyard hobbyist doing a single head, it's just that the MAF bench doesn't have the resolution of an orifice bench. I can do 20 port tests removing the head, porting, and replacing the head again, and get the exact flow at 0.050" every time - not up and down 1cfm, exactly the same number every time -.

[RL]

Lets face it....all flowbenches are highly inaccurate.

Laminar flow element (LFE) benches are highly accurate. Because they are truly linear, you could get down to .05 cfm with .001 inch H2O across the LFE from a manometer that has a resolution of .001 inches H2O on a 50MC2-4 LFE.

The history of LFA benches is that they were popular in the 90's - 6 time NHRA Prostock champion Warren Johnson bought 3 benches from an the sole Australian manufacturer -, but it was noted on the Flowbench forum that the LFA element's readings go off in time, supposedly as the element got dirty, so it lost popularity after that and the manufacturer switched to orifice.

[Barrt]

I'm looking at a calibration certificate for a 4" orifice. The first data point is 0.0 inches H20 with a 0 cfm, and the next data point is .1 inches H2O with 86 cfm.
This is really interesting. I have never thought about how low an orifice can really go.

I checked the CFM at the other data points up to 5 inches H2O and the efficiency is around 77 - 78% for all points.

[Barrt]

Lets face it....all flowbenches are highly inaccurate.

Laminar flow element (LFE) benches are highly accurate. Because they are truly linear, you could get down to .05 cfm with .001 inch H2O across the LFE from a manometer that has a resolution of .001 inches H2O on a 50MC2-4 LFE.

The history of LFA benches is that they were popular in the 90's - 6 time NHRA Prostock champion Warren Johnson bought 3 benches from an the sole Australian manufacturer -, but it was noted on the Flowbench forum that the LFA element's readings go off in time, supposedly as the element got dirty, so it lost popularity after that and the manufacturer switched to orifice.

If you are looking for the most imperfect world, the measurement of moving compressible gas is a good place to start.

[1960FL]

It was YOU who said the FP1 couldn't do single orifice. And it was YOU that said that only the PTS DM could do a single orifice - unfortunately YOU didn't mention that it was on the proviso that you didn't do a leak test -.

Rl, Well then I apologize as I would have never told you, you could do it if I new you were trying to measure as low as .25cfm.

Rick

[SWR]

I always leak test if I am removing the head / test piece, even considering I have double O-rings on every bolted joint on my stuff. Valves do get sucked open, that spark plug might be in, but not tightened, a scratch in the head surface to be machined later might make a tiny gap the O-ring can't seal.. I have a custom sharp edged orifice for that. It's full range is a whopping 5 cfm. I have an inclined almost a yard long with 1.000 (yes, ONE THOUSAND) increments. I can see 0.0316% of flow...

That said, I do not leak test at the test pressure. I leak test with as much pull as I have height in the manometers for. If I read a 1.63 cfm leak at 70"... I can back-calculate it to know the actual - it will not be worse at less pressure - maximum leak I have.

I would think I have a good procedure? Or?

[larrycavan]

What you can do with a Digital Manometer on a flow bench is dependent to the design of the digital manometer and the software that comes with it.

While the design function is the same, that is to say, to measure a pressure differential, there are pluses and minuses in various designs of digital manometers that will either aid you in our work or hinder you from accomplishing what you're trying to get accomplished....Conduct an accurate flow test, analyze & archive your data and get on with your business.

It's possible to conduct your flow tests using one large orifice and get accurate, repeatable readings well below 50CFM if you have the right box and the right software.

Photos of tests and results can be found at http://the-bluebox.com

[gnash]

I'm curious about the different brands of flowbench digital manometer and their sensitivity at low flows using a single large internal orifice, and I was wondering if people could run a simple test on their FB.

Using a FB with a DM and the orifice set to one of the larger sizes. The test is the standard inlet cyl head leak test that you do before every flow test, but with slowly opening the inlet valve from closed - no masking tape on the ports - while watching the numbers rise on the DM. Something like setting a SF600 to orifice 5, turning the FB on, and slowly opening a valve off it's seat.

Do the cfm numbers go up gradually like 0, 0.25, 0.5, 0.75, 1, etc.
Or do they go from 0 and jump straight to something like 3.5cfm.

As an example :-
Using a 2.54" internal orifice my FP1 starts at 0, and after 0.016" lift of the inlet valve it jumps straight to 3.5cfm, with no numbers in between.

Just wondering about the sensitivity and resolution of the PTS, Black Box, Audie, FP, etc DM's.

What is happening with the FP1 is that the FP1 has a feature where it will not calculate CFM until a certain test pressure is achieved. As sent from the factory, this minimum test pressure is usually set at .1"wc. So, until you reach .1"wc test pressure, the CFM will not be calculated. When you do reach .1"wc test pressure, you are probably flowing 3.5CFM

This feature is like a squelch control on a radio. It filters out random ambient air pulses that show up on the pressure sensors, like static on a radio.

To get your lower CFM readings, you can set this minimum test pressure to a lower value, like .01"wc or .005"wc for instance. But you may get spurious CFM readings when your flow bench is turned off.