A gas flowmeter measures the amount of a substance that flows through the device and would be expressed some amount of volume in a given time e.g gallons/min.
Remember the ideal gas law:
n= number of moles
R= Universal gas constant
T= temperature in K
Volumetric measurements must compensate for pressure and temperature.
A Mass flowmeter actually measures the amount of mass that flows through the device .
Mass is not dependent on temperature of pressure.