Abstract: Aspects of the present invention relate to method of controlling a supply of purge gas to reduce leakage past a dynamic seal (10). The dynamic seal (10) comprising at least a first sealing member (12-1) having opposing first and second sides (12-1A, 12-1B). A pressure differential (?P) is determined between a first operating pressure (P1) on the first side of the first sealing member (12-1) and a second operating pressure (P2) on the second side of the first sealing member (12-1). The supply of purge gas to the second side of the first sealing member (12-1) is controlled in dependence on the determined pressure differential (?P). Aspects of the invention relate to a controller (23) for controlling a supply of purge gas; a dynamic sealing system (1) for a vacuum system (2); and a vacuum system (2).