Method of introducing and controlling compressed gases for impurity analysis

A method for analyzing compressed gas for particle concentrations comprising introducing a source of compressed gas into a system, said system comprising a sensor means for measuring particle concentration within the gas, means to pressure balance the system which comprises first valve means located between said source of compressed gas and said sensor means, second valve means for introducing said gas to said system, filter means located downstream of said second valve means for substantially removing particle impurities in the source gas and/or introduced by the upstream components, third valve means located downstream of said filter means for controlling the exhaust of said gas from said system, and a critical orifice located between said filter means and sensor means for back-filling said gas to said sensor until there is a pressure equilibrium across said first valve means, said method comprising introducing said compressed gas while said first valve means is closed, causing said compressed gas to pass through said second valve means, filter means and critical orifice for a length of time sufficient to equalize pressure across said first valve means while said third valve means is closed and opening said first valve means and third means once pressure across said first valve means has been equalized enabling said compressed gas to pass through said sensor means and critical orifice prior to being expelled from said system through said third valve means.