text: Using this logic, the improvement in reliability gained by use of voting logic where at least two sensors must function for the system to function is greatly reduced compared to the fully redundant version. Improvements are only in the order of one to two orders of magnitude. This would be great improvement over the single sensor, but the analysis above has assumed statistical independence of the failure of the sensors, which is generally not realistic.
  Factors which make the cell outputs in a rebreather statistically dependent include:
  Common calibration gas - They are all calibrated together in the pre-dive check using the same diluent and oxygen supply.
  Sensors are often from the same manufacturing batch - Components, materials and processes are likely to be very similar.
  Sensors are often installed together and have since been exposed to the same PO2, temperature profile over the subsequent time.
  Common working environment, particularly with regards to temperature and relative humidity, as they are usually mounted in very close proximity in the loop, to ensure that they measure similar gas.
  Common measurement systems
  Common firmware for processing the signals
  This statistical dependency can be minimised and mitigated by:
  Using sensors from different manufacturers or batches, so that no two are from the same batch
  Changing sensors at different times, so they each have a different history
  Ensuring that the calibration gases are correct
  Adding an statistically independent PO2 measuring system to the loop at a different place, using a different model sensor, and using different electronics and software to process the signal.
  Calibrating this sensor using a different gas source to the others