Actual and Standard Flow Rates

There are two ways that practitioners of air sampling measure and talk about flow rate.
Those doing compliance sampling for Industrial Hygiene/Occupational Health speak of Volumetric Air Flow rates. It is the volume of air at the existing pressure and temperature at the sampling site. The US EPA also specifies this type of measurement for PM_2.5. Electronic meters of the bubble, piston, venturi and orifice type “read out” in volumetric flow rate or Q_a. There are exceptions which do both.

EPA, for Politico Legal, reasons uses QS known as Standard Air flow rate for reporting PM₁₀. This means that the flow rate is reported to Standard conditions. For the US EPA, these conditions are 25° C and 1 atmosphere pressure. (1 Atmosphere = 760 mm of Hg = 29.92 in of Hg = 1013.25 millibars = 1013.25 hecto Pascals).
Because the mass of air flowing could be calculated from QS it has come to be called Mass flow.
Throughout most of the world Q_s is not usually referred to as mass flow and it is to a different standard. The conditions outside of the U.S. are 0° C and 1013.25 mb.

Instruments for calibrating Q_s devices are usually Mass flow sensors. Caution is needed in applying these devices as you must have knowledge of what temperature base (25°, 20° or 0° C) they are set up for.

Volumetric devices are in the majority and any of them may be used to measure Q_s if there is also knowledge of the T and BP, in the immediate locale, either from the instrument itself or supplemental instruments.

Q_s = Q_a * (BP_a/760)*(298.15/T_a+273.15) For U.S. applications when T_s=25° C and BP_s=760 mm of Hg

Q_s = Q_a * (BP_a/1013.25)*(273.15/T_a+273.15) For world applications when T_s=0° C and BP_s=1013.25 mb

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