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polyphenyl ether synthetic diffusion pump fluids capable of producing ultra-high vacuum in the 10-10 torr range. Thermally stable with exceptionally low volatility, these non-halogenated water white fluids exhibit extraordinary resistance to degradation from heat, oxygen, radiation, and chemical attack. Due to this fluids extremely low vapor pressure, backstreaming characteristics are lower than any other fluid, resulting in less contamination and longer operation.