Abstract: An unkown signal is sampled by simultaneously comparing its value with that of each of a plurality of predetermined monitoring levels. An independent time variable is recorded that represents the instant when the signal has a value equal to that of a given monitoring level. Equality of the signal value to that of a given level is detected regardless of whether the signal has increased or decreased to that value. The values of the monitoring levels are changed automatically whenever the signal has a value greater than the highest value monitoring level or less than the lowest value monitoring level. Signal sections having infinitely high rates of change over time may thus be accurately sampled and reconstructed.

Abstract: An analytical apparatus for determining the cryolite concentration in a cryolite-alumina electrolysis cell. The apparatus uses a reference electrode comprising a solid oxygen electrolyte tube containing a reference electrode of a cryolite melt supersaturated with alumina. A measuring electrode is combined with such a reference electrode to provide a potential representative of alumina concentration in the alumina electrolysis cell.