Abstract: A self-cleaning probe for a drilling fluid or drilling mud density-testing apparatus comprises an elongated tube through which pressured air is supplied to a nozzle disposed in the mud-immersed end of the tube. A cleaning plunger is normally disposed in an axially spaced, inoperative position relative to the nozzle and is reciprocated by an actuator mounted on the other end of the tube to pass through the nozzle and thereby remove any deposits of congealed mud from the nozzle bore. In a preferred embodiment, a small quantity of water is introduced in the upper end of the tube concurrently with the actuation of the cleaning plunger. This ensures that the congealed mud in the nozzle bore will be sufficiently hydrated to be easily removed by the cleaning plunger. Additionally, the cleaning plunger may be passed through an elastomeric wiping element which removes congealed mud from the plunger before it is again retracted to its inoperative position relative to the nozzle.

Abstract: A drilling fluid density measurement system uses a continuously operating bubble measurement system for an oil or gas well. A microprocessor correlates the signal output from a differential pressure transmitter connected to a pair of self-cleaning pressure probes which are positioned in a circulating drilling fluid. By manipulating solenoid valves, the microprocessor controls the intermittent zeroing of the output signal from the differential pressure transmitter and controls a periodic level check to determine if the pressure probes are submerged in the drilling fluid.

Abstract: A multi-drop communication link, consisting of a master and a plurality of slaves is intrinsically safe for use in a hazardous environment. Slaves are in parallel and communicate with the master through transmit and receive shunt-diode safety barriers. An opto-coupler interface is provided between each slave and the transmit and receive circuits communicating with the master. The opto-couplers comprise infrared emitter-detector pairs which can be spaced apart by an intrinsically safe distance.