Computer + Plan or 2 Computers for Deco Diving

[markmud]

I think that @JohnnyC said it best: "There's literally no good reason in this modern age to CHOOSE a bt/slate IN LIEU OF a second computer. You can certainly choose to do so, but there's no valid argument other than, 'I choose to do it this way.' "

Hi Kohanbash,

Thanks for starting this thread. I really enjoyed it. I won't answer your question because I am not a tech diver. I am a certified tech40 diver; however, that does not make me a tech diver. I recommend Tech 40 (or equal) for all Rec divers because it is truly the first comprehensive dive training I received. And now, I understand tech lingo a little better while I am reading Scubaboard stuff.

I agree with @JohnnyC and @doctormike when they argue the cost-to-benefit factor for tech diving with a second computer. The reliability point that Dr. Mike brought up is also very valid.

I spent a considerable amount of time (and effort) lobbying for diving the front side of Santa Cruz Island on a live aboard trip on DV Vision out of Santa Barbara. I spoke with biologists from the California Department of Fish and Wildlife and created a PowerPoint slide show to further bolster my opinion regarding potential dive sites.

A consensus of crew, passengers and organizers formed; barring Santa Ana or prevailing winds from the northwest, we were diving the steep topography of the front side of Santa Cruz Island.

Then we started diving; I was on my third dive of the first day. My HP hose failed. I was using a Pro Plus 3 and a wrist mounted Atmos 2 computer. I re-boarded Vision, we scrounged up some parts, and I was diving with my Atmos 2 and an SPG in 15 minutes. I would have been screwed for rest of the day without that second computer. It was the best dive day of the trip.

I dive with a Perdix AI and a Pro Plus 3. Overkill for sure. But I like them both. The money I spent on the Perdix AI to replace my Atmos 2 is a small percentage of the outlay for the dive travel we will do next year.

I can imagine, that a second computer may increase your odds of making subsequent tech dives more safely if one of your computers craps-out (especially for a tech diver who does not follow a strictly square profile). Is this a true assumption on my part?

cheers,
markm

 

[doctormike]

I can imagine, that a second computer may increase your odds of making subsequent tech dives more safely if one of your computers craps-out (especially for a tech diver who does not follow a strictly square profile). Is this a true assumption on my part?

cheers,
markm

Exactly, and while it's a bit OT for this thread, that's another excellent reason for having and wearing two dive computers, even on NDL dives. Having a backup computer that hasn't been diving with you is useless unless you take a very long break from diving.

If you finish a tech dive with one functional computer and do the appropriate deco, there is no reason why failure of one of your two computers would make you more at risk for subsequent tech dives. The only issue would be that you now have to change from the OPs option 2 to option 1 (with the implications discussed upthread) for deco backup.

This would involve replacing your dead backup computer with another computer in gauge mode (or an actual BT), and then having your ascent plan on a slate. You would plan the second dive with the data from the first dive's surviving computer.

If you are doing option 1 and that one dive computer craps out, you really don't have the option of using a new computer for the second dive at all. I suppose you could go totally old school and just use your BT (or another computer in gauge mode), tables and a square profile, but then you have no backup deco information (yes, I know, two failures). Also, your planning would be affected since you would not know exactly what you did on the first dive; you would plan assuming first dive square profile (at max depth and total runtime) even if you hadn't done that. I think in that situation, I would just sit by the pool and offgas for the next day or two.

 

[Jim Lapenta]

When I teach AN/DP my students have to have a primary computer and some type of back up. That back up can be a bottom timer or other computer in gauge mode.
We cut tables using multi deco. The problem in some instances with relying on a computer is that when the team has different computers, schedules may not match up.
So we use the tables, in training, to see the differences and develop the discipline to stick to an agreed-upon plan.
It does no good to have new tech divers at different spots on the upline.
I prefer all my students to have shearwaters as their primary but even then, depending on their own acceptable level of max po2, ascent rate, GF or conservatism factors, they may still end up with different run times.
So we pick the most conservative plan, agree on it, write the tables out, and stick to those for our run time schedules.
Another reason I do this is that too often I see that people wanting to get into tech diving really shouldn't at that point. They don't have the discipline to stick to a plan, they want to do it and cut corners, and some of them don't have the basic skills necessary for it.
So I have to make sure they understand that it takes discipline and the ability to stick to a plan. A written plan with clear run times removes the tendency to play fast and loose with schedules, team dynamics, and safety.

