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As part of a mixing test, an instrument to measure transient concentration of several different binary gas mixtures was required. A combination sonic nozzle/hot film instrument was selected for this measurement, but for certain gas mixtures the output from this device was unexpectedly small. A study of the dynamics of the instrument reveals that this is caused by the compensatory nature of the two principal contributors to convection: change in constant velocity convection coefficient and change in sonic velocity. The problem is addressed by introducing a physical delay line between the hot film element and the sonic nozzle. Results show that a finite interval of constant velocity thermal convection data can be taken before sonic velocity effects are introduced. A second variation is proposed where the hot film sensor is replaced by a simple orifice flowmeter. Orifice pressure drop and sonic velocity effects, normally strongly compensatory, are also separated by a delay line such that nearly constant velocity orifice discharge coefficient data can be taken over a finite time interval. Results show that this instrument design has higher sensitivity and lower cost than the hot film variant, albeit with slower time response. © 1996 American Institute of Physics.