In earlier posts, we considered gas meter types and the difference between volumetric and mass flow rates; here, we show the gas mass flow rate units of measure.
Gas Mass Flow Measurement
Flow measurement is the measure of bulk fluid movement and determined through:
- Positive displacement meters, which collect a fixed volume of fluid, release and refill the fluid, then tally the times the amount fill to quantify flow.
- Flow measurement devices which rely on the strength produced by the flowing stream, as it prevails over a known constriction, and indirectly calculates flow.
- Flow may also be determined by measuring the velocity of fluid over a known area.
Units of Measure
Both gas and liquid flow can be expressed in volumetric or mass flow rates, and the quantities can be converted between one another if the substance’s density is known. The density for a liquid is mostly independent of the liquid circumstances, while, for gas, the density is contingent on pressure, temperature, and to a smaller degree, the gas composition.
When gases or liquids transport for sale, the flow rate is often conveyed by energy flow, such as BTU/day or GJ/hour. The energy flow rate equals:
Volume flow rate X energy content/unit volume
Mass flow rate X the energy content/unit mass
While flow meters calculate the volume flow rate or mass flow rate, a flow computer determines the energy flow rate.
The volumetric flow rate is usually denoted by the symbol, and the symbol is used for the mass flow rate.
Gases are compressible and change volume when:
- Under pressure
In other words, a volume of gas at established pressure and temperature conditions does not equal the same gas under different pressure or temperature conditions. Because of this, meter flow rates are referred to “actual” and “standard” or “base” flow rates with units such as:
- acm/h (actual cubic meters/hour)
- kscm/h (kilo standard cubic meters/hour)
- LFM (linear feet/minute)
- MSCFD (million standard cubic feet/day)
Gas mass flow rate can be directly measured, independent of pressure and temperature effects, with thermal mass flow meters, Coriolis mass flow meters, or mass flow controllers.
For a review of mass, volume and density, this video may interest you.