Measurement of a gas characteristic

Abstract

Apparatus to measure the calorific value and/or the Wobbe index of fuel gas supplied along a pipe. A control can operate two outlet valves and cause an inlet valve to be locked in a closed position or be unlocked to stay in a normally open state until the gas pressure in a chamber of predetermined volume reaches a preset pressure difference above the pressure in the pipe. Under direction of the control, a pump moves fuel gas from the pipe into the chamber whilst the outlet valves are closed. Once the pressure in the chamber attains the preset difference, the inlet valve closes automatically to be held locked closed by the control. Certain properties of the gas in the chamber, such as thermal conductivity, specific heat capacity and speed of sound, are measured and corrected to STP conditions. The pump moves two samples of gas into the chamber at two sequential times, each time until the gas pressure therein attains the preset value and the inlet valve again closes automatically in a locked position. Then 1 of 2 outlet valves is opened to allow gas from the chamber to discharge to the pipe through either an orifice place or a capillary passage. This discharge allows the control to measure the gas density or viscosity at STP, from which the calorific value and/or the Wobbe index of the gas is/are determined.