Non-linear cells do not perform in the expected manner across the required range of oxygen partial pressures. Two-point calibration against diluent and oxygen at atmospheric pressure will not pick up this fault which results in inaccurate loop contents of a rebreather. This gives the potential for decompression illness if the loop is maintained at a lower partial pressure than indicated by the cell output, or hyperoxia if the loop os maintained at a lower partial pressure than indicated by cell output.