Deco option for 55-60m dives

[nickb]

Up to now I’ve been using 50% and/or O2 for deco as my usual buddies are DIR-inclined although they are not wedded to standard gases, e.g. we would use best-mix nitrox or 25/25 to 45m and 21/35 to 55m(ish).

We’ve only this year started to push the limits of our TDI trimix tickets and I’m now thinking about other choices for deco, with an eye on bail-out options should things go tits-up.

Playing with some numbers in V-Planner this morning (slack day at work, as usual ) I’ve come to the conclusion that using 32% for a 55/60m will offer more options than 50% with little or no penalty in deco obligation. I’ll get a slightly lower CNS loading and, more importantly, it will allow me to switch at 40m, or even 50m if I’m prepared to push the PO2 in an emergency. I can also save a shitload of backgas into the bargain (even more if I use the 32% as a travel mix, which would offer a good practice opportunity for an advanced ticket).

I can’t see a downside to this, aside from a longer deco obligation if I should lose one of my deco gasses.

Any comments?

[Jetsam]

I wouldn't be too happy with the bit of PPN2 spike at 40m and then the PPO2 spike at 6m, but it's all theory anyway...

[milldog]

If you run the figure try 40% and 80% deco gases gives you a better depth switch and allows for a bit of O'F*ck

Using 50% and 100%

Dec to 60m (3) Triox 21/35 18m/min descent.
Level 60m 26:40 (30) Triox 21/35 1.46 ppO2, 29m ead, 35m end
Asc to 36m (32) Triox 21/35 -9m/min ascent.
Stop at 36m 0:20 (33) Triox 21/35 0.96 ppO2, 16m ead, 20m end
Stop at 30m 2:00 (35) Triox 21/35 0.84 ppO2, 12m ead, 16m end
Stop at 27m 2:00 (37) Triox 21/35 0.77 ppO2, 11m ead, 14m end
Stop at 24m 2:00 (39) Triox 21/35 0.71 ppO2, 9m ead, 12m end
Stop at 21m 4:00 (43) Nitrox 50 1.54 ppO2, 10m ead
Stop at 18m 1:00 (44) Nitrox 50 1.39 ppO2, 8m ead
Stop at 15m 3:00 (47) Nitrox 50 1.24 ppO2, 6m ead
Stop at 12m 4:00 (51) Nitrox 50 1.10 ppO2, 4m ead
Stop at 9m 6:00 (57) Nitrox 50 0.95 ppO2, 2m ead
Stop at 6m 21:00 (78) Oxygen 1.60 ppO2, 0m ead
Surface (84) Oxygen -1m/min ascent.

40% and 80%

Dec to 60m (3) Triox 21/35 18m/min descent.
Level 60m 26:40 (30) Triox 21/35 1.46 ppO2, 29m ead, 35m end
Asc to 36m (32) Triox 21/35 -9m/min ascent.
Stop at 36m 0:20 (33) Triox 21/35 0.96 ppO2, 16m ead, 20m end
Stop at 30m 2:00 (35) Triox 21/35 0.84 ppO2, 12m ead, 16m end
Stop at 27m 3:00 (38) Nitrox 40 1.47 ppO2, 18m ead
Stop at 24m 1:00 (39) Nitrox 40 1.35 ppO2, 16m ead
Stop at 21m 1:00 (40) Nitrox 40 1.23 ppO2, 14m ead
Stop at 18m 2:00 (42) Nitrox 40 1.11 ppO2, 11m ead
Stop at 15m 3:00 (45) Nitrox 40 1.00 ppO2, 9m ead
Stop at 12m 5:00 (50) Nitrox 40 0.88 ppO2, 7m ead
Stop at 9m 7:00 (57) Nitrox 80 1.51 ppO2, 0m ead
Stop at 6m 21:00 (78) Nitrox 80 1.28 ppO2, 0m ead
Surface (84) Nitrox 80 -1m/min ascent.

