When calibrating a polarographic oxygen analyzer, if room air reads 21% O2 but 100% O2 reads inaccurately, the recommended action is to

• A. Reset the zero point
• B. Replace the battery
• C. Replace the fuel cell
• D. Add electrolyte solution

Answer: (B)

In a polarographic oxygen analyzer, the sensing element relies on a stable power supply to drive the electrochemical reaction and produce a current proportional to oxygen tension. If the room air reading (around 21%) is correct but the 100% O2 reading is inaccurate, the issue is typically related to the power available to the sensor rather than the sensor chemistry itself. A weakening battery can reduce the current the cell can deliver, especially at high O2 concentrations, causing the high-range reading to be off while the room-air reference remains valid. Replacing the battery restores the necessary voltage/current, bringing the high-range response back in line with the calibration. Replacing the fuel cell would be more appropriate if the sensor showed broad, persistent drift across the entire range, not just the high end. Adding electrolyte or resetting zero wouldn’t fix the underlying power-related limitation affecting the high-point measurement.