This document addresses the specific application of using two PG7000 piston gauges to calibrate low differential pressure sensors which operate at elevated line pressure. Two PG7102, PG7202 or PG7302 piston gauges are typically used.
This application is commonly encountered in the natural gas transmission industry where a large gas transmission pipeline is operated at several hundred psi but flow inside the pipe is calculated as a small pressure drop across an orifice restriction. The pressure sensors may see a maximum differential pressure of a few inches of water relative to an upstream pressure of several hundred (or more) psi. Another application is steam that is sold to customers for heating buildings.
This application note also details the uncertainty analysis for operating in High Line Differential Mode. Also see Sections 3.9.4 and 220.127.116.11 in the PG7000 Operation and Maintenance Manual.
Application Note is here,
High Line Differential Pressure Application Note
PG7000 Manual is here,
PG7000 Operation and Maintenance Manual