The quartz sleeve separates the ultraviolet lamp from the water while passing UV-C into it. Length and glass grade both identify a sleeve — length alone does not, and this catalogue stocks more than one sleeve at several of these lengths. This is the 815 mm sleeve listed against the B42VI. Note that this catalogue also lists an 867 mm sleeve for the B42VI under item 1016-0504; measure the sleeve actually fitted, or quote the reactor serial number, before ordering.