In brief, the small valve is left slightly open during filling. It's positioned at the 80% level. During the filling process, at first it discharges vapor, which is invisible, just like air escaping.
At the full level (80%), it begins to discharge liquid, which looks and sounds similar to a CO2 fog from a fire extinguisher. At that point stop filling and close the indicator valve.
I encourage everyone to know the correct process, because you'll meet plenty of workers who've not been trained or don't know the correct method.
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