No Deco on Trimix?

[ivankahn]

I am not a Trimix diver nor in a class, but I have been doing some reading on Trimix and doing a little math, and so a question.

Assume a dive to 165 feet using 21% O2, 45% N2, and 34% He. At 165 feet the equivalent N2 air depth is 80 feet and the NDL is 30 minutes. For the He, the equivalent Heliox (80% He, 20% O2) is only 50 feet and the NDL is 2+ hours.

And so it would seem that a diver on this Trimix could diver to 165 feet and as long as the dive was less than 30 minutes the diver not incur a mandatory deco stop.

What have I missed?

 

[nilsdiver]

What have I missed?

Yes - you can not do air equivalent depth for deco or NDL.

165 fsw for for 30 minutes will get you something like 40 minutes of deco on 50% 02.

 

[PerroneFord]

There is no free lunch. You're still pushing inert gas into tissue and it still must come out with deco. The difference is that you won't have the narcosis at that depth.

 

[4sak3n]

The short (and dirty) answer is that helium is an inert gas, just like nitrogen, so it needs to be eliminated from your body just like nitrogen. Just because you don't have enough nitrogen in your body to require deco, it doesn't mean that you don't have enough helium to require deco.

Added to that it is absorbed (and released) by your body quicker than nitrogen so it complicates any NDL and/or deco calculation you might do.

Suffice it to say that if you go any deeper than 130 ft for extended periods of time you WILL accrue a [possible substantial] deco obligation whether you are using helium or not.

 

[DA Aquamaster]

Helium is a "faster" gas that both enters and leaves tissues faster.

So you acquire a deco obligation quicker and ascent rate is more critical and initial stop depths are almost always deeper. But on some shorter "moderate" deco profiles, you will actually end up doing more deco, not less, than on an air profile.

For example with a particular bubble gradient model, 30 minutes at 165' on air with 50% O2 for a deco gas would require:

2 min @ 80' (on air)
2 min @ 70' (50% O2 for all remaining deco)
2 min @ 60'
2 min @ 50'
4 min @ 40'
5 min @ 30'
9 min @ 20'
17 min @ 10'

For a total of 45 minutes of deco

In contrast, 30 minutes at 165' on 18/45 trimix with 50% O2 for a deco gas would require:

2 min @ 90' (on 18/45)
2 min @ 80' (on 18/45)
2 min @ 70' (50% O2 for all remaining deco)
1 min @ 60'
2 min @ 50'
4 min @ 40'
6 min @ 30'
9 min @ 20'
18 min @ 10'

For a total of 47 minutes of deco.

The major deco advantages have occurred in terms of deco gasses using higher precentages of O2.

Using air for deco on the 165' for 30 minute air dive requires 79 minutes of deco compared to 45 with 50% and only 35 using both 50% and 100%.

Closed circuit rebreathers take this one step farther by maintianing a maximum PPO2 of 1.2 to 1.4 on the working portion of the dive and then maintaining a PPO2 of 1.4 to 1.6 during the entire deco phase of the dive which gets the diver out of the water even faster - but with a greater O2 exposure.

 

[HowardE]

This same dive (using V-Planner) and 21/35

There's only 12 minutes of deco @ 10'

 

[Bismark]

This same dive (using V-Planner) and 21/35

There's only 12 minutes of deco @ 10'

I strongly suggest that you re-check that........

I think most will read your post as your suggestion that you only require 12 minutes of deco at 10' for this dive. However, if what you really mean is you are only disagreeing with the 10' deco time, then you should probably state that. If not, and you really are suggesting this dive only requires 12 min at 10' for the deco obligation, you should probably put DAN on speed dial and ask them to get the local chamber warmed up....... You don't really mean 12 min of deco do you? (please say no.........)

 

[Major Restriction]

This same dive (using V-Planner) and 21/35

There's only 12 minutes of deco @ 10'

V-Planner 3.84 by R. Hemingway, VPM code by Erik C. Baker.

Decompression model: VPM - B

DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 165ft (3) Triox 21/35 50ft/min descent.
Level 165ft 26:42 (30) Triox 21/35 1.23 ppO2, 77ft ead, 95ft end
Asc to 90ft (32) Triox 21/35 -30ft/min ascent.
Stop at 90ft 0:30 (33) Triox 21/35 0.77 ppO2, 35ft ead, 47ft end
Stop at 80ft 1:00 (34) Triox 21/35 0.71 ppO2, 30ft ead, 40ft end
Stop at 70ft 2:00 (36) Triox 21/35 0.64 ppO2, 24ft ead, 34ft end
Stop at 60ft 3:00 (39) Triox 21/35 0.58 ppO2, 18ft ead, 27ft end
Stop at 50ft 4:00 (43) Triox 21/35 0.52 ppO2, 13ft ead, 21ft end
Stop at 40ft 7:00 (50) Triox 21/35 0.46 ppO2, 7ft ead, 14ft end
Stop at 30ft 9:00 (59) Triox 21/35 0.40 ppO2, 2ft ead, 8ft end
Stop at 20ft 18:00 (77) Oxygen 1.59 ppO2, 0ft ead
Surface (77) Oxygen -30ft/min ascent.

