Advances in ultrasonic flow measurement technology have reached a point where it is now feasible
to consider ultrasonic flowmeters for custody transfer measurement of natural gas. The American
Gas Association has released a report specifying the use of multipath (transit-time) ultrasonic
flowmeters in this capacity (Report #9). Just like the AGA’s #3 (orifice plate) and #7 (turbine)
high-accuracy gas flow measurement standards, the AGA9 standard requires the addition of pressure
and temperature instruments on the gas line to measure gas pressure and temperature in order
to calculate flow either in units of mass or in units of standardized volume (e.g. SCFM). The
measurement of temperature and pressure for a transit-time ultrasonic flowmeter has nothing to do
with correcting errors within the meter itself, since we know transit-time flowmeters are inherently
immune to changes in gas density or composition. Temperature and pressure measurements
are necessary for custody transfer applications simply because ultrasonic flowmeters, like turbine
flowmeters, measure only volumetric flow. The fair sale and purchase of a gas requires measurement
of molecular quantity, not just volume, which is why a flow computer requires measurements of
pressure and temperature in order to convert the ultrasonic flowmeter’s volumetric output to either
mass flow or standardized volumetric flow