These are positivedisplacement meters (nutating disc or ovalgear types), where each rotation moves a fixed amount of fluid. The rotation drives a set of mechanical odometer-style wheels amazon.com+12flowfitonline.com+12fillrite.com+12bellflowsystems.co.uk+3fillrite.com+3fillrite.com+3.
The display normally features:
A 3-digit resettable counter for batch measurement.
A separate 6digit (or more) non-resettable totalizer recording cumulative volume
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