Trimix Computers

[nereas]

By scanning the previous posts, I see that there are several culprits that I have muted in the past for their in-flame-atory posts. Therefore I am not surprised!

I agree that you don't NEED a computer for trimix, even so, I also use one. I dive with a DiveRite NitekHE, and I love it. These are hard to find now, while everyone waits for the newer versions to be manufactured by DiveRite.

I also cut summarized conservative tables with multiple "what-ifs" from V-Planner as backups (thus, 2 of everything).

The backups I record on the wrist slate only in terms of MOD, time at MOD, total deco time before surfacing, and stop pattern. This allows me to load more backups onto the slate, and rely on the NitekHE for actual deco data at various depths during decompression.

If the NitekHE were to fail, I would fall back to the slate, with deep stops of 1 min each, nitrox stops of 2 mins each, then specifically prestated stops at 50 ft, 40 ft, 30 ft, and the balance of deco at 20 ft on 100% O2. A typical sketch would show 1/2/3/4/10/20 or 1/2/4/8/15/30 etc for the various MODs and respective times. That is how the slate shorthand works.

My backup timer is a SUUNTO in gauge mode, and it fits onto the same elastic wrist strap as the NitekHE comes with. (Two of everything.)

To switch gasses underwater with the NitekHE, you press the big left button A once, then scroll to the next mix with the big right button B, then press A again to select your mix. It will hold 7 mixes, therefore I put travel mix in #1, bottom mix in #2, TMX 20/40 in #3, TMX 30/30 in #4, EAN 50 in #5, and 100% O2 in #6 for a really deep dive at the edge of the limits for open circuit. For shallower dives, I will put bottom mix in #1, EAN 50 in #2, and 100% O2 in #3. Thus, the computer is very versatile. It does all the work for you, but you still need to plan the backups in case the electronic device fails, as they surely can, anytime.

The NitekHE (or any helium computer) is nice because it gives you precision and flexibility with a rather conservative deco schedule, for any combination of depth and time. By following it, you tend to eliminate making mental errors that are otherwise possible with a slate and timer(s) alone.

In your trimix classes (basic normoxic and advance hypoxic), you will want to set up the slates and use those as your primary method, and refer to the helium dive computer as your backup. This way, you will learn everything properly, and not become dangerously dependent on the computer alone.

Therefore you will need 4 things, if you chose to dive with a helium computer now: (1) the computer, (2) a backup timer or other computer in gauge mode, (3) a wrist slate, and (4) software or preprinted deco tables. You cannot rely on a computer alone, since your life depends on everything functioning properly, and therefore the redundancy rule of 2 of everything.

Being dependent on the computer alone is what all the in-flame-atory remarks are probably all about, it would seem. These are probably from cave divers, who can get agitated rather easily, even before the actual dive begins! Cave diving is a high stress sport.

 

[nereas]

You're welcome, sunkmail. BTW if you are diving a CCR (the KISS) then you would probably be better off with the VR3 rather than my NitekHE.

 

[Tomeck]

My backup timer is a SUUNTO in gauge mode, and it fits onto the same elastic wrist strap as the NitekHE comes with. (Two of everything.)

Why I must wait 48 hours to be able to change on air or ean mode, if I use a gauge mode in water?

 

[DA Aquamaster]

Neareas, good post.

I use a ECM-20H set at 30% (which per the Rubicon article link is about right) to more or less conform to the general deco schedules I get with D-Plan.

I also use my computer very similar to how you use yours. I like the ability to cut deco schedules on my palm and then transfer them to a multileaf wrist slate with contingency plans for next deepest depth and time as well as for the loss of either deco gas.

A downside of the EMC-20H is that gas switching is automatic so if you are on a lost gas contingency you are strictly on the table as the computer will quite happily continue assuming you are on a different deco gas. But then again it is also not much of a downside as you have the tables and have done the contingency gas planning as well.

It is the planning, or more correctly the temptation to skip the gas planning ad contingency planning, where I think the biggest risks lie in using a computer as a primary deco tool. What happens in a lost gas scenario if the diver did not do the planning?

I also carry an Uwatec bottom timer to provide a backup for depth and time information as any table is very hard to follow without depth and time references.

On the other side of the coin, if the dive is turned early due to an emergency, equipment failure, etc, a computer can get you out of the water a lot quicker without having to actually bring tables along and figure out a new shorter profile on the fly.

 

[Bismark]

Why I must wait 48 hours to be able to change on air or ean mode, if I use a gauge mode in water?

Used in gauge mode, your computer has no idea of your last dive as far as residual n2, as it did not "know" what gas your were using. Therefore, to be ultra cautious, 48 hours to clear the computer pretty much means you are starting with a clean slate as far as N2 goes.

 

[Scared Silly]

Why I must wait 48 hours to be able to change on air or ean mode, if I use a gauge mode in water?

Because 48 hours is what it wants for you to be totally off loaded of any N2. Otherwise if you were to have a bit of a load the computer would not be to give an accurate NDL.

Although I do not dive Trimix, I do use a computer but as my backup. I cut my tables and contingencies and then see how they match what computer gives me.

 

[sunkmail]

You're welcome, sunkmail. BTW if you are diving a CCR (the KISS) then you would probably be better off with the VR3 rather than my NitekHE.

I wasn't thanking for the particular computer, but for the well thought out and rational reasoning.

I actually do have a VR3, and a Shearwater attached to my rig. Along with that, I cut tables for anything I'm doing beyond the NDL.

(The Shearwater Pursuit is another Trimix computer to consider - esp if you may be heading to CCR in teh future .. it has an upgrade path with PINs similar to the VR3)

As for where to put the tables ...

White Electrcial tape and a fine point sharpie.
The tape is stuck to the computer, beside the LCD. I calculate each dive and write out a new deco schedule, taking into account my SI and mix I'm diving that day.
As long as the comp. face is clean before attaching the tape, it'll do a few dives no problem without any sign of peeling.

Personally, I like this more than using a slate ... it's right where I'm looking all the time anyway, and I can compare the table and comp. during ascent to make sure they're lining up fairly well

 

[Tomeck]

Which dive computer for trimix gives the best precision of depth?

 

[Scared Silly]

White Electrcial tape and a fine point sharpie.
The tape is stuck to the computer, beside the LCD. I calculate each dive and write out a new deco schedule, taking into account my SI and mix I'm diving that day.
As long as the comp. face is clean before attaching the tape, it'll do a few dives no problem without any sign of peeling.

I do the same except I use yellow duct tape and put it on my slate. I use the yellow duct tape for marking my MOD as well.

Which dive computer for trimix gives the best precision of depth?

My understanding is that most pressure transducers used for dive computers are good for +/-0.25% of their max range. As such, I doubt that you see much difference between any of them.

 

[Meng_Tze]

My understanding is that most pressure transducers used for dive computers are good for +/-0.25% of their max range. As such, I doubt that you see much difference between any of them.

More like +/- 3%.