The best replacement for flow meter is spirometer.

The best alternatives to flow meter are rated as following:
★★★★★  spirometer
★★★★★  regulator
★★★  flow transmitter
★★★  flow switch
★★★  flow gauge
★★  incentive spirometer
★★  flow sensor
★  volume flow meter
★  spirometry
★  magnetic flow meter
★  mass airflow sensor
  coriolis
  flowmeter
  venturi flow meter
  flow meter

Your alternative for flow meter is not listed? Suggest!
Speaking about flow meter, one probably shouldn't forget about valve.

Other search terms related to flow meter and their ratings are:
★★★★★ valve
regulator
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Google Trends

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flow meter
spirometer
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