Request for comments on article - mixed OC / CCR teams

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I don't know any CCR divers that use ditchable weights, but some do add weights when diving in salt water in a dry suit, if the rig is well weighted in fresh water or with less exposure suit buoyancy. I do that with mine (JJ), there are these tubes that you can put lead weights into and take out when you need less ballast. They are perfectly positioned - it's a well designed rebreather.

I use a weight belt. In fresh water I might get away without one if I had a full setup with two bailouts and a suit inflate cylinder but usually I will have a couple of kg. My most recent (ha months ago!) buddy was also using a weight belt, unusually heavier than mine due to diving a prototype undersuit.

I have some tube weights in my garage but have not been brave enough to try them out yet. They went to Scapa and back but stayed in the car :)

I did once lose a weight belt on a CCR dive. I was at 12m on a shallow scenic no stop dive, heard a clang as the weight hit my bailout and ended up at the surface and managed to get back to the belt by bailing out, emptying the counter lungs and squeezing my suit. Luckily my buddies stopped so finding the belt was not too hard.

For some reason I was using a club belt which I threw away afterwards, the buckle was rubbish. A lesson in making sure of small details and not becoming complacent.
 
DoctorMike,

That brings up one other difference between OC divers and CCR divers; exercise (or rather, lack of exercise). OC divers usually are not concerned about the work of diving, whereas CCR divers seem obsessed with not exerting. This is because CO2 buildup is very hazardous, and also decreases the “run time” of their absorbent. So when doing a mixed dive (CCR & OC divers), the pace is set by the slowest. This is something from hiking that needs, I think, special emphasis when CCR and OC divers dive as a buddy team.

SeaRat
This is really only a concern for deep dives with excessive gas density. I find that by the time I have a heap of helium in my mix the OC divers are more worried about gas consumption and so quite measured in their exertion.

I am only doing relatively short dives, less than two hours, and so for me the life of the sorb is well down the list of priorities. Typically I get two dives from one fill, maybe four if shallow and short.
 
DoctorMike,

That brings up one other difference between OC divers and CCR divers; exercise (or rather, lack of exercise). OC divers usually are not concerned about the work of diving, whereas CCR divers seem obsessed with not exerting. This is because CO2 buildup is very hazardous, and also decreases the “run time” of their absorbent. So when doing a mixed dive (CCR & OC divers), the pace is set by the slowest. This is something from hiking that needs, I think, special emphasis when CCR and OC divers dive as a buddy team.

SeaRat

@John C. Ratliff

1. Exercise
The main issue is to not 'over breath' the scrubber. Effectively push too much CO2 through the scrubber too fast.
if we use 20 litre/min, you expire 4% CO2. There needs to be sufficient scrubber material so that the 20 litres passes through the scrubber and all the CO2 is 'caught' by the scrubber.
So -
If we where to say that there are in total 200 molecules, 8 of these are CO2, hidden in the 200, these 8 must be caught by the scrubber material, or you get 'break through'.
Now -
The perceptive will note this is at the surface. At 40m the absolute pressure is 5 bar.
So -
The 200 molecules are now 1000, but we do NOT produce any more CO2 for the same level of exercise. So we still have 8 molecules hidden in the 1000. These still must be caught by the scrubber material, but they are far better hidden among the 1000.
So -
There must be sufficient depth of scrubber material to ensure you 'catch' those pesky 8 molecules of CO2.
ASSUME -
that for every 25 mm (1") of scrubber depth you will catch 10 molecule of CO2 in 1000 molecules of Gas if we pass 1000litres/min through the scrubber.
So -
If we are certain that the scrubber will catch the CO2.
BUT -
If we start to work hard and double our breathing rate. to 40 litres/min (on the surface). At 40 litres/min we are now pushing 2000 litres/min through the scrubber with 16 molecules of CO2.
NOW
With a 25mm (1") depth of scrubber at 40m we WILL get break through.

Thats why CCR divers don't like heavy levels of exertion at depth.
- A new scrubber is 'safer' because there is a maximum depth of unused scrubber material
- That is why CCR divers have a maximum time at depth
(APD Inspiration - At 50m (or deeper) the diver must leave the bottom at 100min. At 20m the diver must leave the bottom 140 min. Total scrubber time is 180mins)
(This is for 1.6lpm of CO2).
2. Descent / Ascent Speed.
DESCENT
CCR divers do not like to descend too quickly. To avoid the risk of 'spiking' the PO2.
Most CCR divers have a low set point of 0.7.
From the surface to 10m the pressure doubles - 0.7 x 2 = 1.4!
From the surface to 20m the pressure tripples - 0.7 x 3 = 2.1!

Obviously, the O2 is being consumed, so it will drop, and the unit will 'top up' the O2 as it is metabolised to bring it back to set point.
In addition, 'DIL' is being added to maintain sufficient volume of gas in the counterlung, so further diluting the gas in the loop.
A slow(ish) descent, allows the O2 to be used (metabolised) during the descent, avoiding 'spiking' the PO2.
At the same time, the Unit is attempting to maintain the set point.

ASCENT
During ascent, the task loading for a CCR diver is quite high.
The PO2 is dropping, and the unit is topping up the O2 to attempt to maintain the SP. (so ADDING gas to the counter lung).
The Gas in the counterlung is expanding (Increasing buoyancy), which needs to be dumped
As per normal the Drysuit & / or Wing needs to be managed to maintain neutral buoyancy.

3. Sawtooth Profiles.
CCR divers HATE sawtooth profiles.
They want to go to the deepest part of the diver early, then progressively ascend.

This is not new - all divers OC and CCR should avoid sawtooth profiles.

