Rebreather Question

[Sidediver77]

Rebreather Question. If I have a reading on my primary that says .65 .63 .80 the set point is .7 and I do a dil flush. How will I know if .65 and or .63 are wrong and .80 is right? Or if .65 and .63 are right and .8 is wrong.

 

[sdj42]

Use your current depth and your diluent FO2 to figure out what your diluent PO2 should be, and compare that to the sensor readings. To use really simple numbers, if you were using air dil (21% FO2) at 66 ft (3 ata), you would expect to see a PO2 reading of 0.63 on good sensors after an effective dil flush. With the readings you gave, that would suggest that sensors 1 and 2 are good and sensor 3 is out. If you calculate that your dil PO2 should be closer to 0.8 based on depth/FO2, then it could be that sensors 1 and 2 are both out instead.

 

[northernone]

Here's a rough reply:

You'll need to calculate your dill's PPO2 at that specific depth, some handsets provide this info. The cell which matches is likely the working cell. Provided you get a proper flush.

Is this what you are wondering about?

Regards,
Cameron

 

[Sidediver77]

so im at 66ft 3ata. I do the dil flush. While I do the flush I should see .63 assuming I have 21% as a dil.

 

[OceanEyes]

so im at 66ft 3ata. I do the dil flush. While I do the flush I should see .63 assuming I have 21% as a dil.

One important detail, which will depend on the specific unit you are using:

Prior to performing the dil flush, set your controller to manual. This will prevent a solenoid from firing as the unit attempts to maintain your designated set point. It will take a little time for your cells to accurately register the loop’s PO2 after the flush, but your presumption that they should read near .63 with your given parameters is correct.

 

[Tassi Devil Diver]

If you are using a Shearwater computer the dil PO2 at your current depth can be displayed with the push of a button or two.

 

[doctormike]

A bit off topic, but you should also note that the dil flush does not 100% guarantee that your sensors are OK. A common failure mode is current limitation (failure to respond in a linear fashion at higher PO2s). So even if they show the predicted dil PO2, you also might try to bump the O2 manually slightly above your high setpoint (say, moving from 1.2->1.4) to see if they all respond and if one of them is lagging.

 

[davehicks]

If your sensors are that far off in the middle of a dive, then end the dive. Don't put yourself in a critical deco situation if things are square. Back on the surface Recalibrate at 100% o2 and check that they read 21% in air. Then you will know if your sensor is failing or just had an intermittent wet connection.

 

[fsardone]

If your sensors are that far off in the middle of a dive, then end the dive. Don't put yourself in a critical deco situation if things are square. Back on the surface Recalibrate at 100% o2 and check that they read 21% in air. Then you will know if your sensor is failing or just had an intermittent wet connection.

Sorry but I have to disagree. A current limited cell at 1.1 or 1.2 might as well calibrate correctly at surface with 100% oxigen and give you an hiperoxic loop when at dept and set point 1.3 ...
This is the same issue doctormike highlighted above.

 

[wedivebc]

If your sensors are that far off in the middle of a dive, then end the dive. Don't put yourself in a critical deco situation if things are square. Back on the surface Recalibrate at 100% o2 and check that they read 21% in air. Then you will know if your sensor is failing or just had an intermittent wet connection.

Sorry this info is WRONG and dangerous. As mentioned above cell limiting will not be detected or corrected by a calibration and cell current limiting is the number one killer of rebreather divers! Please know what you are talking about before posting false and potentially dangerous information.