ICD and gas swaps

[KevinNM]

Yes, maybe ...
but if you always dive using these mixes with your buddies ....

That’s why they are ‘standard gases’.

 

[TheSnake]

That’s why they are ‘standard gases’.

Standard or not, you must plan your dive and you need to check and label gasses after the blender checks.

Now, if a gas is named 35/30 instead of 35/25 used at same depth, I do not mind.

 

[TrimixToo]

It depends on the diving for the entire trip and how you get your gas. If you are blending your own gas, have a booster, and doing multiple dives over several days, it is cheaper to go with the extra gas bottle. That way you are keeping more high helium back gas for the next dive and using less helium in the middle range of the dive.

What I was trying to say is that, compared to using EANx for deco, using Trimix for deco costs more. Nitrogen is effectively free.

It is certainly true that adding a deep deco gas saves on He costs (and cuts deco time as well), whether one chooses EANx or Trimix mixes. It does, however, add another gas switch, adds another tank to be managed, and add to the number of potential failure points, all of which increase complexity and some aspects of risk. I should mention that adding He to deco mixes can also increase deco time somewhat, which adds environmental risk and increases the probability of equipment failure slightly.

But there is a balance to be managed between minimum deco time (for a given algorithm and conservatism), how many gases one brings, and managing ICD risks. As with many things, there can be more than one "right answer," depending on the diving environment and other factors. Something close to the minimum algebraic sum of risks and their probabilities seems to be what we should all be after.

(I am quite certain I am not telling you anything you don't already know.)

 

[TheSnake]

It is certainly true that adding a deep deco gas saves on He costs (and cuts deco time as well), whether one chooses EANx or Trimix mixes. It does, however, add another gas switch, adds another tank to be managed, and add to the number of potential failure points, all of which increase complexity and some aspects of risk. I should mention that adding He to deco mixes can also increase deco time somewhat, which adds environmental risk and increases the probability of equipment failure slightly.

But there is a balance to be managed between minimum deco time (for a given algorithm and conservatism), how many gases one brings, and managing ICD risks.

I completely agree.

Note that adding much more He than needed on back gas also increases total deco time.
Just for example, if you calculate a 18 mins dive @100m/328f, without using decos ( just to see the impact of different mixes ), you have

Using 12/60: Runtime 647 minutes
Using 10/85: Runtime 1125 minutes
Using 9/87: Runtime 1220 minutes

the same dive, using standard gasses: 21/35 + 35/25 + EAN50 + OXY:

Using 12/60: Runtime: 119 minutes
Using 10/85: Runtime: 139 minutes ( +20 minutes )
Using 9/87: Runtime: 146 mi,utes ( +27 minutes )

So, 27 minutes of extra deco means, much more gas, tanks, more deco time spent at deeper depths ( adding slow tissues additional saturation ) and much more shallow deco stops ( more gas needed ).

There are many factors to be managed, as Narcosis, Oxygen toxicity, deco time, number of tanks, gas quantities, gas density to manage CO2, eventually ICD, CNS, ans so on .... in deeper dives you have also HPNS to be managed, H2 ...

The key, as you said, is a balance to be managed between all these factors. There is not a single solution.

 

[PfcAJ]

You aren’t solely manipulating the helium, but also the oxygen content. Of course the deco times are different if you have less oxygen in your bottom gas.

 

[TheSnake]

Yes, but even if you dont touch at oxygen percentage, you got similar results:

Without decos:

10/60: Runtime = 791 minutes
10/85: Runtime = 1125 minutes

With standard decos:

10/60: RunTime = 120 minutes
10/85: Runtime = 141 minutes

 

[PfcAJ]

Dang 20mins to avoid a 98ft narcosis equivalent and 140ft gas density. The horror...

 

[TheSnake]

Never seen 98ft narcosis. But if you want avoid it, yu're right.

That said, Merry Christmas to all and have good bubbles.

 

[JohnnyC]

Those decompression schedules don't necessarily reflect the most current research re:helium "penalty" btw.

And you've never seen somebody narc'd at 30m? Seriously?

 

[Germie]

12/60 is in my eyes a good gas at 100m if you dive OC.
EADD is shown in the app 'CCR Mixer' too. But 43m as EADD is ok for me.
If you use 12/60 you can do a dive to 100m with 15 minutes bottomtime with 1 travelgas and 2 decogases. Then no 4th stage needed.
The PO2 in the bottomphase is important too for the deco. A PO2 of 1.1 (10% oxygen) will give you more deco than a PO2 of 1.3 (12%).
If you put in 12/60, 21/35, 40% and 80% with a 40/70 conservatism, the divetime will be around 105 minutes (I am not discussing icd here, just some examples of some gases for a 100m dive , no standard 'DIR' gases). If you use 10/60 instead of 12/60, the divetime will be around 110 minutes. Not a big difference, but in cold water it can be an issue, or with the gasreserves it can be a between 'possible' or 'better not'.