Compounds of Formula are disclosed. The controller cyclically modulates the power between a predetermined first voltage and a predetermined second voltage that is less than the first voltage to control the rate of fuel injected by the piezoelectric fuel injector during the portion of the injection. The silicon surface of the active layer substrate 1 is exposed by removing a spontaneous oxidation film 7 formed on the surface. The top end of the pole is affixed to a nonconductive head that is shaped to cradle electrical cables. The controller may be configured to selectively communicate a flow of calibration gas with the sensor during operation of the combustion source, and compare the signal received during the selective communication with a reference value.