I noticed condensed water vapor collecting at the exhaust of the QDP80 -> My measuring of the purge air dew-point prior to pumping yesterday was of the air exhausting into the room from the check valve (acting as a point-of-use pressure relief).  This had been the nominal procedure until recent years after we had observed that the purge air drying towers could be "starved" in a maximum demand case, such as full-open purge air valves in conjunction with one or more chamber doors being removed.  We found then that the measured dew-point of air sourced only from the VE (during pressure equalization "blow-down" prior to pump-down) could be different than that of the air sourced only from the purge air header.  This could result from those scenarios where the drying towers were starved at the time that the door(s) were reinstalled.  Once the doors are installed, there is minimal air exchange between the relative "wet" air in the VE and the relatively "dry" air that the, now not-starved drying towers, were supplying to the header.  

This has not been observed at the Corner Station as we haven't been able to "starve" the drying towers with nominal purge air demands.  

We intended to make it standard practice to measure the dew-point of the air exhausting into the room during the "blow-down" step when preparing to pump down a -mid or -end volume.