Use a Computer or BT?

[EFX]

I'm confused about the use of a computer for tech diving. I've read that computers are too conservative and possibly not safe and therefore one should develop deco on the computer and use a BT and calculations. There was one post in the DIR section where it was mentioned that a computer shouldn't be used. However, I see pictures of tech divers with big rectangular computers on their wrists and looked at computers specifically for tech diving. I think one can develop a custom deco schedule and download it to the computer. So, if this is true why wouldn't the computer be used instead of the BT/calcs?

 

[rivers]

The answer to your question is tech divers use one or the other or both even. But, chances are, you won't see a technical diver using a standard recreational computer unless it has been put into gauge mode. The algorithms rec computers use just aren't conducive to technical dives. Computers from shearwater, ostc, and liquivision (a few models) are developed for technical divers. They are programmed with both VPM and gradient factors, with the diver deciding how aggressive or conservative they want to be with their deco. I haven't used any of the tech computers (i dive with a bottom timer), so i can't comment on what else they are capable of doing besides deco, gas switches, and trimix capable. I'm sure someone will be along to explain more in depth. On top of the primary computer, a diver either has a back-up computer or a back-up bottom timer in case of failure. If you're using a bottom timer as a back-up, the diver has also cut various tables with bottom times, stops, gas switches, etc. If you're using only a bottom timer, then you've cut tables based on your planned dive, but also tables in case SHTF, and you've lost gas, encountered some problem and your bottom time is longer than planned, etc. There are various programs you can use to cut your own tables, but you really need to know what you're doing.
Whether a computer or bottom timer is used, there is a lot more planning that goes into a technical dive than a normal recreational dive. Things like bottom time, depth, gas, etc are planned for in advance, and divers tend to know what their stops are going to be beforehand.
I myself am just starting down the tech path at the moment (all my dives are in recreational depths, but some of them require deco and have just taken an accelerated deco course as well).

 

[Tortuga68]

Short answer: BT/computers in gauge mode (same thing really) are great for square profile dives; technical computers, where you can vary the parameters yourself, are great for non-square proflile dives. Same as using tables vs a computer for rec dives.

 

[PfcAJ]

Somehow I manage without a computer doing non square profiles. Weird.

 

[FM1520]

Short answer: BT/computers in gauge mode (same thing really) are great for square profile dives; .

Not really, bottom timers are great for any profile.

 

[Nasser]

There is also a different approach in how computers are used in tech diving. In recreational non deco-stop dives, most people jump in and swim around following the NDL limits on their computers... ascending accordingly. In tech dives, much more emphasis is placed on pre-dive planning. Generating your own tables, figuring out gas consumption, CNS etc... The BT/Computer is then used to follow the deco plan and Run Times. Even for those using "tech" computers, many are essentially still following Run Times and a pre-planned deco schedule... basically using the computers as "reality checks" and backups. You won't find too many tech divers just taking a tech computer down and diving "on the fly" like in regular recreational sport diving.

BTW it's also not difficult generating custom tables for multi-level tech dives if the profile is not too complicated (at least with respect to open water - I don't cave dive so that's maybe a different story)

 

[EFX]

... The BT/Computer is then used to follow the deco plan and Run Times. Even for those using "tech" computers, many are essentially still following Run Times and a pre-planned deco schedule... basically using the computers as "reality checks" and backups. You won't find too many tech divers just taking a tech computer down and diving "on the fly" like in regular recreational sport diving.

So then, manually setting or downloading the run times and deco schedules to the dive computer is not possible, or at least to the granularity required for the profile. Or perhaps, for some simpler profiles it can and "on the fly" diving could be done. If so, would you trust it. If you normally dive with tables using it as a backup makes sense. But as a reality check trust becomes an issue. For example, my tables say I deco for 10 minutes at depth1 but my computer requires 15. Which do I trust? Since custom tables are designed for specific profiles, if you deviated from that profile would you switch to the computer and finish the dive with it?

 

[rongoodman]

Modern technical dive computers have the ability to run the exact same deco software as your home computer/tablet/smartphone to generate a schedule, so there really is no conflict between the two approaches.(This hasn't been possible for all that long and some of the on-line rhetoric still hasn't caught up.) For any significant dive, I'd certainly still pre-plan it in order to look at gas requirements, contingency deco, etc. Generating a schedule and downloading it to your dive computer doesn't make any sense and can't be done, although the various software packages have parameters for conservatism, stop depths, etc. which you can adjust in advance. The computer has enough power to continually generate a schedule in real time, adjusting not only for time and depth, but also available gases, so if you blow through a stop or lose a deco gas, it will keep on doing its best to get you to the surface alive and unbent.

 

[Dr. Lecter]

My approach is to plan the dive, cut tables, and then dive within the plan using the realtime profile created by the computers. If they keep working, they'll give me a deco schedule that fits the less demanding dive actually conducted; if they fail, I can fall back on the cut tables/timer/depth gauge I brought secure in the knowledge that they're "set" for the maximum limits I planned for that dive.

 

[Nasser]

So then, manually setting or downloading the run times and deco schedules to the dive computer is not possible, or at least to the granularity required for the profile. Or perhaps, for some simpler profiles it can and "on the fly" diving could be done. If so, would you trust it. If you normally dive with tables using it as a backup makes sense. But as a reality check trust becomes an issue. For example, my tables say I deco for 10 minutes at depth1 but my computer requires 15. Which do I trust? Since custom tables are designed for specific profiles, if you deviated from that profile would you switch to the computer and finish the dive with it?

I haven't seen a computer that will allow you to download your custom tables, but as has been mentioned, this is not really necessary as many tech computers and deco software programs share the same algorithms and allow you to match the Gradient Facotrs and degree of conservatism.

This brings up another aspect... that we are still essentially dealing with theoretical models and to a certain degree, the level of conservatism as far as the user is concerned is arbitrary. There is still no guarantee that you won't get bent or that you are anywhere near close to tracking the actual nitrogen levels in your body. These models are theories that have generally proven to have fairly good track records - but not 100% guarentees. Indeed, in a recent article I read about DCI, it was stated that about 70% of actual cases of DCI were "undeserved hits"

If I found that my computer gave me a different reading than what my plan called for, I would stick with the more conservative of the two. I could probably get out of the water sooner if my computer cleared me, but as an additional precaution, I would do the extra deco. In actuality, this is really no different conceptually than setting your own Gradient Factors or conservatism on your deco software (or computer) as you plan dives and cut tables. One can asses the diving environment and at best make an informed guess on what degree of conservatism to apply... so perhaps the question as to whether to trust the computer should be extended further as to whether to trust the decompression model you're using?

Anyway the main focus of my first post is to point out that tech divers tend to place much more emphasis on pre dive planning and don't use their computers in quite the same way as recreational sport divers generally do - the approach and mindset is a little different.