How do you calibrate a flow bench

[stihlls45]

OK, so here is a question. #4 on a SF600 is 2.507..

I have a plate that is rated 236 @ 25"

Flows 229.50 on the bench at 25" with the numbers posted from the bench.

77.9 manometer x 294.9 = 229.70

Is the bench low? Is the plate wrong? Leave it? Change it?

Where'd the plate come from and who rated it? for what purpose?

It it came from SF for the express purpose of calibrating (not just checking) then I would adjust your range. If it came from anybody else then the posted number on it would mean nothing to me.

Rick

So I guess you would adjust the bench.

Orifice plates wear. The edges erode. When the edge erodes, the Cd changes. A study done by the University of Illinois found that their steel test orifices needed regrinding after a few hours of use to retain their original Cd. They also found the Cd changed with differential pressure. If I recall correctly, it was about .01 change from 5" to about 30" of water differential. There is a company called Bird Precision that makes orifices from ruby. They rate the Cd erosion on their orifices by hours.

[user-9274568]

Where'd the plate come from and who rated it? for what purpose?

It it came from SF for the express purpose of calibrating (not just checking) then I would adjust your range. If it came from anybody else then the posted number on it would mean nothing to me.

Rick

So I guess you would adjust the bench.

Orifice plates wear. The edges erode. A study done by the University of Illinois found that their steel test orifices needed regrinding after a few hours of use to retain their original Cd. They also found the Cd changed with differential pressure. If I recall correctly, it was about .01 change from 5" to about 30" of water differential. There is a company called Bird Precision that makes orifices from ruby. They rate the Cd erosion on their orifices by hours.

Ahh.. Guess that means a flow bench can't last 30 years!

[GARY C]

So I guess you would adjust the bench.

Orifice plates wear. The edges erode. A study done by the University of Illinois found that their steel test orifices needed regrinding after a few hours of use to retain their original Cd. They also found the Cd changed with differential pressure. If I recall correctly, it was about .01 change from 5" to about 30" of water differential. There is a company called Bird Precision that makes orifices from ruby. They rate the Cd erosion on their orifices by hours.

Ahh.. Guess that means a flow bench can't last 30 years!

Oddly enough I have one of the earliest SF110's made and my port numbers repeat on a SF600.

[gmrocket]

im suprised high end head flow equipment has not gone with ultra sonic to measure flow...it eliminates all the guess work, no need for a calibrated flow plate ever. its pretty amazing technology and they are even available as clamp on....could just be used under the head on the test cylinder adapter and thats it.

we use large calibrated orifice plates in natural gas transmission lines because they are maintenance free, no moving parts and simple to replace. but,,they are sensitive to the pipe length before and after the plate, it needs to be a spec machined diameter and length on either side of the plate and only used horizontally. we are gradually going to ultra sonic, even on small residential gas meters

www.rshydro.co.uk/flow-meters/clamp-on-gas-flow-meters/

[Rick360]

Here is the chart from the SF300 book that shows the range corrections necessary when testing at other than the rated pressure.

SF300_page.JPG

Rick

[pcnsd]

Where'd the plate come from and who rated it? for what purpose?

It it came from SF for the express purpose of calibrating (not just checking) then I would adjust your range. If it came from anybody else then the posted number on it would mean nothing to me.

Rick

So I guess you would adjust the bench.

Orifice plates wear. The edges erode. When the edge erodes, the Cd changes. A study done by the University of Illinois found that their steel test orifices needed regrinding after a few hours of use to retain their original Cd. They also found the Cd changed with differential pressure. If I recall correctly, it was about .01 change from 5" to about 30" of water differential. There is a company called Bird Precision that makes orifices from ruby. They rate the Cd erosion on their orifices by hours.

We are not flowing corrosive or dirty gases, or contending with significant pressures and velocities. Using the SF formula posted by cspeier, the velocity through the orifice is >220fps@28"H2O. Significant orifice wear on a stainless steel plate flowing free air seems outside the realm of a risk requiring management. At least to me.

[ole4]

The orfice wear and ruby orfice pertain to sharp edged orfices used in water jets cutters. They use ruby and industrial diamonds to slow wear. Does not pertain to our flowbench orifices.

[stihlls45]

I found the University of Illinois study paper on square edge orifice. Orifice sizes ranged from 1/2 inch to 2.5" diameter. Air pressure differentials were from about 2" to 36" of water. For all orifices, the Cds showed a straight line increase in Cd from 2" to 36" of about .015. So if an orifice had a Cd of .61 @ 2", the Cd was about .625 @ 36" air pressure differential. While each orifice had a different Cd, all had nearly the same increase in Cd over pressure.

Used orifices decreased their Cd by about .012 when reground.

