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