Cheaping out on Trimix - travel gas

[boulderjohn]

TDI, NASE, PADI, GUE, IANTD, NSS-CDS just to name a few..... None of them promote 1.4 bottom PPO2 for trimix dives.

A few will promote up to a 1.4PPO2 in their basic nitrox course (less than 30m/100ft) on a RECREATIONAL dive.

I only have trimix training through two of those agencies, but both of them identified 1.4 as the standard for trimix dives.

 

[tbone1004]

No. It's a damn nuisance. Drives up the CNS, is sensitive to the stop levels (what if you're decoing out on a shot line?) and you need a booster.

80% is far easier: need 170 bar of O2 then top off with 62 bar of air for 232 bar.
80% is quicker to get on to at 9m
80% is safer if you've a bouncy stop or cannot stop at 6m (if you're on a shot line!)
80% doesn't need gas breaks
80% doesn't kill your lungs
80% makes negligible difference to deco times (as you're on it earlier)

Easier-not necessarily, you have to hook it up to two fill whips. One thing if you are cheap and don't have access to a booster, but I find plugging into a booster to be much easier
Quicker-quicker to get on, not to get out
Safer-not for bouncy stops, you aren't going to kick it if you drop to 30ft for a few seconds, and if you can't stop at 6m, use 50%
Gas breaks-gas breaks are a function of ppO2 not fO2, and your body will appreciate a break from extended periods of 1.3 @20ft
Kill lungs-see above, it's about ppO2, not fO2
Deco times-see above, example below.

150ft for 30 minutes on 21/35 using ZHL 50/80
100% -37 minutes total
80% -33 minutes getting on at 30ft
50%- 33 minutes

So why would I use 80% when I can get the same total deco time from 50% which is much easier to blend than 80% and using your logic, you can get on it at 70ft instead of 30? There is a reason 50% is the standard deco gas for those types of dives.

 

[Wibble]

I only have trimix training through two of those agencies, but both of them identified 1.4 as the standard for trimix dives.

Thankyou

I believe that this number is being scrutinised along with the gas densities (and GFs, deep stops, longer deco...).

Having recently been through some advanced CCR training, a lower setpoint (than 1.3) is recommended for deeper dives.

 

[Wibble]

Easier-not necessarily, you have to hook it up to two fill whips. One thing if you are cheap and don't have access to a booster, but I find plugging into a booster to be much easier
Quicker-quicker to get on, not to get out
Safer-not for bouncy stops, you aren't going to kick it if you drop to 30ft for a few seconds, and if you can't stop at 6m, use 50%
Gas breaks-gas breaks are a function of ppO2 not fO2, and your body will appreciate a break from extended periods of 1.3 @20ft
Kill lungs-see above, it's about ppO2, not fO2
Deco times-see above, example below.

150ft for 30 minutes on 21/35 using ZHL 50/80
100% -37 minutes total
80% -33 minutes getting on at 30ft
50%- 33 minutes

So why would I use 80% when I can get the same total deco time from 50% which is much easier to blend than 80% and using your logic, you can get on it at 70ft instead of 30? There is a reason 50% is the standard deco gas for those types of dives.

With access to a booster, high pressure O2 is readily available.

I'll reduce my comment down to "I don't like 100%" for all of those reasons listed. But most of all it means that 80% is less likely to have problems - with no discernable difference in deco times against 100%.

 

[tbone1004]

With access to a booster, high pressure O2 is readily available.

I'll reduce my comment down to "I don't like 100%" for all of those reasons listed. But most of all it means that 80% is less likely to have problems - with no discernable difference in deco times against 100%.

with your arguments though, why not use 50% which is an accepted standard deco gas and makes all of your points that much stronger.

 

[grantctobin]

with your arguments though, why not use 50% which is an accepted standard deco gas and makes all of your points that much stronger.

And a better emergency surface gas (or at least extra volume) when someone takes a hit.

 

[Wibble]

with your arguments though, why not use 50% which is an accepted standard deco gas and makes all of your points that much stronger.

That's easy... it's a rubbish deco gas!

To expand, it's great as a first deco gas which you can get on to at 21m/70', or even a bit deeper if you're in a mess (bailing out). However, half of all your deco will be at 6m/20', and at that depth the PPO2 of 50% is (mix x depth) is 0.8 which is lousy (half of what you're breathing is nitrogen which you're trying to off gas).

At 6m, or 1.6ATA, the Rolls Royce is 100% O2 meaning the nitrogen's nowhere to hide except floating upwards. This has its drawbacks (it's a very hot gas so CNS, OTUs, although this can be mitigated if you can finish deco shallower than 6m). 80% at 1.6ATA is a PPO2 of (1.6 x 0.8) is 1.28, which is enough to quickly get rid of the excess N2, but not enough to rack up the CNS and OTUs.

