Pressure gauges read relative to atmospheric pressure (gauge pressure). The ideal gas law requires absolute pressure. Absolute pressure is gauge pressure plus the local atmospheric (barometric) pressure. Standard atmospheric pressure at sea level is 14.7 psia; this default is appropriate for most industrial sites.

\begin{equation}\label{eq:orifice-method-abs-pressure} P_{abs} = P_{atm} + P_{supply} \end{equation}

Symbols

\(P_{abs}\)	Absolute supply pressure \([\unit{ \psia}]\)
\(P_{atm}\)	Local atmospheric (barometric) pressure \([\unit{ \psia}]\)
\(P_{supply}\)	Compressed air supply pressure (gauge) \([\unit{ \psig}]\)