The most common type of oxygen sensor is an electrochemical one based on the solid electrolyte ZrO2. These sensors are used in various industries, including in cars to monitor oxygen content in exhaust gas which is important for optimizing air-fuel ratios to minimize emissions. The sensors operate as a concentration cell which separates a compartment with a known reference oxygen partial pressure PR from the gas stream to be monitored (which has an unknown oxygen partial pressure of P). The following half-cell reaction occurs at each of two Pt electrodes separated by the ZrO2 solid electrolyte which is permeable to only O2- ions: O2 4e- (Pt) ↔ 2O2- Determine the unknown partial pressure in atm if the device reads 65 mV and Pr = 1 atm.