Mass flow meter
The mass flow meter does not measure the volume per unit time (e.g., cubic meters per second) passing through the device; it measures the mass per unit time (e.g., kilograms per second) flowing through the device. Volumetric flow rate is the mass flow rate divided by the fluid density. If the density is constant, then the relationship is simple. If the fluid has varying density, then the relationship is not simple. The density of the fluid may change with temperature, pressure, or composition, for example. The fluid may also be a combination of phases such as a fluid with entrained bubbles.
Actual density can be determined due to dependency of sound velocity on the controlled liquid concentration.
Gravitic density meter
A Gravitic density meter is a type of density meter used in multiple industries to measure the density of a slurry flowing through a pipe line. It consists of a flexible rubber hose that deflects when weight is flowing through the hose. A displacement measurement device, usually a high precision laser or load cell, is used to measure how much change has occurred in the system. 
By calculating the change of deflection of the hose, a gravitic density meter can be used to 
find the real time, continuous density.