restrictor 32 which is preferably a 0 020 inch diameter orifice or other square root restrictor at a sub sonic flow rate restrictor the time required for pressure decay in reference chamber 26 over the predetermined pressure span will be related to the density of the gas according to an empirical relationship based on calibration tests using a pressure span of 4 inches of water 5 34 h20 to 1 34 h20 and a reference chamber with a volume of 0 25 cubic feet show that the time varies from 7 55 to 8 95 seconds as the specific gravity of the gas increases from 0 5 to 0 7 this amount of time difference allows acceptably accurate measurement of gas density by my declining pressure method the time difference can be varied by changing the volume of reference chamber 26 and or the size of restrictor 32 computer controller 22 has a readout 34 for plotting pressure v time turning to fig2 the correlation between gas density and rate of gas pressure change through a restrictor at a sub sonic flow rate can be utilized when the gas pressure in a reference chamber is rising rather than decaying in fig2 gas flowing through gas pipeline 40 is introduced into my density determining system by opening manual valve 42 electric valve.