A dynamic sealing system 1 comprising a seal 12-1 with two opposed sides 12-1A, 12-1B; a system 11 for supplying a purge gas to the second side 12-1B of the seal; and a controller 23 for controlling the purge gas supply system 11. A method of controlling the purge gas involves calculating a pressure differential ΔP between the operating pressures P1 and P2 on the respective sides 12-1A and 12-1B of the seal 12-1; and supplying purge gas based on the value of ΔP. The pressure differential ΔP may be calculated by subtracting P1 from P2; and the purge gas may be supplied if ΔP is greater than, less than, or equal to, either of two predetermined values. Also disclosed is a vacuum system 2 comprising the dynamic sealing system 1. In some embodiments, the supply of purge gas increases when a vacuum pump 3 is deactivated, or when an operating speed of the vacuum pump 3 decreases.
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