Less Deco time with Air vs Tx? iDeco Pro

[Tom Kreider]

Hello everyone!

I'm prepping for my TDI Entry-Level Trimix class in May and I'm also learning to use the iDeco Pro software on my iPhone. I've run into something odd.

According to the iDeco Pro software, I'll need less deco time for a deep dive using air, than I would using trimix. (see below) Now, I know I don't want to use air @ 200ft because of narcosis, but are these numbers true? In class, we'll be using V-planner and other software to plan the training dives and cut tables. We'll be diving that plan. Again, we'll be using Trimix, not air, but I have a reason for looking into the numbers with air. I don't own a Trimix computer. I own an Atmos Elite T3 that handles 3 Nitrox blends, but not Tx. I'm taking this class in Bonaire as part of a 2 week dive vacation and want to use the same computer for the entire 2 weeks. So... If the deco profile is this similar, and I will pre-plan the training dives and cut tables for the training dives, could I just tell my computer that I'm using air as my bottom gas, along with the EAN 50, and the O2? If so, that would save me from renting a Nitek X for the entire 2 weeks ($250). Also, the remainder of the 2 weeks will be kept within Rec limits.

Help and suggestions from trimix divers are greatly appreciated.

Dive #1 - Air
Dive: 200ft for 20 [ 20 ] on Air
Asc.: 170ft for 1 [ 21 ] on Air
Dive: 150ft for 15 [ 36 ] on Air
Asc.: 120ft for 1 [ 37 ] on Air
Asc.: 90ft for 1 [ 38 ] on Air
Deco: 90ft for 1 [ 39 ] on Air
Deco: 80ft for 1 [ 40 ] on Air
Deco: 70ft for 2 [ 42 ] on EAN50
Deco: 60ft for 3 [ 45 ] on EAN50
Deco: 50ft for 4 [ 49 ] on EAN50
Deco: 40ft for 4 [ 53 ] on EAN50
Deco: 30ft for 8 [ 61 ] on EAN50
Deco: 20ft for 8 [ 69 ] on 100% O²
Deco: 10ft for 13 [ 82 ] on 100% O²

Gas Consumption:
Air 135.3 cuft
EAN50 24.4 cuft
100% O² 14.9 cuft

Dive #1 - Tx
Dive: 200ft for 20 [ 20 ] on Tx 21/35
Asc.: 170ft for 1 [ 21 ] on Tx 21/35
Dive: 150ft for 15 [ 36 ] on Tx 21/35
Asc.: 120ft for 1 [ 37 ] on Tx 21/35
Asc.: 90ft for 1 [ 38 ] on Tx 21/35
Deco: 80ft for 4 [ 42 ] on Tx 21/35
Deco: 70ft for 2 [ 44 ] on EAN50
Deco: 60ft for 2 [ 46 ] on EAN50
Deco: 50ft for 4 [ 50 ] on EAN50
Deco: 40ft for 5 [ 55 ] on EAN50
Deco: 30ft for 8 [ 63 ] on EAN50
Deco: 20ft for 10 [ 73 ] on 100% O²
Deco: 10ft for 17 [ 90 ] on 100% O²

Gas Consumption:
Tx 21/35 138.9 cuft
EAN50 24.1 cuft
100% O² 19.1 cuft

Runtime on air: 82 minutes
Runtime on Tx: 90 minutes

Thanks very much!

Tom

 

[LiteHedded]

buhlmann penalizes you for helium. it's pretty silly

 

[Blackwood]

buhlmann penalizes you for helium. it's pretty silly

Going a little further, standard Buhlmann (like that found in iDeco - BTW if I misspeak hopefully Brock will correct me) uses gas density as a variable, and since helium is less dense than the gas it's replacing (i.e. nitrogen), Buhlmann thinks it move in and out of solution quicker and form bubbles more readily.

In other decompression models which address gas solubility as a variable, since helium is relatively insoluble in our tissues it's not such a significant issue.

 

[TSandM]

There are two schools of thought about helium . . . one is that it forms bubbles faster than nitrogen, and the other is that, because of its poor solubility, there are so many fewer helium molecules that it's actually a BETTER gas for decompression. Programs that work with the first assumption come up with longer deco times for trimix, and those that work with the second come up with shorter ones.

 

[Peter Guy]

Tom K wrote

I don't own a Trimix computer.

But yes you do, your iPod which gave you the profile you quoted.

Obviously TK meant to write he didn't own a trimix diving computer but what I think this shows is that people are confused about what "dive computers" do -- they are but "stupid instruments doing what some programmer wrote for them to do" just as the iPod or my laptop does.

When people say "I don't use a dive computer when I tec dive" they lie (or at least the smart ones are lying [note, people who are using RD are a different breed and at least one generation removed from someone's computer profile]) because they DO use a "dive computer" which just happens to stay on shore. We are all trusting our lives (to some extent) to some unknown person or group of people who have written programs which we hope somehow approximate what our bodies actually do. The only difference is whether we take our dive computer with us to actually know what dive we actually do.

 

[PfcAJ]

Well, ya, I generate a base ascent profile with software, then I modify the crap out of it based on the latest deco theory and information.

To the OP, consider backing off the ppo2 a bit. 21% oxygen gives you a ppo2 of roughly 1.5, try to stick to about a 1.2 or 1.3 for the bottom portion. Jack up the helium to 45% or more, its better for deco at the end of the day (assuming you deco out the right way) and keeps your END shallow.

Buhlmann vs VPM shows about a 13 minute difference for this dive (with 18/45 as the bottom gas, which is more reasonable than 21/35, and certainly more so than air) and is more inline with what I would do as far as total ascent time is concerned.

 

[DrAlberto]

I have a Liquivision X1 and Ideco and the simple answer for that problem is:insert more deep stops than both and you have the best of both worlds that is a like run time for relatively shallow,short and low He.