Gas sharing at deco stops

[lermontov]

Buddy breathing deco gas is stupid. I'm required to teach it, but I explain that skills that are not regularly practiced degrade and RARELY does anyone actually practice buddy breathing. Buddy breathing a deco gas if you haven't practiced the skill opens divers up to making other mistakes, such as blowing a deco stop (either up or down) due to task loading.

The best answer is to never be in this situation to begin with by properly maintaining your gear, but if one buddy runs out of deco gas for some reason, the next best option is for one diver to deco on the richer gas while the other is using the richest "lean mix" to continue off-gassing while waiting for the first diver to clear the stop, then diver B goes on the rich mix.

my initial post was more an interest in the theoretical side of off gassing than a scenario event of losing all gasses. As has already mentioned back gas will be available even if it means an extended stop.

If as Norhrneone suggests if its a regular breathing pattern then theres no need to add extra deco time although its would be a PINA to do this for extended time - theres more efficient ways to handle this

If PfcAj suggests
Intervals like "you breath for 5mins, then I breath it for 5mins" is the move. Depending on length of deco stops you could even alternate stops.
would you also switch your computer gas f02 choices to keep track of correct deco times

adding deco times x1.5 or any combo of time extrensions are based on what?

 

[doctormike]

my initial post was more an interest in the theoretical side of off gassing than a scenario event of losing all gasses

If you lose all of your gasses, offgassing is the least of your worries!

If as Norhrneone suggests if its a regular breathing pattern then theres no need to add extra deco time although its would be a PINA to do this for extended time - theres more efficient ways to handle this

As we discussed, the breathing pattern doesn't affect offgassing or deco obligation. Of course, there are all sorts of unknown variables that contribute to a given diver's chance of getting bent, and maybe stress is one of them. But as long as you are breathing, you are offgassing, and you offgass proportional to the PO2 differential between your tissue compartments and that of whatever you are breathing at ambient pressure. No matter what your RMV is.

If PfcAj suggests
Intervals like "you breath for 5mins, then I breath it for 5mins" is the move. Depending on length of deco stops you could even alternate stops.
would you also switch your computer gas f02 choices to keep track of correct deco times

If you are making an ascent using your computer to generate stops, you obviously have to let it know what you are breathing at all times.

 

[PfcAJ]

my initial post was more an interest in the theoretical side of off gassing than a scenario event of losing all gasses. As has already mentioned back gas will be available even if it means an extended stop.

If as Norhrneone suggests if its a regular breathing pattern then theres no need to add extra deco time although its would be a PINA to do this for extended time - theres more efficient ways to handle this

If PfcAj suggests
Intervals like "you breath for 5mins, then I breath it for 5mins" is the move. Depending on length of deco stops you could even alternate stops.
would you also switch your computer gas f02 choices to keep track of correct deco times

adding deco times x1.5 or any combo of time extrensions are based on what?

You're on the deco gas for less time, so you need to make up that time.

If you put in something like 150' for 30mins and compare using 50% to just using backgas, 50% roughly halfs your total time from 70' to surface.

64mins with no deco gas vs 34 with 50%. Pretty close to half.

If you're on the bottle 50% less, I think it's reasonable to do 50% more deco.

that ratio kinda breaks down on deeper dives and with more gases, but it's decent enough to get you through those stops where you're having a deco gas issue. It's simple, and generally fairly conservative if you breath your other working/ breathable deco gases during the "off bottle" time.

For instance, 270 for 25mins with 12/65, 35/25, 50%, 100%.

Oxygen time would be 61mins. Delete oxygen and do the last stop(s) on 50% and that time goes up to 92mins. 1.5x.

Could you get away with doing something like 1.25x? Yea, I think so. But it's a simple strategy that will get you out of the water. Additionally, maintaining 1.5x the needed deco gas gives you some wiggle room if you need the gas and some contingency if something breaks.

 

[lermontov]

You're on the deco gas for less time, so you need to make up that time.

If you put in something like 150' for 30mins and compare using 50% to just using backgas, 50% roughly halfs your total time from 70' to surface.

64mins with no deco gas vs 34 with 50%. Pretty close to half.

If you're on the bottle 50% less, I think it's reasonable to do 50% more deco.

that ratio kinda breaks down on deeper dives and with more gases, but it's decent enough to get you through those stops where you're having a deco gas issue. It's simple, and generally fairly conservative if you breath your other working/ breathable deco gases during the "off bottle" time.

For instance, 270 for 25mins with 12/65, 35/25, 50%, 100%.

Oxygen time would be 61mins. Delete oxygen and do the last stop(s) on 50% and that time goes up to 92mins. 1.5x.