Off gassing starts at 119.9ft

OTU's this dive: 79
CNS Total: 42.9%

 

[HowardE]

I strongly suggest that you re-check that........

I think most will read your post as your suggestion that you only require 12 minutes of deco at 10' for this dive. However, if what you really mean is you are only disagreeing with the 10' deco time, then you should probably state that. If not, and you really are suggesting this dive only requires 12 min at 10' for the deco obligation, you should probably put DAN on speed dial and ask them to get the local chamber warmed up....... You don't really mean 12 min of deco do you? (please say no.........)

Don't everyone get in a tizzy...

with 50% for deco. Just citing the previous post.... to show that the deco time with a better mix (trimix) would in fact be shorter for the dive in question.

Decompression model: VPM - B

Dec to 165ft (3) Triox 21/35 50ft/min descent.
Level 165ft 26:42 (30) Triox 21/35 1.26 ppO2, 77ft ead, 96ft end
Asc to 90ft (32) Triox 21/35 -30ft/min ascent.
Stop at 90ft 0:30 (33) Triox 21/35 0.78 ppO2, 35ft ead, 47ft end
Stop at 80ft 1:00 (34) Triox 21/35 0.72 ppO2, 30ft ead, 40ft end
Stop at 70ft 1:00 (35) Nitrox 50 1.56 ppO2, 32ft ead
Stop at 60ft 2:00 (37) Nitrox 50 1.41 ppO2, 26ft ead
Stop at 50ft 2:00 (39) Nitrox 50 1.26 ppO2, 20ft ead
Stop at 40ft 3:00 (42) Nitrox 50 1.10 ppO2, 13ft ead
Stop at 30ft 4:00 (46) Nitrox 50 0.95 ppO2, 7ft ead
Stop at 20ft 8:00 (54) Nitrox 50 0.80 ppO2, 1ft ead
Stop at 10ft 12:00 (66) Nitrox 50 0.65 ppO2, 0ft ead
Surface (66) Nitrox 50 -30ft/min ascent.

Off gassing starts at 120ft

OTU's this dive: 68
CNS Total: 25.3%

 

[Bismark]

Don't everyone get in a tizzy...

with 50% for deco. Just citing the previous post.... to show that the deco time with a better mix (trimix) would in fact be shorter for the dive in question.

Decompression model: VPM - B

Dec to 165ft (3) Triox 21/35 50ft/min descent.
Level 165ft 26:42 (30) Triox 21/35 1.26 ppO2, 77ft ead, 96ft end
Asc to 90ft (32) Triox 21/35 -30ft/min ascent.
Stop at 90ft 0:30 (33) Triox 21/35 0.78 ppO2, 35ft ead, 47ft end
Stop at 80ft 1:00 (34) Triox 21/35 0.72 ppO2, 30ft ead, 40ft end
Stop at 70ft 1:00 (35) Nitrox 50 1.56 ppO2, 32ft ead
Stop at 60ft 2:00 (37) Nitrox 50 1.41 ppO2, 26ft ead
Stop at 50ft 2:00 (39) Nitrox 50 1.26 ppO2, 20ft ead
Stop at 40ft 3:00 (42) Nitrox 50 1.10 ppO2, 13ft ead
Stop at 30ft 4:00 (46) Nitrox 50 0.95 ppO2, 7ft ead
Stop at 20ft 8:00 (54) Nitrox 50 0.80 ppO2, 1ft ead
Stop at 10ft 12:00 (66) Nitrox 50 0.65 ppO2, 0ft ead
Surface (66) Nitrox 50 -30ft/min ascent.

Off gassing starts at 120ft

OTU's this dive: 68
CNS Total: 25.3%

Okay, so I guess we can sleep easier now.........

I would suggest that there are other reasons for selecting a high He content other than just how fast or slow you get out of the water. END, PP02, etc are other factors to consider. Also, my choice to use 18/45 at this depth would be to deal with unforeseen circumstances like current, etc. I would also like to keep my END lower than 21/35 at 165' (END of 103') but that is just my choice and doesn't have to be yours. Getting out of the water quicker is not necessarily the most important factor.