Flowmeters push out a pulse (or two) per revolution. So the resolution is the volume per pulse. However, and this is where inaccuracies come from, flow isn't linear. So that at the extremes, while the paddle rotates, the volume isn't as the flowmeter claims. So flowmeters have orifices that linearize some section of the pulse-per-flow curve.
What does this mean? Well - there are two main use cases for flow meters:
1. Inter shot repeatability (same dose, same grooming, same packing) - same water and same bev weight.
2. Measuring water volume.
For 1 - as long as you are close to the linear zone, it should be okay. On the GS/3 AV it seems the choice was for a rather high flow flowmeter so it work reasonably well - but not perfectly well.
For 2 - Same GS/3AV Gicar flowmeter was useless for evaluating flow volume for extremely long air purge and Scott Rao Fingering Pause (what we used to call PreInfusion back in the days...). The Gicar was very much non-linear in those ranges and even when using a 2 or 3 step calibration curve proved to be inadequate for the task (my measurements were posted here in other threads).
The Digmesa I use now (kind gift of member Piezo) send order of magnitude more pulses per flow and I can get accuracies down to a single ml over very low flows (30ml/min or so). My PI is now fully based on volume.
Much more on this on Jakes PI thread, on the Chimera thread and on the Flowmeter accuracy threads:Gicar flowmeter accuracyMy long and rambling path to preinfusion/pressure profiling
1. Obviously, using a water debit starved gear pump is by far the best mod... For any number of reasons. I'll never use a rotary vane ever again. An Ulka would be an upgrade...
2. Accurate flowmeter - PI progression metering when combined with manometer.
3. Slayer style bypass comes in a distant third. Over the last month my bypass wiring became loose and I ended up using the metering valve and gear RPM as my sole control and it was sweet. I may bypass the bypass altogether (if it weren't for the effort put into building the damn thing; but it works well and extends the dynamic range of the flow paddle).
4. PP - sucks. pressure profiling sucks since the gear pump is volumetric. Manometer as feedback is cool - but pressure sensor is rather overkill.
Scraping away (slowly) at the tyranny of biases and dogma.