I switched to a 12-bit ADC that I had laying around from another project (this one). You can supply it with a range of voltage up to about 5.5 V i think, but it has it’s own voltage reference. That cleaned up the noise I was getting, and I’m now getting voltage readings across the resistor in my current loop that put me very close – it’s right on at 0 gal/min at the bottom end and within about 0.05 gal/min if I set my top end at 50 gal/min. I did have to apply a small correction factor to the output voltages (let’s call this a single point calibration), which I’m assuming is due to the fact that my shunt resistor is not exactly 240 ohms (its a 5% resistor). I just got some precision resistors that will probably take care of that.
So, now on to some live testing in the lab. I’ve got the guys downstairs in our hydrualics lab plumbing one of the 2 inch meters into a line that should give me up to 200 gal/min of flow, which is right near the top end of the manufacturer’s stated accurate operation range of the meter.
Your information about the depth sensors is interesting. I’m dubious about how well I will be able to do with flows at the very low end. For testing, I’m not sure yet how much control I will have over getting really low flow through the meter in the lab, but I’m going to see what I can do.