Abstract: A distance measuring apparatus and a differential pressure transmitter utilizing the same having an ultrasonic element reflector mounted between a pair of ultrasonic transducers. The alternate operation of the ultrasonic transducers produces a pair of transducer energizing signals which are related to the relative separation of the reflector element from the transducers. In one embodiment, the reflector element is mounted within a closed pressure vessel divided internally by an ultrasonic reflector diaphragm with a pair of fluid inlet lines admitting respective pressurized fluids to corresponding sides of the reflector diaphragm.

Abstract: A process control system having a backup controller which is substituted for one of a plurality of primary process controllers upon the detection of a failure of one of the primary process controllers. The substitution of the backup controller is controlled by a backup director which is arranged to sense the operation of a primary process controller and to transfer the operating data from the failed controller into the backup controller to enable the backup controller to take over the operations being performed by the failed controller.