(Recently been though these discussions regarding bailout gasses when doing a deep bailout -- 50% is fine and dandy except it is too slow to deco. Even 60% is significantly better. Of course the best is to have two deco bailouts, 50% plus 80%/100%, but that's three cylinders worth with the bottom gas)

 

[tbone1004]

That's easy... it's a rubbish deco gas!

To expand, it's great as a first deco gas which you can get on to at 21m/70', or even a bit deeper if you're in a mess (bailing out). However, half of all your deco will be at 6m/20', and at that depth the PPO2 of 50% is (mix x depth) is 0.8 which is lousy (half of what you're breathing is nitrogen which you're trying to off gas).

At 6m, or 1.6ATA, the Rolls Royce is 100% O2 meaning the nitrogen's nowhere to hide except floating upwards. This has its drawbacks (it's a very hot gas so CNS, OTUs, although this can be mitigated if you can finish deco shallower than 6m). 80% at 1.6ATA is a PPO2 of (1.6 x 0.8) is 1.28, which is enough to quickly get rid of the excess N2, but not enough to rack up the CNS and OTUs.

(Recently been though these discussions regarding bailout gasses when doing a deep bailout -- 50% is fine and dandy except it is too slow to deco. Even 60% is significantly better. Of course the best is to have two deco bailouts, 50% plus 80%/100%, but that's three cylinders worth with the bottom gas)

how can you say it's too slow? the example that I gave has an identical deco time to your 80% gas and that wasn't even a strategic example, just a random choice of depth/time.

 

[rjack321]

This is like the wibblelocks!

50% too slow!
100% too rich!
80% just riigghhttt!

Sorry i'm laughing so hard at these contrived arguments which are almost entirely tangential to the OP. Except that 50% saves him using 21/35 for a few extra stops - which has the nifty benefit of minimizing his minimum gas requirements. For the OP's diving I would use 50%, and at his level get 2 deco dives (~15-25mins TTS) out of an AL80 before getting it refilled.

 

[Wibble]

how can you say it's too slow? the example that I gave has an identical deco time to your 80% gas and that wasn't even a strategic example, just a random choice of depth/time.

(Not on for an argument!)

If you've a fair bit of deco to do, for example if you're doing 40 mins at 60m/200' this would give you roughly 40 x 2 = 80 mins of deco (60m's about 2:1, for every min at 60, it's 2 mins of deco). Of that deco time, roughly half will be at 6m/20', so about 40mins.

If we did the runtimes for a full deco on 50% (as you'd be doing on a bailout) this would be a lot slower than if you were doing a deco with 50% and 80% (obviously).

Turns to Multideco... for ease I'll use an OC dive to 60m/200' for 40mins on 18/45...
GF50:80, SAC = 15 litres/min bottom and 12 litres/min for deco
(showing only the last 3 stops).

50% only
12m 7 (70) Nitrox 50 1.10 ppO2
9m 12 (82) Nitrox 50 0.95 ppO2
6m 64 (146) Nitrox 50 0.80 ppO2
Surface (148)
OTUs: 140
CNS: 50%
2093 ltr Nitrox 50


60% only
12m 7 (78) Nitrox 60 1.31 ppO2
9m 11 (89) Nitrox 60 1.14 ppO2
6m 63 (152) Nitrox 60 0.96 ppO2
Surface (154)
OTU's this dive: 155
CNS Total: 56.8%
1838 ltr Nitrox 60


50% + 80%
12m 7 (70) Nitrox 50 1.10 ppO2
9m 10 (80) Nitrox 80 1.51 ppO2
6m 47 (127) Nitrox 80 1.28 ppO2
Surface (129)
OTUs: 174
CNS: 67%
574 ltr Nitrox 50
1147 ltr Nitrox 80


50% + 100%
12m 7 (70) Nitrox 50 1.10 ppO2
9m 12 (82) Nitrox 50 0.95 ppO2
6m 42 (124) Oxygen 1.60 ppO2
Surface (126)
OTUs: 181
CNS Total: 112% <Over 100>
846 ltr Nitrox 50
825 ltr Oxygen


Thus:
50% alone uses a full ali80; little over for contingency.
60% alone is slower than 50% (as breathing 8 mins more 18/45)
50% + 80% is 21 mins "more efficient" than 50% only.
100% is most efficient but blows the CNS limit - nearly double the 80% for no real time benefit. Balances the 50% and Oxygen usage.


(Or have I misunderstood?)