This work presents the (SNV) approach for feed-forward gas flow rate control, the considerations included in the design of a prototype, and the results of its evaluation using a bell prover. The SNV uses a pulse-width modulation approach to provide inexpensive feed-forward gas flow rate control in contrast to closed-loop systems with thermal mass flow sensors. Attempts for feed-forward digital control of gas flow rate with throttling is usually frustrated by the difficulty of precisely and repeatably locating a throttle body to the precision required. The SNV uses a statically machined transonic nozzle to provide a mass flow rate insensitive to back-pressure for the on portion of the switching cycle. The severe pressure drop usually associated with sonic flow is almost entirely recovered by using a converging-diverging nozzle instead of a plain orifice, so that excellent metering can be achieved with drops as low as 11%.
Uncertainty in the actual delivered flow is found to be ±1% of full scale. Back-pressure insensitivity is found to be ±0.5% up to 88% of the supply pressure. The operator design predicted that operation would be possible up to 100Hz, but actual performance was limited to roughly 33Hz. Unexpected minor quadratic behavior in the valve calibration and a single errant data suggest that the degraded performance is due to migration of lubricant inside the assembly.