What is the primary objective of pre-breathing a manual CCR?

[sea_ledford]

I'm working on updating our dive manual on CCR diving, and I'm pretty ignorant on manual CCRs. My take on pre-breathing is it is primarily a function check of the solenoid to reach and maintain set point and verification that cells are behaving as expected in the .7 to .98 range. Secondarily it is a function check of MAV, ADV and BOV. I don't consider it a useful verification of the scrubber. There is plenty of evidence that the variability of individuals to tell if their scrubber is working or not in 5 minutes makes that a non-functional test.

For those of you that dive manual CCRs, what are you looking for during a pre-breathe? Is an IP check of the O2 side a critical step? How about leaky valves, is there something in particular that you are looking for from a functionality check?

How about hybrid systems? Anything unique to them that isn't captured in the eccr and mccr pre-breathe?

 

[Mobulai]

those of you that dive manual CCRs, what are you looking for during a pre-breathe? Is an IP check of the O2 side a critical step? How about leaky valves, is there something in particular that you are looking for from a functionality check?

Pre breathing hasn’t been stressed in my SW mod1 class (but I have some qualms about the instructor so I’m not sure if more should have been clarified

I looked at the leaky valve function mainly, and that the ppo2 didn’t drop like lead— but I’m also quite sure these are things others have discussed more thoroughly in mod2/3 classes (they even looked at orifice cleaning and related stuff)

A couple years ago in prepp I nerded out on all the aspects of ccr and I read (an assumingly urban myth) that part of the function is to also „check for brakethrougs“ but I know now that this is quite impossible to pin down that way; esp after the ppCO2 paper/discussion by @Dr Simon Mitchell on the topic (not prebreathing)

 

[OTF]

I am an ECCR diver and generally of the school that pre-breathes are of marginal value, unless performed far longer than most divers are willing to do them. Catching a breakthrough is a nice idea but it would take a long time (like 10 minutes +) sitting there drooling into the mouthpiece. That said:

- A pre-breathe is one more general function check to look/listen/feel the unit and see that cells are responding in a normal timely fashion.

- When done immediately prior to diving, a pre-breathe can help get the sorb reaction started and warm up the scrubber, which MIGHT be marginally helpful under certain conditions. Like if the diver will be working hard right at the beginning of a dive in very cold conditions on a partially used scrubber, as in cold water wreck diving.

 

[LFMarm]

Agree on the above.

After RF4 I don’t do anymore 5 mins pre-breathe as it’s mostly inconclusive on CO2 breakthrough. For me it’s about 1 min to check basic solenoid function. Everything else gets tested before. This is a lower importance pre-dive check than others like calibration, negative, positive, DSV.

 

[OTF]

It fundamentally sucks that we don't have a reasonable breakthrough test or reliable CO2 sensing in these machines.

 

[Mobulai]

It fundamentally sucks that we don't have a reasonable breakthrough test or reliable CO2 sensing in these machines.

Someday hopefully soon

 

[kierentec]

One of the most common cause of fatalities on a CCR is beginning a dive with the oxygen cylinder closed. The prebreathe is an effective (and critical) last check to make sure the unit is adding oxygen, and should be done on the surface before entering the water. On a manual unit, you are checking to make sure the PO2 is stable, ensuring oxygen is flowing into the loop. With a CMF (leaky valve), PO2 will drop very slowly. With a needle valve, the prebreathe is a perfect time to tune in the flow rate to your resting metabolic rate so the PO2 remains perfectly stable. The duration of the prebreathe is based on how long it takes for the PO2 to stabilize, and then breathing for a minute to verify O2 is flowing. Once the prebreathe is complete, the oxygen cylinder should stay on and the needle valve (if equipped) should stay set to what it was adjusted to on the prebreathe. O2 cylinder stays on until you exit the water and remove the rebreather.

 

[nadwidny]

I do the prebreathe for about 3-5 minutes. To get the scrubber working, especially in cold weather, and as a final check to make sure my solenoids are working and O2 is on.

 

[LFMarm]

One of the most common cause of fatalities on a CCR is beginning a dive with the oxygen cylinder closed. The prebreathe is an effective (and critical) last check to make sure the unit is adding oxygen, and should be done on the surface before entering the water. On a manual unit, you are checking to make sure the PO2 is stable, ensuring oxygen is flowing into the loop. With a CMF (leaky valve), PO2 will drop very slowly. With a needle valve, the prebreathe is a perfect time to tune in the flow rate to your resting metabolic rate so the PO2 remains perfectly stable. The duration of the prebreathe is based on how long it takes for the PO2 to stabilize, and then breathing for a minute to verify O2 is flowing. Once the prebreathe is complete, the oxygen cylinder should stay on and the needle valve (if equipped) should stay set to what it was adjusted to on the prebreathe. O2 cylinder stays on until you exit the water and remove the rebreather.

Only for the solenoid or leaky valve. The fact that the tank is open is typically ensured by doing manual O2 addition (which is more safety critical than leaky valve or solenoid).

 

[kensuf]

I use it as a function check. Start with checking valves are on, then testing the regs (and BOV if you have one), then go for inflation devices, next do a DIL flush to make sure DIL add works (PO2 drops), then an O2 flush to see if you can bring it back up to 0.7-0.8, finally keep breathing for a minute or two to make sure the unit is holding PO2 or if it's crashing.