Pfeiffer Vacuum

2.1.7 Water vapor tolerance / water vapor capacity

Water vapor tolerance pw0 is the highest water vapor pressure with which a vacuum pump can continuously intake and displace pure water vapor under standard ambient conditions (20 °C, p0 = 1,013 mbar). It can be calculated from pumping speed, gas ballast volume, relative humidity and saturation vapor temperature and is indicated in mbar [7].

Formula 2-6:

Water vapor tolerance

DIN 28426 describes the use of an indirect process to determine water vapor tolerance. Water vapor tolerance increases at higher pump outlet temperature and greater gas ballast volume qpB. It declines at higher ambient pressure.

Without gas ballast, a vacuum pump having an outlet temperature of less than 100 °C would not be capable of displacing even small amounts of pure water vapor. If water vapor is nevertheless pumped without gas ballast, the condensate will dissolve in the pump oil. As a result, the base pressure will rise and the condensate could cause corrosion damage.

Formula 2-7:

Water vapor capacity

is the maximum volume of water that a vacuum pump can continuously intake and displace in the form of water vapor under the ambient conditions of 20 °C and 1,013 mbar.

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