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A microhygrometer has been developed at JPL's Microdevices Laboratory based on the principle of dewpoint/frostpoint detection. The surface acoustic wave device used in this instrument is approximately two orders of magnitude more sensitive to condensation than the optical sensor used in chilled-mirror hygrometers. In tests in the laboratory and on the NASA DC8, the SAW hygrometer has demonstrated more than an order of magnitude faster response than commercial chilled-mirror hygrometers, while showing comparable accuracy under steady-state conditions. Current development efforts are directed toward miniaturization and optimization of the microhygrometer electronics for flight validation experiments on a small radiosonde balloon.

An engine-dynamometer study was performed to quantify the air-fuel ratio (A/F) offset between the window of a three-way catalyst (TWC) and the closed-loop control point of an exhaust gas oxygen (EGO) sensor. In this study, the effects of rpm, torque, EGR, and A/F modulation were explored along with the age of the TWC and EGO sensor. In general, it was determined that the closed-loop EGO sensor control point shifts lean as a function of increasing feedgas NOx concentration, thus causing the engine A/F to move away from the high NOx conversion efficiency regime of the TWC.

The transient response of ZrO2 and TiO2 EGO sensors has been investigated under actual engine operating conditions. The results of this study show that the response of an EGO sensor is dependent upon the characteristics of the engine and feedback control system with which it is used. Specifically, sensor response time is a function of the magnitude and frequency of the A/F changes and the initial and final values of A/F to which the sensor is exposed. ZrO2 and TiO2 sensors show similar transient behavior and have practically equivalent response times.

This paper discusses the properties of EGO sensors based on thick TiO2 films. These devices are fabricated by depositing a TiO2 film on an insulating or conducting substrate. In addition to being inherently inexpensive, planar techniques provide flexibility in the fabrication of multicomponent structures incorporating temperature compensation and heating elements. Furthermore, TiO2 film sensors are found to possess faster transient response than available ZrO2 and ceramic TiO2 sensors. Results of engine studies of the properties of these new TiO2sensors will be presented and discussed.