Could you get away with doing something like 1.25x? Yea, I think so. But it's a simple strategy that will get you out of the water. Additionally, maintaining 1.5x the needed deco gas gives you some wiggle room if you need the gas and some contingency if something breaks.

yes all makes sense- cheers

 

[TrimixToo]

As I understand it, the rate of offgassing is proportional to the differential partial pressure of the dissolved gas in the bloodstream and the inspired gas in the lungs. If you do a reasonably normal buddy breathing protocol (2 breaths, pass the reg), are you offgassing quickly enough to change the partial pressure of the gas in the lungs to any significant degree, or is it so little that it's statistical noise lost in normal operational variance?

I'd always assumed offgassing was slow enough that padding stops would not be needed in this case, but I am happy to be corrected if someone can demonstrate otherwise.

 

[Dan_P]

What is the physiology regarding off gassing at deco stops when sharing deco gas? is there a rule of thumb regarding extended time for reduced off gassing due to irregular breathing?

Great question!

There is a general concern with work of breathing/breathing resistance due to risk of hypercapnia, for a few reasons including N2 off-gassing.

In open cirquit diving, it's best managed by refraining from high-density gas choices at depth.

Several agencies advocate using air/nitrox no deeper than 30m, to stay within 5g/L gas density - with the first deco stop on a dense mix at 21m.

As for rules of thumb, the above may serve very well.
(You might be interested in Dr. Simon Mitchell's presentation "Respiratory Failure in Technical Diving", which is available on youtube)

More relevant to the procedural aspect of your question, I don't personally subscribe to buddy breathing - it's a technique that 1) has an increased complexity level, and 2) is no longer taught to the diver from the onset of diver training (making it inconsistant with your training).

In the days of diving with one 1.-stage and one 2.-stage, it made sense to solve a gear problem with that procedure, but since large-scale implementation of an extra 2.-stage, it's obsolete.

Personally, I dive and teach a solution that is taught to rote from Open Water level through tech training - longhose donation.

If I dive sidemount, I donate the longhose and switch to necklace - standard longhose donation - and both diver A and B complete their deco stops at the same time.

If I dive on a backmount rig, I would donate and then complete deco stops/cycles in rotation (first diver A completes a stop, then diver B, then next stop diver B first, then diver A, etc., etc.), with the downside that this will double those deco stop times in and also consume more backgas (accounted for in gas planning).
It's imperfect to be sure, but more viable than buddy-breathing, in my view.

 

[PfcAJ]

Great question!

There is a general concern with work of breathing/breathing resistance due to risk of hypercapnia, for a few reasons including N2 off-gassing.

In open cirquit diving, it's best managed by refraining from high-density gas choices at depth.

Several agencies advocate using air/nitrox no deeper than 30m, to stay within 5g/L gas density - with the first deco stop on a dense mix at 21m.

As for rules of thumb, the above may serve very well.
(You might be interested in Dr. Simon Mitchell's presentation "Respiratory Failure in Technical Diving", which is available on youtube)

More relevant to the procedural aspect of your question, I don't personally subscribe to buddy breathing - it's a technique that 1) has an increased complexity level, and 2) is no longer taught to the diver from the onset of diver training (making it inconsistant with your training), and for good reason;

In the days of diving with one 1.-stage and one 2.-stage, it made sense to solve a gear problem with that procedure, but since large-scale implementation of an extra 2.-stage, it's obsolete.

Personally, I prefer a solution that is taught to rote from Open Water level through tech training - longhose donation.

If I dive sidemount, I donate the longhose and switch to necklace - standard longhose donation - and both diver A and B complete their deco stops at the same time.

If I dive on a backmount rig, I would donate and then complete deco stops/cycles in rotation (first diver A completes a stop, then diver B, then next stop diver B first, then diver A, etc., etc.), with the downside that this will double those deco stop times in and also consume more backgas (accounted for in gas planning).
It's imperfect to be sure, but more viable than buddy-breathing, in my view.

And you're carrying 2x the required deco gas for your SM option?

 

[Germie]

Just do 1.5 times every stop and switch between the available gases. Then no buddybreathing needed. stop 1 diver 1 has decogas, diver 2 has decogas, diver 3 stays on backgas or travelgas. Next stop, diver 1 on backgas, diver 2 on decogas, diver 3 on decogas. All stops done 1.5 time planned time.

 

[Dan_P]

And you're carrying 2x the required deco gas for your SM option?

I'm carrying the same gas amount, which I think is a fair basis for comparison.

Whether on backmount or sidemount, I will run out of deco gas at the same depth, and swap to backgas - which at that point, I'll have more of on sidemount than backmount because I haven't greatly increased my previous - deeper - deco stops.

And, I'll have used the same donation technique I've learned from Open Water Diver-level.

 

[UWZen]

I'm carrying the same gas amount, which I think is a fair basis for comparison.

Whether on backmount or sidemount, I will run out of deco gas at the same depth, and swap to backgas - which at that point, I'll have more of on sidemount than backmount because I haven't greatly increased my previous - deeper - deco stops.

And, I'll have used the same donation technique I've learned from Open Water Diver-level.

I don't understand this. How do you have more backgas in sidemount if your gas planning is the same?