As a manufacturer of thermal mass flow meters, we are often asked, “What is the thermal mass flow meter correction factor? Why do we need to measure mass flow? What is the difference between ACFM and SCFM? Is pressure or temperature correction needed for thermal mass flow meters?

## ACFM vs. SCFM

Many gas flow meters measure the actual flow rate (ACFM) at operating conditions rather than the desired mass flow rate. Volumetric flow is the actual volume flow of gas leaving the flow meter.

Standard flow rate (SCFM) would be the corresponding flow rate the gas would be flowing if the temperature and pressure were at standard operating conditions. It is usually the most useful gas flow measure because it defines the mass flow, the number of molecules, and the heat delivery capacity of the gas.

## Pressure and Temperature Correction

To obtain the mass flow measurement of gas, it is necessary to correct for pressure and temperature. The most common device used to measure the flow rate is a differential pressure flow meter. In this case, the differential pressure device delivers the actual flow rate, which is then corrected for pressure and temperature. Unfortunately, to do this requires the differential flow meter as well as a temperature and pressure device. All three signals transmit to the flow computer, which determines the gas’s mass flow rate. Therefore, four separate devices are used to determine the corrected gas flow.

Is external pressure or temperature correction needed for thermal mass flow meters? No. Thermal mass flow meters replace all of this with one instrument that directly measures the mass flow rate of the gas.

While you may be interested in our paper, Fundamentals of Thermal Mass Flow Measurement, Thermal Flow Working Principle and Theory, may provide the answers you seek.

Units of Measure: Gas Mass Flow