The three readings every fire hydrant flow test depends on — and why mixing up which one came from where makes the whole test meaningless.
A hydrant flow test looks simple from the sidewalk: someone reads a gauge, someone else opens a hydrant down the block, water comes out, and eventually a number comes back about how much flow the water system can supply. What that simplicity hides is that the test only works because three distinct pressure readings are taken at two different hydrants, at two different points in time, for three different reasons. Get any one of those wrong — the wrong hydrant, the wrong moment, the wrong gauge — and the flow-availability number that comes out the other end isn't just a little off. It's meaningless, because the whole calculation depends on correctly relating a pressure drop measured in one place to a flow rate measured in another.