Venturi and Orifice plate flow meters
Discussion
This is going to be slightly embarrassing because I should know this...
However, I am marking some 1st year Fluid dynamics lab sheets and so far nearly everyone has commented that the discharge coefficient is somehow a representation of how accurate the flow rate measurement by each is. Cd for a orifice plate is of course in the 0.6 region (typical 0.61-62) and for the Venturi meter we are up in the very high 0.9's
From my understanding, Cd has not really got much to do with it. So long as the discharge coefficient for an orifice plate has been well quantified for that particular plate, then the accuracy of the flow measurements will be pretty good (I remember somewhere in the region of 2-4% inaccuracy). The Cd is just a way of accounting for the incalculable losses introduced by each measurement method and because one is essentially a bluff plate with a smaller hole it then it causes a fair bit of incalculable loss compared to the other. It doesn't necessarily make it a wholly inaccurate device though, and its a nice cheap solution for flow rate measurement so long as you are ok with the amount of head loss it induces.
If you want minimal head loss and a slightly more accurate measure and you have money to burn, you get a venturi meter.
Tell me I am remembering this correctly!
However, I am marking some 1st year Fluid dynamics lab sheets and so far nearly everyone has commented that the discharge coefficient is somehow a representation of how accurate the flow rate measurement by each is. Cd for a orifice plate is of course in the 0.6 region (typical 0.61-62) and for the Venturi meter we are up in the very high 0.9's
From my understanding, Cd has not really got much to do with it. So long as the discharge coefficient for an orifice plate has been well quantified for that particular plate, then the accuracy of the flow measurements will be pretty good (I remember somewhere in the region of 2-4% inaccuracy). The Cd is just a way of accounting for the incalculable losses introduced by each measurement method and because one is essentially a bluff plate with a smaller hole it then it causes a fair bit of incalculable loss compared to the other. It doesn't necessarily make it a wholly inaccurate device though, and its a nice cheap solution for flow rate measurement so long as you are ok with the amount of head loss it induces.
If you want minimal head loss and a slightly more accurate measure and you have money to burn, you get a venturi meter.
Tell me I am remembering this correctly!
jshell said:
I can assure you that many, many global gas exporting offshore platforms still use OP meters for their fiscal accounting. This is 'real money' meters, so the accuracy is or can be very, very high indeed.
I wrote the software for a field calibration system for the pressure meters used on these. It was nearly 20 years ago though so cannot remember any detailed figures. What I do remember is that the pressure differential versus flow was very well known and stable so that the main requirement was to calibrate the pressure meters regularly not the plate itself. cheers chaps
I have to say... The grades necessary to get on this course are now something stupid. Last I saw it was AAB and I am pretty sure there will now be an A* in there some where and you'll need an A in maths. On a lab sheet where you are essentially told what to do and even given the formulas to use, you'd expect perfect results. All you have to do is put some numbers in a calculator!
nope.
About 75% of them couldn't be arsed to show a sample calculation
Nearly all of them have pretty high errors due to generous rounding of previous calculations (why they don't just leave the full number in their calculator or better yet, do it in a spread sheet, is beyond me).
About 30% clearly copied answers off another guy because they all have the same obvious mistake when calculating velocity from pressure differentials.
About 50% think hand drawing a graph on graph paper is alright. I mean ok they did do a graph and in the old days, this was the only way. But today it shouldn't be beyond anyone at this level to knock a graph up in Excel. Speaking of knocking a graph up. Some of them did! no title, no axes labels, plotted experiment and theory results with same colour lines (experimental data should be discrete X, Y points as well).
Then we get on to the analysis section where they have to write about what the found during the experiment and what their results mean. Wow, very few show any kind of understanding, clearly parroting what they were told (and often parroting it wrong, like Chinese whispers). For some, english clearly isn't a first language which I guess means they have some kind of excuse, and some even went so far as to blame differences between theory and reality on the lab technician helping them do the lab!
Then there is formatting. No justification, varying font sizes, 2 x line spacing, no equation numbers etc. Some have section headings, some don't, some are just one never ending paragraph. One guy thought it was ok to do his analysis in numerous text boxes dotted about a page filled with a graph and excel tables!
Truly dire, and I know I wasn't that bad when I started and neither where my friends. We only needed BBC to get in all those years ago, so surely these A* kids should be showing us the way!
I have to say... The grades necessary to get on this course are now something stupid. Last I saw it was AAB and I am pretty sure there will now be an A* in there some where and you'll need an A in maths. On a lab sheet where you are essentially told what to do and even given the formulas to use, you'd expect perfect results. All you have to do is put some numbers in a calculator!
nope.
About 75% of them couldn't be arsed to show a sample calculation
Nearly all of them have pretty high errors due to generous rounding of previous calculations (why they don't just leave the full number in their calculator or better yet, do it in a spread sheet, is beyond me).
About 30% clearly copied answers off another guy because they all have the same obvious mistake when calculating velocity from pressure differentials.
About 50% think hand drawing a graph on graph paper is alright. I mean ok they did do a graph and in the old days, this was the only way. But today it shouldn't be beyond anyone at this level to knock a graph up in Excel. Speaking of knocking a graph up. Some of them did! no title, no axes labels, plotted experiment and theory results with same colour lines (experimental data should be discrete X, Y points as well).
Then we get on to the analysis section where they have to write about what the found during the experiment and what their results mean. Wow, very few show any kind of understanding, clearly parroting what they were told (and often parroting it wrong, like Chinese whispers). For some, english clearly isn't a first language which I guess means they have some kind of excuse, and some even went so far as to blame differences between theory and reality on the lab technician helping them do the lab!
Then there is formatting. No justification, varying font sizes, 2 x line spacing, no equation numbers etc. Some have section headings, some don't, some are just one never ending paragraph. One guy thought it was ok to do his analysis in numerous text boxes dotted about a page filled with a graph and excel tables!
Truly dire, and I know I wasn't that bad when I started and neither where my friends. We only needed BBC to get in all those years ago, so surely these A* kids should be showing us the way!
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