“…Disc variance, reported as a complication of those valves manufactured before 1968, the disc of which was of extruded Teflon (Beall, Bricker, and Messmer, 1970;Robinson, Hildner, and Greenberg, 1971), was thought to be less likely when the disc was replaced by a significantly more dense, compression-moulded Teflon twice as thick as that previously used. However, Berroya and Escano (1972) report two examples of variance of the newer, thicker, compression-moulded discs. Still more recently, the problem of disc variance is believed to have been overcome by the use of Pyrolite carbon (Beall et al, 1972).…”