Definitions for "Mass flow"
The weight of a gas or airflow going into a vacuum system. Usually expressed in SCFM or Pounds per Hour and is then converted to volume flow (ACFM) for pump sizing.
Also called normal flow or standard flow. The mass flow rate can be thought of as what the volume flow rte would be if the gas flowing through the line were at standard conditions. Actual line pressure and temperature affect the density of the gas, which contracts (above atmospheric pressure and / or low temperature) or expands (under vacuum and / or high temperature), and thus affects the measured volume flow rate. This means that the exact same number of molecules of gas flow can be measured as radically different volume flows when the temperature or pressure is fluctuating. Some mass flow meters have an absolute pressure sensor, temperature sensor or other technique to determine and compensate for variable gas density on the fly. This means that a change in mass flow reading is known to mean an actual change in the number of gas molecules as opposed to a simple change in the gas density. In addition, when the volumetric flow rate is corrected to standard conditions, it is a simple matter to multiply the mass flow rate by the density of the gas at standard conditions (commonly published) to determine actual mass flow rate (e.g. grams / minute)
Mass flow is the flow pattern in a hopper or silo characterised by the fact that no stagnant zones are present, i.e. that all parts of the particualte solid including those adjacent to the wall, are moving when the hopper is discharged.