Pump fluids for oil diffusion pumps must exhibit a low vapor pressure at room temperature and must be able to resist thermal decomposition and oxidization to a large extent. Surface tension of the pump fluids must be high to reduce creep of oil films. They must be chemically inert, exhibit a high flash point and evaporation heat must be low. Moreover, the pump fluids should permit high pumping speeds over a wide range of pressures and be cost effective.

One type of pump fluid alone cannot meet these comprehensive requirements. It is therefore required to select a pump fluid according to the operating pressure and the requirements of the application in each case.