Abstract: A three-wire digital atmospheric sampling system, particularly adapted to detect methane in mines, wherein accuracy is increased by ratiometric analog-to-digital conversion synchronized by a microprocessor. A reset signal, and read-out pulses indicative of the atmospheric condition, are transmitted along the same signal wire as are digital ratiometric signals, thus enabling the use of three conductors only between an atmospheric sampling probe, a microprocessor, and a visual read-out console.

Abstract: A party line telephone system capable of effecting selective calling from one station to another and particularly adapted for use in battery-powered two-wire telephone systems such as those used in underground mines where consideration of intrinsic safety, low power consumption and compatibility with existing equipment are paramount. The system of the invention is such that the receiver at each station in the network need not be connected to a battery-powered source for any prolonged period of time except when that station is being called by another station for a private conversation, thereby conserving battery power.