If you are looking for better than +/-1% then go with a Mass Flow Controller. Check Tescom, Dwyer, Sierra Instruments. I have a nice unit from Thermal Instrument company but it just measures the gas flow.
I really don't like how the term flow meter often means devices that both control and meter flow....its the last day of the week for me so I feel like ranting a bit.
You may need to do a little digging on the webpage to find what you need. I have the catalogue in my office and it has a few devices along with specs (rotometers, mass flow controllers, etc.).
Measuring the flowrate of a compressible is dependent on the density, the pressure and the temperature of the gas.
Rotameter flow indicators are good to 1-2%, but the repeatability is almost perfect. To get the best accuracy, you must know the pressure at the discharge of the meter.
You get the STANDARD-CONDITIONS air capacity of a rotameter from the capacity tables in the catalog. The denser compressed flow is slower and will not provide as much lifting force on the rotameter float.....that's why the correction is made.
To correct for different gas density, press, and temp use the following equation:
CF=sqrt((SG*Tr) /(36*Pabs)
SCFM selected x CF =rated Air capacity
Example: If your nitrogen is at 60 degreesF and 50 psig
The SG of N2 is .97, 60 F is 520R, and 50 psig is 64.7 psia.
CF=sqrt 0.2165 =.465
So to read 100 scfh at full scale, pick a meter as close as possible to 46.5 SCFH AIR at atmospheric, 60F.
You can get Roatameters both with valves and with flow controllers. A flow controller is a constant differential relay that looks at the differential across the valve seat, and opens or closes to maintain a constant differential across the restriction-hence constant flowrate.
