Abstract: IA purge block is disclosed in which inlet, process, purge and vent ports are series connected in a ring or bridge. The channel or passageway connecting the inlet and purge ports comprises a flow sensor. The impedance to gas flow of each channel is different from the others. The impedances are chosen such that improper operating conditions cause an imbalance in the flow which is detected by the flow sensor. During normal operation of the system in which the purge block is used, the flow sensor acts as an excess flow detector by comparing the flow through itself with the flow through a parallel, but more restrictive, path within the purge block.

Abstract: Control system for indicating which, of a plurality of sensed pressure control fluid functions, first malfunctions. The system includes a plurality of series connected devices, each such device includes a shiftable valve which, in one position activates an indicator, and, in another position deactivates said indicator. Signalling connections link the devices such that all indicators, save the one governing the first malfunction, remain in the indicating condition they were in prior to such malfunction occurring.