CCR divers particularly dislike ascending then descending (sawtooth profiles).
Every time they descend they use DIL - to maintain enough volume of gas in the breathing loop (counterlung), and to adjust buoyancy. They have a SMALL amount of Diluent (3 litres), which is used both for buoyancy and the counterlung.

If you descend to your deepest part of the dive and then progressively ascend. Then DIL is only used ONCE, at the start. In addition keeping the PO2 steady is easier, once established on the bottom.
Constant ascents and descents at depth use the limited supply of Gas in the Diluent cylinder quickly.

Hope that helps John.

However, we are progressively complicating Mikes 'briefing'.
Although knowing that CCR divers want to keep workload to a minimum, and don't wish to descend too quickly are relevant to the dive briefing .

4. Low / High Setpoint
Because it is not possible to maintain a Hight set point of 1.3 bar on the surface, most units have two set points.
Setpoint High is generally set at 1.3 PO2, and is selected either automatically by the unit or manually by the diver. (It can be lower, especially were the UPTD or CNS dose is of particular concern or where workload is high).
Setpoint Low is generally set at 0.7 PO2, and is the default. This set point must be selected when on the surface. It is selected at then end of the dive before the last part of the ascent from the last stop.


ADDENDUM Add to descent section GJL + Explanation on Low/High set point
 
Gareth,

Thank you. That was a fine, in-depth explanation of what I had causally observed.

John
 
My pleasure John

Although we are high jacking Mikes thread, and adding more and more detail.

Explaining how and why things are of particular importance, as we are doing here, remove mysticism helps everyone involved. I remember when people said Nitrox was too complicated for recreational divers, then trimix was too complicated.

My view is that the 'briefing' needs to be simple, with minimal action points to ensure it is remembered and practical in the real world. Also, there is no standardisation between the different manufacturers and the different units. So keep it minimalist and simple.

1. Know how to put me on the surface.
2. Know how to switch the BOV and / or close the loop.
3. Know how YOU are going to get gas from ME if you need it.

Gareth
 
My pleasure John

Although we are high jacking Mikes thread, and adding more and more detail.

You have my permission! I actually like "organic" threads that grow, within reason. It's more like a nice conversation in a pub between friends, less like a legal deposition.


My view is that the 'briefing' needs to be simple, with minimal action points to ensure it is remembered and practical in the real world. Also, there is no standardisation between the different manufacturers and the different units. So keep it minimalist and simple.

Right but some things are the same in all CCRs , so I do think that it helps to explain that.


1. Know how to put me on the surface.
2. Know how to switch the BOV and / or close the loop.
3. Know how YOU are going to get gas from ME if you need it.

Gareth

Yeah, I think that it's a good idea to streamline the summary at the end into a checklist. I think that one of the most important things is that the OC diver needs to be able to differentiate between distracted, unresponsive and unconscious.
 
OK, new question!

Our underwater photography club just had a great zoom session with Peter Symes, the editor-in-chief of X-Ray magazine. One of the things that I took home from the talk was that opening with an anecdote sometimes grabs the readers attention. What do you guys think about adding this to the opening of the text, assuming that I'm going to submit this for publication?


Bill and Tony had been dive buddies for several years, frequently visiting the local wrecks near their shore town. Tony had been content doing single tank recreational dives, but Bill had gotten into technical diving and had recently certified on a closed circuit rebreather. Given their longstanding friendship and the lack of other local rebreather divers, Bill had continued to dive with Tony, visiting the same wrecks and adjusting the dive plan so that Tony would never get close to his gas or decompression limits.

At 90 feet, about 10 minuets into their first dive on beautiful morning with spectacular conditions, Tony tried to get Bill’s attention and couldn’t. Bill was staring right at Tony. He seemed to be breathing normally, but he didn’t respond to hand signals. Tony reached out and grabbed Bill’s shoulder, and he still didn’t respond.
 
OK, new question!

Our underwater photography club just had a great zoom session with Peter Symes, the editor-in-chief of X-Ray magazine. One of the things that I took home from the talk was that opening with an anecdote sometimes grabs the readers attention. What do you guys think about adding this to the opening of the text, assuming that I'm going to submit this for publication?


Bill and Tony had been dive buddies for several years, frequently visiting the local wrecks near their shore town. Tony had been content doing single tank recreational dives, but Bill had gotten into technical diving and had recently certified on a closed circuit rebreather. Given their longstanding friendship and the lack of other local rebreather divers, Bill had continued to dive with Tony, visiting the same wrecks and adjusting the dive plan so that Tony would never get close to his gas or decompression limits.

At 90 feet, about 10 minuets into their first dive on beautiful morning with spectacular conditions, Tony tried to get Bill’s attention and couldn’t. Bill was staring right at Tony. He seemed to be breathing normally, but he didn’t respond to hand signals. Tony reached out and grabbed Bill’s shoulder, and he still didn’t respond.
that will want me to know a) what the problem was and b) what did tony do - so yes a great hook line
 
that will want me to know a) what the problem was and b) what did tony do - so yes a great hook line

Right, and then the question is whether or not to go back to it in the end... I guess the rest of the article is really the answer to what Tony should have done - before getting in the water and once this happened.

Thanks!
 
DoctorMike,
I enjoyed the article as someone who occasionally dives mixed team. Diving wrecks in the Great Lakes I often worry more about other people who are on the boat and making sure they understand where my alternate is they run out of gas. I have visions of someone grabbing my loop then needing to buddy share from my bailout to the surface. Since buddy sharing isn't a skill that isn't often taught I'm sure it wouldn't be pretty. These are all worries on dive within the recreational limit.

As a Poseidon diver (I know, I'm going to die with so few O2 sensors) one thing to note is that many of the units don't have MAV's and training has a strong mantra of when it doubt bail out. For those units the best you could do for someone is to switch the BOV to open circuit.
 
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