VPInstruments flow meters measure mass flow (shown by a “n” like in mn/sec or S as in SCFM). Mass flow enables you to measure with our equipment in different pipe diameters, with different flow-, pressure-, or temperature ranges. Compressor output is also given in mass flow, so this makes your job easier. You can compare the results of the VPI flow meters directly to the output of your compressor. There are a view things to take into account though. In this application note: more about the principle of mass flow, what to be careful about and how to calculate with mass flow.

## Definition of mass flow

Mass flow rate is the mass of substance which passes through a given surface per unit time. Its unit is mass divided by time. This means kilogram per second in SI units, and slug per second or pound per second in US customary units. Mass flow rate can be calculated from the density of the substance, the cross-sectional area through which the substance is flowing, and its velocity relative to the area of interest:

= ρ * v * A where: = mass flow rate

ρ = density

v = velocity

A = flow area

## Definition of a Normal cubic meter (m3n)

A normal cubic meter is used to relate mass flow to a volumetric unit. A normal cubic meter is the volume of 1 m3 of air at temperature of 0°C and pressure of 1013 ,25 mbar abs. The mass of 1 m3n of air is 1293 gram. The normal cubic meter is officially indicated with a subscript “n” behind the volumetric unit. The indication Nm is not correct.

Other common normal conditions:

- 20°C reference (FAD): See ISO 1217
- 15° reference: See DIN 1533

## VPFlowScope and mass flow

The VPFlowScope insertion probe is calibrated in normal meters per second (mn/sec). A normal meter per second is a meter per second under the same reference conditions as a normal cubic meter. This enables you to use the VPFlowScope in various pipe diameters. The VPFlowScope only needs to know the inner tube diameter to calculate the mass flow rate.