An orifice plate who's required turn down is 10:1 means that the transmitter has to have a turn down ratio of at least 100:1 (Since flow is proportional to dP^1/2). Although some transmitter manufacturers are going to boast that they have transmitters with far more rangeability than this, are you going to take their word for it?
Your situation is something that should be studied from an economic point of view. You need to ask yourselves the following questions:
1) What is your acceptable loss? Assume the flow rates that this flow meter is registering compared to what flow rates you're being charged at. How much difference is between the two? This difference is what you can assume is your metering systems accuaracy. Is this amount acceptable? If this is > 2% then you should seriously consider means and methods of proving this metering system or ask for justifications on the discrepancies.
2) Whatever technology you use to measure flow, in replacement of what you presently have, you will need to perform an accuracy calculation to determine the economics of switching. Will the accuracy you achieve eventually pay for the meter as well as save you money? How long will it take for you to start saving money. Just a word of caution - the economics should be based on total installed cost (TIC) or plant life cycle cost rather than just the price of the meter because there will probably changes to your piping, etc. Also, the meter may require maintence, etc.
3) Is your supplier charging you based on a standard flow (a temperature corrected flow) or are they charging you based on actual flow? This makes a significant difference. Put it this way, when you buy gas for your vehicle you are buying it based on a standard temperature (15 Deg C) and maybe even pressure. Think about what happens when there is a change in temperature each and every time you buy gas and what it does to the actual amount of gas you buy . . .
4) Is your custody transfer meter a calibrated meter run or simply a mass flow measurement? If it's a calibrated meter run, what standard was used to certify it? In your industry API isn't really relevant, but you should check out AGA Report 3. This report specifies what needs to be done in order to certify a calibrated meter run, according to the American Gas Association. Calibrated meter runs have a high accuracy of measurement but only as long as all the rules have been adhered to. If not, then there is no real way to quantify the measurement error without a proving system.
5) Calibrated meter runs are, in essence, an exact replica of a laboratory setting where they've proved an accuracy to within an acceptable percentage of error. Therefore all variables that could possibly introduce errors are controlled. If any of the lab conditions aren't met (including the ambient air conditions and the temperature of the measured fluid) then there is room for error. Remember that you are only as accurate as your least accurate measurement. Does the temperature of the O2 vary? If so, by how much does it swing? What is the accuracy of the temperature and pressure measurement? All of these introduce more errors if the O2 temperature varies significantly. There are correction factors built into the AGA 3 formula to correct for this, but these are just correction factors, not accurate measurements.
6) What is your turn down ratios and are they within the accepted turn down ratios as stated by AGA report 3? Turn-down ratios are a factor in custody transfer and you should see what certification was used for your custody transfer and if your turn-down ratios are within the limits specified.
7) Traditionaly turn down ratios of 3:1 are what's been specified as the maximum turn down for dP devices. With new smart transmitters it might be acceptable to say that 5:1 is achievable within a high degree of accuracy but I'd steer clear of anyone who says you can achieve 10:1 turn downs with one dP transmitter because of the reason I stated at the begining. Even if you use two transmitters to satisfy a 10:1 turn down, you're looking at doing some fancy logic in your control system so that it knows when to switch from one transmitter to the other.
Ok, now for coriolis mass flow meters:
Advantages:
1) Gives you true mass flow without temperature and pressure compensation.
2) Is impervious to changes in density (which could occur if the temperature of the O2 varies significantly) as well as viscosity.
3) Has a fairly high turn down ratios.
4) No straight run or upstream/downstream requirements.
Disadvantages:
1) Max meter size is 6", ANSI CL 600. You might have to put two 6" meters in parrallel (or two 5", something like that), as well as change your piping system to accomodate.
2) Because you're in O2 service, special cleaning and materials selection is required. This could cost you a pretty penny.
3) These are 4-wire devices so you'd probably have to bring out 120 Vac to the device, as well as your signal wiring.
4) Coriolis meters have a fairly large "foot print", as a ball park estimate in size - think of an equal size globe valve with a pneumatic actuator. If you have tight piping contraints then this would become a factor.
Just of the top of my head, I'd say if the economics suggest it, put in some coriolis meters. Although they'd be fairly pricey - they were designed for your type of service. If your O2 supplier doesn't approve of them then you should ask why and what basis do they have? Talk to the AGA guys and ask them if they have a report on using coriolis meters for custody transfer. I'm almost positive they do or at least they could provide you their points of view on it. In the oil and gas industry colriolis meters are gaining in poplarity over calibrated meter runs when it comes to custody transfer because this meters is robust.
The only limitation this meter has, and why some people give it a bad name, is because of it's supposed ability to measure two-phase or multi-phase flow. Based on the principle of how this meter works, what it's actually measuring is a change in density. It has no idea what is flowing through it, only that it's density has changed. Based on this notion coriolis vendors keep trying to sell this meter as a multi-phase flow measurement system but don't buy that arguement. Unless you know your % compositions up front, all this meter will do is tell you that there has been a change in density. If all you care about is what the flow rate is, then this meter works wonders. But, if you want to know the void fractions of each composition - then you're in trouble.
Hope this helps.
