Flow measurement play an important role from calculating mass flow to volumetric flow which are critical and important parameter to know about actual total flow generated.flow can be measured for liquid or gas. both can be measured in mass or volumetric flow. in both case material related bases on flowing fluid’s density. however, in case of liquid it is totally independent of density and in case of gas density mostly depends upon temperature and pressure.
let distinguish between mass flow and volumetric flow by definition,
Mass Flow  Mass flow rate is a mass of fluid which passes per unit of time. mass flow measured in kg/s or kg/h represented by symbol m.
Volumetric Flow  volumetric flow is a volume of fluid which passes per unit of time. volumetric flow measured in m3/s or m3/hr denoted by Q.
effect on measurement due to different measuring condition,
Gas  gase are compressible and change its volume due to different pressure and temperature. so volume at one temperature and pressure can never be same at different temperature. in case of actual flow rate measured through meter and standard flow rate can be measured through meter using unit like actual cubic meter per hour or standard cubic meter per hour. mass of gas can directly measured measure independent of temperature and pressure using different flow measuring technique.
Liquid  liquid has no effect of any condition whatsoever which can be measured in variety unit out them most commonly used unit is gallon per min.
Types of Flow Measurement Method
there are various method to measure flow based on condition we can use appropriate method which can give best result. method are,
1) Mechanical Flow Meter

Turbine Flow Meter

oval gear meter

paddle wheel meter

current meter
2) Obstruction Based Meter

Orifice plate

venturi tube

wedge type

annubar

pitot tube

Flow nozzle
3) Variable Area Flow Meter
 Rotameter

Thermal Mass Meter

Ultrasonic Flow Meter

Electromagnetic Flow Meter

Vortex Flow Meter

Coriolis Mass Flow Meter

Laser Doppler Flow Meter

Sonar Flow Meter

Open Channel Flow Meter
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