The source of the dry air used in the flow standard is a Joy oil-free two stage reciprocating compressor and a Zurn refrigeration dryer. The mole fraction of water in this air source is 0.0010 ± 0.0005. This uncertainty in composition leads to uncertainty in the compressibility of only 1 × 10-6 Z. The uncertainty in compressibility due to impurities is smaller for the other gases.