From the study:
"Square-edged orifices should be very carefully used and preserved.
Very slight injury to the entrance edge may cause a change
in the coefficient of from 2 to 3 per cent, which may not be detected
until the orifice is recalibrated."

[ole4]

What were those orifices used for?

[Rick360]

I found the University of Illinois study paper on square edge orifice. Orifice sizes ranged from 1/2 inch to 2.5" diameter. Air pressure differentials were from about 2" to 36" of water. For all orifices, the Cds showed a straight line increase in Cd from 2" to 36" of about .015. So if an orifice had a Cd of .61 @ 2", the Cd was about .625 @ 36" air pressure differential. While each orifice had a different Cd, all had nearly the same increase in Cd over pressure.

Used orifices decreased their Cd by about .012 when reground.

From the study:
"Square-edged orifices should be very carefully used and preserved.
Very slight injury to the entrance edge may cause a change
in the coefficient of from 2 to 3 per cent, which may not be detected
until the orifice is recalibrated."

Interesting article. I'll need to read more when I can. Amazing how careful and well thought out these types of tests were back then.

The change in Cd from 2" to 36" would be ~2.5%. That's real close to the SF chart. SF internal orifice is square edge. Maybe when its a sharp edge internal orifice (like the PTS bench) they don't have as much Cd change with pressure change. If you have a SF600 or similar with a square edged orifice you need to use that correction to get most accurate flow numbers.

Rick

[stihlls45]

"7. Orifices and Orifice Tank.-The orifices were bored in saw-steel
plates 0.057 in. thick, and ground to obtain a sharp square edge."

[stihlls45]

What were those orifices used for?

The research

[user-9274568]

I found the University of Illinois study paper on square edge orifice. Orifice sizes ranged from 1/2 inch to 2.5" diameter. Air pressure differentials were from about 2" to 36" of water. For all orifices, the Cds showed a straight line increase in Cd from 2" to 36" of about .015. So if an orifice had a Cd of .61 @ 2", the Cd was about .625 @ 36" air pressure differential. While each orifice had a different Cd, all had nearly the same increase in Cd over pressure.

Used orifices decreased their Cd by about .012 when reground.

From the study:
"Square-edged orifices should be very carefully used and preserved.
Very slight injury to the entrance edge may cause a change
in the coefficient of from 2 to 3 per cent, which may not be detected
until the orifice is recalibrated."

Interesting article. I'll need to read more when I can. Amazing how careful and well thought out these types of tests were back then.

The change in Cd from 2" to 36" would be ~2.5%. That's real close to the SF chart. SF internal orifice is square edge. Maybe when its a sharp edge internal orifice (like the PTS bench) they don't have as much Cd change with pressure change. If you have a SF600 or similar with a square edged orifice you need to use that correction to get most accurate flow numbers.

Rick

Even with my electronics I rotate the disk. I know some just use the biggest hole for the entire test. I also know that the PRO Bench is a single orifice type.

[pcnsd]

I found the University of Illinois study paper on square edge orifice. Orifice sizes ranged from 1/2 inch to 2.5" diameter. Air pressure differentials were from about 2" to 36" of water. For all orifices, the Cds showed a straight line increase in Cd from 2" to 36" of about .015. So if an orifice had a Cd of .61 @ 2", the Cd was about .625 @ 36" air pressure differential. While each orifice had a different Cd, all had nearly the same increase in Cd over pressure.

Used orifices decreased their Cd by about .012 when reground.

From the study:
"Square-edged orifices should be very carefully used and preserved.
Very slight injury to the entrance edge may cause a change
in the coefficient of from 2 to 3 per cent, which may not be detected
until the orifice is recalibrated."

That is interesting and follows my testing/calibration observations that within any given test orifice range selection, after calibration by square root of the sum of squares, (How I define my flow test plate CD) my low end flow always indicate higher flow than the calibration plate and my top end flow always indicate lower. I guess I have to wonder why this isn't addressed by the DM software. Does anyone use software with a single plate system that addresses the CD change with rising test pressure across the flow element?

[midnightbluS10]

Also I calibrate it so I have an idea of what I am getting, as I see some numbers that defy logic sometimes. People claim flow numbers that if you do the math are getting 150CFM per sq inch @28", which is defying the laws of physics. Here is a pic of the baffle box which inside looks just like Chad's.
P1020973.JPG

Is it normal for the incline of the inclined manometer to change angles midway up? If so, is that built in as a function of the bench for some reason?


Edit: the picture that was with this quote shows the inclined manometer and it doesn't appear to be linear from one end to the other. The slope of manometer appears to change about half way up. That's why I was asking. Thought it might just be the angle of the picture, possibly.

Thanks.
.