Calibration of the RPC working voltage in the CMS experiment

The Resistive Plate Chambers (RPCs) are employed in the CMS experiment at the LHC as dedicated trigger system both in the barrel and in the endcap. The working voltage of all the RPCs installed in CMS has been tuned in 2011 with a dedicated High Voltage scan with LHC collisions during the low luminosity operations. This note presents procedures and results of the High Voltage calibration, with emphasis on the RPC detector uniformity and stability during the 2011 data taking period.