Deco basics & twin tank spg

[johnnyblackau]

Hey all I've got 2 questions:

1st.
I'm under the understanding that when a technical dive is planned (even an advanced nitrox dive with gas changes) it's designed off software or from tables and a "Script" or schedule is run through the dive at different times and depths the changes occur and deco's take place. Is that even anywhere near right? if it is then why have a computer at all and not just have one in gauge mode for the whole dive?

2nd. I have an interest to side mount dive however with twin tanks I understand the isolation manifold, it's purpose. I'm curious to know if you do isolate the tanks and are forced to use the regs without thespg, how do you keep an eye on you pressure. I know at this point the dive is as good as over but I would like to be able to know how ugh air I've got.

thanks

 

[Teller]

Alot of tech divers only use a bottom timer or computer in gauge mode with their cut tables. Some also use a computer that allows for gas switches and just follow the computer while using cut tables as a backup. If you need to isolate for any reason your dive is over and you begin your ascent to the surface or first deco stop. If you planned your gas correctly you dont need to monitor your SPG.

 

[Superlyte27]

If you are sidemounting tanks, you have an SPG on both tanks. They tanks are already isolated, as there is no manifold between them. Unless of course you buy that POS sidemount manifold thing that no self respecting sidemount diver would ever use.

 

[mdax]

As posted, many tech divers only use a bottom timer/depth gauge. I use a liquivision x1 which is set to the same conservatism and formula that my computer/mobile program uses, VPM +3, with a backup timer/depth gauge in my pocket. The reason I run the x1 is to back up my brain and confirm things are going as planned.

There are very few situations which would cause me to isolate and if that happens things have gone badly. I'm exiting the cave or ending the dive immediately and preparing to share air with my buddy if things continue down the spiral of doom.

 

[DevonDiver]

4 Basic Options for Running a Technical Dive:

1) Primary Bottom Timer & Tables + Back-Up Bottom Timer & Custom Cut Tables

2) Primary Bottom Timer & Custom Cut Tables + Back-Up Technical Computer

3) Primary Technical Computer + Back-Up Bottom Timer & Custom Cut Tables

4) Primary Technical Computer + Back-Up Technical Computer

There are pros and cons for each option.

Use of SPG with Isolated Back-Mount Cylinders:

One SPG provides full function of gas monitoring throughout the technical dive whilst back-gas is used. The SPG is not life-support apparatus. If the left-side cylinder is shut-down, the the diver can no longer monitor their gas levels. However, they have precision-planned their dive with ample reserve (1/3rds minimum). Should the SPG fail, they would immediately abort the dive, confident that they retain sufficient gas for ascent. Should gas have been lost, now insufficient for ascent to their first gas switch, the possession of a second SPG would not change anything. Addition of a second SPG therefore provides no tangible life-support benefit. It does however present several further risks of failure - extra hose, extra o-rings etc. On balance of risk versus benefit, most technical divers agree that a second SPG is not desirable. This is virtually a 'universal standard' now in technical diving.

 

[Sevenrider860]

I'm under the understanding that when a technical dive is planned (even an advanced nitrox dive with gas changes) it's designed off software or from tables and a "Script" or schedule is run through the dive at different times and depths the changes occur and deco's take place. Is that even anywhere near right? if it is then why have a computer at all and not just have one in gauge mode for the whole dive?

During training many instructors will require you to put your computer in gauge mode and dive the tables you cut. One reason is that there is a tendency to dive the computer rather than your plan because that behavior is so common in recreational diving.

I prefer the "Primary Bottom Timer & Custom Cut Tables + Back-Up Technical Computer" that DevonDiver has listed. I use the GAP RGBM dive planner and a XEO running the Free Phase RGBM firmware. The two do not match exactly. Usually the computer will call for a deeper stop than the GAP generated plan. During my Technical training I allowed my self to start to follow the computer rather than the plan...once. I had a great instructor (DevonDiver) who gave me one of those "What the %$&* do you think you are doing?" looks. I ignored the computer stop and followed the plan. In the end, the computer caught up with the plan and was happy. I have not let the computer change my dive plan since. ​

 

[DaleC]

Here's a link to a recent discussion about SPG's and doubles: http://www.scubaboard.com/forums/ad...0700-question-about-doubles-config-spg-s.html

As for the computer question. I wonder how many divers who poo poo computers would also give up their smart phones and GPS's when doing a road trip - a map is all you really need.
Just a thought. The valid point is that one doesn't become dependent on the technology to do the thinking. In that regard, using a computer too early can make one mentally lazy.

 

[mathauck0814]

4 Basic Options for Running a Technical Dive:

...

4) Primary Technical Computer + Back-Up Technical Computer

There are pros and cons for each option.

Pray tell the "con" of a shearwater backed up by a shearwater (backed up by two other people in your dive team who've backed up their shearwaters with more shearwaters). I'll take my chances with the odds of all six going out on a dive

 

[PfcAJ]

Its just extra junk. Why take extra junk? Same with a dive team bringing 6 computers. Its diving, not a moon landing.

 

[DevonDiver]

Pray tell the "con" of a shearwater backed up by a shearwater

Expense, in comparison with other options.