32% and 80%


Dec to 60m (3) Triox 21/35 18m/min descent.
Level 60m 26:40 (30) Triox 21/35 1.46 ppO2, 29m ead, 35m end
Asc to 36m (32) Triox 21/35 -9m/min ascent.
Stop at 36m 2:20 (35) Nitrox 32 1.46 ppO2, 30m ead
Stop at 30m 1:00 (36) Nitrox 32 1.27 ppO2, 24m ead
Stop at 27m 1:00 (37) Nitrox 32 1.18 ppO2, 22m ead
Stop at 24m 1:00 (38) Nitrox 32 1.08 ppO2, 19m ead
Stop at 21m 1:00 (39) Nitrox 32 0.99 ppO2, 17m ead
Stop at 18m 3:00 (42) Nitrox 32 0.89 ppO2, 14m ead
Stop at 15m 3:00 (45) Nitrox 32 0.80 ppO2, 12m ead
Stop at 12m 5:00 (50) Nitrox 32 0.70 ppO2, 9m ead
Stop at 9m 8:00 (58) Nitrox 80 1.51 ppO2, 0m ead
Stop at 6m 21:00 (79) Nitrox 80 1.28 ppO2, 0m ead
Surface (85) Nitrox 80 -1m/min ascent.

there isnt much diffrence and as for the CNS richard was running 300% CNS on the deco station

Originally Posted by richstevenson

The main boat deployed safety divers after 3 hours as that's when the CNS load starts to rack up and then they swapped out every hour to avoid boredom and excess in water time. CNS loadings were over 300% but we had zero 02 incidents, not even any chesty coughs!

Graham

[Okeanos]

My choice has always been................

Single deco gas 60%

2 deco gases 40% & 80 %, I have and do use 30% but with 20% helium.

I don't use V-Planner, I use Deco-Planner or Decochek, in fact I only use Decochek now!

[Colin Marriott]

My tuppence

Down to 60mtrs I use 20/30 50% and 100% . . . .

My theory being :-
Switching to 50% will then leave the N2 at the same level, and flush out all of the He at maximum rate

Then the 100% will reduce the N2 to an acceptable level at the end of deco.

ColinM

[John Touhig]

Nick,

For the deeper stuff I use 2 deco gases, 50% & 80% I've found that this gives me a nice deco curve without too many CNS issues, although it means I'm not switching until 21m. For bailout I use Paul, that's what he's there for.

The main reason I use 50% and not 40% is that for all my 1 deco gas dives I use 50%. I live about 40 miles from a filling station so its a case of simple logistics. I keep a number of stages full with either 50% or 80% and 1 twinset full of 18/40 and the other full of either 20/30 or air so I'm good to go.

HTH


John

[Mark Chase]

Originally Posted by nickb

Up to now I’ve been using 50% and/or O2 for deco as my usual buddies are DIR-inclined although they are not wedded to standard gases, e.g. we would use best-mix nitrox or 25/25 to 45m and 21/35 to 55m(ish).

We’ve only this year started to push the limits of our TDI trimix tickets and I’m now thinking about other choices for deco, with an eye on bail-out options should things go tits-up.

Playing with some numbers in V-Planner this morning (slack day at work, as usual ) I’ve come to the conclusion that using 32% for a 55/60m will offer more options than 50% with little or no penalty in deco obligation. I’ll get a slightly lower CNS loading and, more importantly, it will allow me to switch at 40m, or even 50m if I’m prepared to push the PO2 in an emergency. I can also save a shitload of backgas into the bargain (even more if I use the 32% as a travel mix, which would offer a good practice opportunity for an advanced ticket).

I can’t see a downside to this, aside from a longer deco obligation if I should lose one of my deco gasses.

Any comments?

On OC I only used three deco gases. 21/35 (60m+), 32% and 80%. For 45-60m dives id use 32% and 80% for the same reasions you have reached on your own. I can bailout on to the 32% deeper if i have a need to and the overall CNS loadings were much better on long dives.

Doing deco on 32 and 80% is a damed site faster than doing the whole deco on 50% and its not much different from running 50 & 100.

I never noticed a problem with coming off 21/35 and switching to 32% at 40m but then again I wasn't there for long. Couple of mins later id be at 21m

ATB

Mark Chase