A sensing head and system utilizes fault tolerant design and self-diagnosis. To prevent the rotor from flying from the base and oil spraying if the cover is removed without shutting off the oil supply via optional valve, a rotor restraint is provided in the form of a ring which surrounds and overlies a flange of the rotor. Walls of the reaction chamber have a temperature gradient in which a temperature of the walls increases in a direction of flow of the aerosol along the reaction chamber. The invention also includes assays and kits for performing the methods described herein. The present invention can improve the performance of the lead-acid battery.