Hi Wu Hao, the flow meter can ...

Hi Wu Hao, the flow meter can be reinstalled so it always runs full by installing the main pipe vertically downwards then horizontally with the meter in the horizontal section before placing a rising section to return to the existing pipe alignment.  The vertical pipe section allows air to rise against the flow direction and be released in the reservoir. At the point downstream of the meter where the pipe rises and returns to the existing pipe a vent pipe should be installed and run to up to the overflow level of the reservoir (perhaps attaching it to the side of the reservoir).  This will release air trapped in the pipeline downstream of the meter.    It is not usually possible to use an air release valve at this point as the pressure is low and ordinary air valves will leak.