Online precision gas evaluation of the ATLAS Muon Spectrometer during LHC RUN1

The ATLAS Muon Spectrometer, a six story structure embedded in a toroidal magnetic field, is constructed of nearly 1200 Monitored Drift Tube chambers (MDTs) containing 354,000 aluminum drift tubes. The operating gas is 93% Ar + 7% CO2 with a small amount of water vapor at a pressure of 3 bar. The momentum resolution required for the LHC physics (dp/p = 2% at 100 GeV) demands that MDT gas gas quality and the associated gas dependent calibrations be determined with a rapid feedback cycle. During the LHC Run 1 more than 2 billion liters of gas flowed through the detector at a rate 100,000 l/hr. Online evauation of MDT gas in real time and the associated contribution to the determination of the time-to-space functions was conducted by the dedicated Gas Monitor Chamber. We report on the operation and results of the GMC over the first three years of LHC running. During this period, the GMC has operated with a nearly 100% duty cycle, providing hourly measurements of the MDT drift times with 1 ns precision, corresponding to minute changes in gas composition.