Questions about External O2 Sensors
What kind of O2 Sensors can I use?
The Predator expects galvanic O2 sensors of the 10mV in air type. It assumes the sensors have linear response to PPO2 and have internal temperature compensation.
I am having problems calibrating. What can I do?
- Always follow the instructions provided by the rebreather manufacturer and training agency when O2 sensors.
- Cable and connector problems are also a common cause of failed calibrations.
- Failed or dying O2 sensors may fail to calibrate properly. Follow the replacement schedule as specified by the sensor manufacturer, the rebreather manufacturer and your training agency. Also, be aware that galvanic O2 sensors can be easily damaged by shock (e.g. when dropped).
- The Predator expects the mV output of the cells to be within 30mV to 70mV in 100% O2 at 1 atmosphere. These mV limits are scaled automatically with pressure changes and the Cal FO2 setting. If the measured mV is outside of the limits, that sensors will fail to calibrate.
I am getting strange or intermittent PPO2 readings. What should I do?
Cables and connectors are the most common points of failure. Inspect cables and connectors carefully. Try to isolate problems. For example, trying a new cable may help.
Failures of the O2 sensor cells themselves are also common . Paul Raymaekers of rEvo Rebreathers has written an excellent article on Understanding Oxygen Sensors used in rebreathers.
What is the pin-out of the Fischer connector?
How do I maintain the Fischer connector?
Have a look at our Fischer maintenance guide.
Is PPO2 displayed in absolute atmospheres or Bar?
The Predator displays PPO2 in absolute atmospheres (ata).
- 1 ata = 1013 millibar = 1.013 Bar
The difference between Bar and ata is only 1.3%, so for practical purposes the displayed PPO2 values can be considered either ata or Bar.
I calibrated with the Fraction of O2 (FO2) set to 98%, but after calibration the PPO2 reads differently (for example, 0.96). Why?
The Predator automatically adjusts the calibrated PPO2 to adjust for variations in current barometric pressure (due to altitude or weather). PPO2 is dependant upon ambient pressure, so the calibrated PPO2 will not always match the Fraction of O2 in the calibration gas.
For example, say we are at sea level (normally 1 atmosphere or 1013 millibars), but a low pressure weather system has the barometric pressure reading 990 millibars. If we calibrate with an FO2 of 98%, then the expected PPO2 is (990/1013) x 0.98 = 0.96. So after calibrating, all three sensors in this case would read 0.96 (not 0.98).
This is the correct way to calibrate PPO2. However, if this difference drives you crazy or you want to match another dive computer that does not adjust for barometric pressure, then set the "Display Setup->Altitude" setting to "SeaLvl". This will assume that surface pressure is always 1013 millibars, and your calibrated PPO2 will match the FO2. Note however, that this is actually an incorrect calibration and you are introducing error into your PPO2 measurements by doing this. DO NOT use the "SeaLvl" setting when diving at altitude. That is, when diving at altitude above sea level, set the Altitude setting to Auto.
How often should I calibrate my O2 sensors?
You should follow your training agency and rebreather manufacturer guidelines regarding calibrations.
In the absence of these, we can offer what we believe to be best practices:
- Always calibrate when a new sensor is installed.
- Calibrating every dive is unnecessary and can actually mask the decline of dying cells.
- Better than frequent calibration is infrequent calibration (perhaps quarterly), but coupled with frequent verification of calibration accuracy.
- Calibration accuracy should be verified for every dive.
- Verification of accuracy should be done at multiple points, ideally with one point above a PPO2 of 1.0.
- For example, verify that the sensors read close to 0.21 in air (at sea level, adjust expected value if at altitude). A pre-dive flush of O2 should show the sensors come close to 1.0. Verifiying above 1.0 can be done by stopping to flush with O2 at 6m during the descent. Alternatively, small pressure chambers are available for testing cells up to a PPO2 of 1.6 on the surface.
The reason verification should be performed above a PPO2 of 1.0 is that O2 sensor current limiting is a common failure mode of these sensors as they age. Current limiting can cause a sensor to behave linearly below a PPO2 of 1.0, but then taper off quickly above 1.0.
Also, CCR divers should always use at least 2 independent PPO2 displays. Calibrations should not be done on both displays at the same time (unless new sensors are being installed, in which case both must be calibrated). When performing the calibration, there is a risk of making a mistake, resulting in an inaccurate calibration. If you perform both calibrations at the same time, both will be wrong and with this error potentially undetectable. If calibrations are done on different days, you still may not know which is correct, but at least you will notice the difference, allowing you to call the dive or bail out to open circuit (Of course if you had verified the accuracy of the calibration, you wouldn’t have gotten into such a situation).
Why don't you use two-point PPO2 calibration?
We do not believe two-point calibration offers any benefit and has some distinct drawbacks over a one-point calibration.
- Two-point calibrations are more complex and time consuming, resulting a higher chance for mistakes.
- Two-point calibrations can mask some types of cell faults.
We believe the best approach is a one-point calibration WITH frequent verification of calibration accuracy.
Frequent verification of calibration accuracy is critical. Verification should be done at a minimum of two known points, ideally with one point above a PPO2 of 1.0. Achieving a PPO2 above 1.0 can be done by flushing with O2 at 6m during the descent or by using small pressure chambers that are available for this specific purpose.