When measuring a flowrate, there are two fundamental types of measurements; volumetric and mass flowrate. Volumetric flowrate is a measure of the volume of liquid flowing through the metering device. The units for this type of flow measure indicate that it is volumetric, i.e. GPM (Gallons Per Minute). Most flowmeters measure volume of a fluid by measuring the velocity through a known area. These meters include orifice plates, fluidic flowmeters, and magnetic flowmeters.
Mass flowmeters measure the mass of the fluid flowing through the meter. The units for this type of meter indicate that it is a mass-based measurement, i.e. #/hr. The use of mass flowmeters is limited because of the relatively high cost of the meters and application restrictions.
Often, a volumetric flowmeter is used in conjunction with a flow computer to calculate the mass flowrate. If the volumetric flowrate, denoted by Q is multiplied by the density of the fluid, the result will be the mass flowrate, denoted by W.
W = ρ.Q
where W is the mass flowrate:
ρ is the fluid density at flowing conditions
Q is the volumetric flowrate