 

[_Ralph]

where are the commonly available $400 Shearwaters at? I routinely see used Perdix in the 700 range but not 400.

.... Petrels and predators

_R

 

[rjack321]

While many would say that if you have two computers, you probably don't really need to write out a set of tables, no matter what, you still have to plan the dive and have an idea of what you're looking at before you splash. Only a fool does a deco dive without knowing what his run-time will be before he gets in the water.

Alas this actually happens fairly frequently.

So I have to make sure they understand that it takes discipline and the ability to stick to a plan. A written plan with clear run times removes the tendency to play fast and loose with schedules, team dynamics, and safety.

Yup​

I had the pleasure(?) of listening to a well known FL instructor talk about his computer failure in Indian (cave) at Cave Adventures a few years ago. OH.MY.GOD he had "no idea" how much deco obligation he'd racked up. All I could do is wonder about the quality and thoughtfulness of this instructor's students. Software and wetnotes/slates tend to reinforce better habits IMO. Or <gasps> ratio deco. A second computer or a helium analyzer is rarely matter of life or death. How many hours did Exley count out second-by-second when his watch failed?

Not confirming the O2 fraction even if the tank is marked and/or analyzed? That's still killing people.

 

[wetb4igetinthewater]

@rjack321

I wish I could like your comment more than once.

 

[100days-a-year]

I concur with some of the other old guys, too often see people have no clue what contingency planning looks like beyond turning to a backup computer. Computers are great tools, but tools without in depth knowledge can be hazardous when they induce a false sense of confidence.

 

[stretchthepenn]

I'm a n00b tec diver, so take this opinion with a full shaker of salt, but I'm seeing a lot of parallels between this thread's topic and the OW-level debate about learning to dive a computer versus learning to dive tables.

For example, I and my dive buddy did a piddly-level 130' tec dive on Saturday, and we both used the same setup: two Shearwaters and an armslate with a table. We could've ridden our computers and guesstimated our deco stops--and at this particular site, relying on computers was a good idea 'cuz we couldn't do a square profile, meaning we needed computers to give us credit for all the time spent getting to depth--but we didn't. We cut tables first. We didn't really use them...but we sure had them, and the planning was valuable.

 

[rjack321]

For example, I and my dive buddy did a piddly-level 130' tec dive on Saturday, and we both used the same setup: two Shearwaters and an armslate with a table. We could've ridden our computers and guesstimated our deco stops--and at this particular site, relying on computers was a good idea 'cuz we couldn't do a square profile, meaning we needed computers to give us credit for all the time spent getting to depth--but we didn't. We cut tables first. We didn't really use them...but we sure had them, and the planning was valuable.

Just curious about this scenario. If you only had one shearwater (each). Plus your table plan. Your shearwater dies (say dead battery) but your buddy is right there and has been at most ~8-10ft away the entire dive. Do you:

give yourself credit a multilevel dive and follow his shearwater?
both revert to a square table plan (assuming its not a multilevel plan on the slate) and he sits there and watches you finish?
split up and he leaves you in the dust while you finish the square plan?

 

[helodriver87]

Just curious about this scenario. If you only had one shearwater (each). Plus your table plan. Your shearwater dies (say dead battery) but your buddy is right there and has been at most ~8-10ft away the entire dive. Do you:

give yourself credit a multilevel dive and follow his shearwater?
both revert to a square table plan (assuming its not a multilevel plan on the slate) and he sits there and watches you finish?
split up and he leaves you in the dust while you finish the square plan?

This is how I dive. If I've cut my tables right, it shouldn't be too different from what the computer spits out. I like to pull up the shearwater generated schedule at my first stop and compare it to my wetnotes to make sure they're aligned. If there are meaningful multi level segments (P3 comes to mind), those are factored in to the tables. Average depth usually works though.

If we're down a computer, I'd compare the schedule on the working computer to the tables and (gas volume permitting) go with the more conservative (more deco) schedule if they're reasonably close. If they differ significantly, try to determine why (shallower average depth, shorter BT, etc) and go with the one that makes more sense. If I can't determine why, go with the more conservative schedule. Either way, deco out as a team. 99% of the time, a bit of extra deco won't hurt you.