Prebreathing Survey

[doctormike]

In his posts here, CO2 monitoring (and the X-CCR), and here, CO2 monitoring (and the X-CCR), Dr Simon Mitchell mentions the rEvo and tempstick monitoring for predicting scrubber breakthrough. His post gives me some bit of confidence that the rEvo rMS is reasonably reliable - WHEN it is actually giving you a prediction. rMS may not be reliable in terms of always giving you a prediction - like mine, which is currently broken. But, when it IS giving the appearance of functioning and giving a "remaining scrubber time", I feel pretty good about it not telling me the scrubber is still good when I'm actually about to experience breakthrough.

In other words, the manufacturer spec may be 4 hours, but if I've got 2 hours on the scrubber and the rMS is telling me I still have 4 hours of remaining scrubber time, I would feel pretty okay about planning another 2 hours of diving without changing the sorb. Without the rMS, if I had 2 hours one day and was contemplating 2 hours the next day, I would probably go ahead and change the sorb.

Right, that makes sense. It doesn't always work, but if it is working then it lets you get more time out of your sorb. And it doesn't really increase safety since it can't pick up channeling.

 

[stuartv]

Right, that makes sense. It doesn't always work, but if it is working then it lets you get more time out of your sorb. And it doesn't really increase safety since it can't pick up channeling.

Well, I feel like it increases the safety of, for example, using your sorb for 6 hours. Because I "know" that the reaction front inside the scrubber is still well away from the threshold for breakthrough...

 

[doctormike]

Well, I feel like it increases the safety of, for example, using your sorb for 6 hours. Because I "know" that the reaction front inside the scrubber is still well away from the threshold for breakthrough...

Right, sort of like the question of whether Nitrox reduces DCS risk. It does if you dive air tables!

Is there any failure mode where the rMS would give you a false negative? Spent scrubber with no indication on the system?

 

[stuartv]

Right, sort of like the question of whether Nitrox reduces DCS risk. It does if you dive air tables!

Is there any failure mode where the rMS would give you a false negative? Spent scrubber with no indication on the system?

Not that I know of. There wasn't any thing like that (a failure with a false negative) covered in my rEvo class. But, I am a rank newbie....

And yeah, but you can dive Nitrox, use a bigger margin for your "turn NDL" (than you would if diving air) and be both safer AND have longer bottom times...

I would just speculate (PURE speculation, mind you) that most experienced rEvo divers who don't have rMS do not strictly limit themselves to the cold water spec for the scrubber - at least, not when in warm water. If they do the same dives with the same scrubber usage, but with rMS, it seems to me like that would be safer.

Or I suppose it could be rMS-induced false security, if it causes you to relax into not noticing that you are starting to experience breakthrough or channeling...

 

[doctormike]

Not that I know of. There wasn't any thing like that (a failure with a false negative) covered in my rEvo class. But, I am a rank newbie....

And yeah, but you can dive Nitrox, use a bigger margin for your "turn NDL" (than you would if diving air) and be both safer AND have longer bottom times...

I would just speculate (PURE speculation, mind you) that most experienced rEvo divers who don't have rMS do not strictly limit themselves to the cold water spec for the scrubber - at least, not when in warm water. If they do the same dives with the same scrubber usage, but with rMS, it seems to me like that would be safer.

Or I suppose it could be rMS-induced false security, if it causes you to relax into not noticing that you are starting to experience breakthrough or channeling...

I speculate one of two things about many experienced CCR divers (which admittedly neither you nor I are at this point!).

1) They put a lot of faith in their body's reaction to hypercapnea as an early warning system

2) They run their scrubbers longer than manufacturer spec in warmer water

It would be really nice to have some of this warm water data, since there is probably more scuba diving done in warm water than cold water overall. Especially if this push to open rebreathers to recreational divers takes off (e.g. the Mares Horizon), then there will be a larger market for sorb. The rebreather manufacturer who does and publishes the testing at several temperatures is likely to get an advantage in that market.

 

[The Chairman]

I want to hear a click or two, see the PO2 maintained and then I'm done.

 

[doctormike]

I want to hear a click or two, see the PO2 maintained and then I'm done.

I have a high frequency hearing loss, and I almost never hear the click of the solenoid. I wonder if there is a way of making your rebreather louder?

I'll bet that no one has ever asked that before...!

 

[The Chairman]

I wonder if there is a way of making your rebreather louder?

Or add an LED?

I have a hard time on noisy boats, but the click seems to be amplified by the SF2.

 

[taimen]

I'll bet that no one has ever asked that before...!

No, you are not. Many people have hearing losses. Also when using dry hoods it may not be that easy to hear the solenoid.

 

[doctormike]

Or add an LED?

I have a hard time on noisy boats, but the click seems to be amplified by the SF2.

Yeah, I was wondering about that... Not sure I want an LED with a cable, but if anyone from @Shearwater is listening, it sure would be nice to have a little blinking red dot on the NERD or the controller that went off when the solenoid fired!