Q154. What additional considerations are necessary when using a mass flow meter for landfill gas?
A154. If a mass flow meter is used, then the terms associated with volumetric flow and conversion to standard conditions (i.e., (V)n × 520°R/(T)n × (P)n/1 atm) must be replaced by the mass flow, M, and a correction to standard volumetric flow using the average molecular weight of the gas stream, MWave, as follows: M × 836.6/MWave, where 836.6 is the molar correction volume (cf/kg-mol) at 60ºF and 1 atm. The average molecular weight can be determined using the GC methods, but the compositional analyses should be provided on a wet basis. If the gas composition is measured on a dry basis, the moisture content must also be measured and the dry gas compositions corrected for the moisture content. If methane composition is measured using an alternative gaseous organic concentration measurement or other infrared method (like LandTec GEM units), then GC methods or ASTMD2503-92 (Reapproved 2007) (see §98.34(b)(5)(iv)) will still need to be performed in order to determine the average molecular weight of the landfill gas during the measurement period. Note that temperature and pressure monitoring is not needed when mass flow meters are used.