With most of the standard flow metering technologies currently in use (like positive displacement meters or turbine meters) deliver a volumetric measurement. That is to say that the value displayed on the equipment's display screen is given in a volumetric measurement unit such as gallons or liters per minute (GPM / LPM). The problem is that many applications that call for flow measurement - batching and mixing chemicals, monitoring emissions, and tracking material custody - rely on having the rate of flow expressed in mass units. Of course, it's mathematically simple to produce a mass flow measurement through multiplication. The measurement produced by a volumetric flow meter needs to be multiplied by the density of the medium to find the weight.

From a practical standpoint, this sort of derived mass flow measurement becomes much more complex. Density is a variable characteristic. In liquids it changes according to temperature; in gas, it changes according to both temperature and pressure. If a fluid undergoes density changes during an industrial process, measuring its derived mass using a volumetric flow meter may produce an inaccurate reading. This is where mass flow meters that measure mass directly are useful. Mass flow meters are not affected by changes in temperature and pressure.

If an industrial fluid process is to be kept efficient, it's vitally important to have accurate measurements and full control overflow conditions. Flow measurements are often used to gauge the performance of a fluid process. This works with mass flow meters thanks to the law of conservation of mass. When measurements are taken over an equal interval, the amount of mass leaving a closed system has to be equal to the amount of mass entering the system. This is why mass flow meters (together with mass flow transmitters) play a vital role in plants which handle liquid materials of virtually any sort.

Most of the standard types of flow meters - e.g. ultrasonic, differential pressure, magnetic, positive displacement, variable area, turbine, and non-compensated vortex meters - strictly measure the volume of flowing material. To produce a mass flow measurement, these meters need to be combined with sensors measuring pressure and temperature. The resulting figures are fed into a flow computer to derive a mass flow figure mathematically. Derived mass flow measurement is sometimes considered to be the only practical option, but SmartMeasurement™ is in the business of bringing direct mass flow measurement to virtually any application.
Direct Mass flow measurement are Coriolis and thermal flowmeters such as SmartMeasurementTM’s  Coriolis flowmeter family of ALCM meters with various flow tube construction to best fit any application used mainly for liquids.  While in gas measurement SmartmeasurementTM’s, ATMF family of thermal mass flow with direct mass flow and temperature outputs are the smart solution for any gas applications.
In steam measurement, the only technology able to measure mass flow is a self-compensated vortex flow metes. The ALVT family of vortex flow meter produced by SmartMeasurement can take direct mass flow readings on steam flow with built-in temperature and pressure sensors well as mass flow transmitter. These flow meters are perfect for steam flow measurement because when steam temperature and/or pressure changes the output is the actual mass flow of the steam.  Non compensated vortex meters or other volumetric flow meters will not measure the true mass flow of steam when steam temperature and/or pressure changes.