Bottom Timer Questions

[Teller]

I just want to make sure I understand. Is it common practice to have 2 Bottom timers when you are cutting your own tables?

If I am doing dives that have a square profile is a computer even worth buying.

Lets say you plan a dive to 130 feet with a bottom time of 10 min and it takes me 3 min to decend to my max depth. Does the 3 min count toward my bottom time or do I have a full 10 min at 130.

 

[fdog]

The descent counts as part of the bottom time. In your example, you would spend 3 minutes descending, and enjoy 7 minutes on the bottom.

You'll also note that programs such as V-Planner calculate bottom time in this way. And the USN tables have been this way forever.

(there is an animated discussion that can be had regarding starting depth averaging when passing the first deco gas depth, but that's not applicable here)

I only carry one BT, my buddy is my backup. A failed BT is a major as far as I'm concerned, and I'll turn the dive.

Also, I carry a knotted spool and SMB, and feel confident in using that (and counting) to complete the deco if everything really went to poo.



All the best, James

 

[fire_diver]

Fdog summed it up pretty well. You should have a backup BT, but that backup can be your buddy's BT.

As for the computer, it depends. If the deco algorithm matches what you are comfortable with, then thats fine. Most computers have a BT function as well.

 

[DevonDiver]

Did you mean to post this in the Technical Diving forum?

I just want to make sure I understand. Is it common practice to have 2 Bottom timers when you are cutting your own tables?

As far as I am aware, most (all?) tech agencies state that time/depth gauges need redundancy. This means 2x depth gauge & 2x timer (or 2x computers).

Fdog summed it up pretty well. You should have a backup BT, but that backup can be your buddy's BT.

You were taught that on a technical diving course?? :shocked2::shocked2::shocked2:

If I am doing dives that have a square profile is a computer even worth buying.

It is debatable.

If your dive is very square profile, then you won't get much advantage from the 'real-time' tracking of a computer. However, you do get flexibility.

Should your actual dive deviate from your plan; shorter or shallower, then a computer could save you from some unnecessary decompression.

If the dive deviated longer or deeper (or both) then you should have back-up contingency tables cut anyway..but a dive computer/s would make that an easier and more accurate contingency.

Lets say you plan a dive to 130 feet with a bottom time of 10 min and it takes me 3 min to decend to my max depth. Does the 3 min count toward my bottom time or do I have a full 10 min at 130.

With ANY dive your 'bottom time' begins upon descent. That applies from PADI OW course onwards...

Bottom time really is a designation of time when you are loading your tissues (rather than off-gassing). So, you descent definitely has to be counted in that. For some tech dive scenarios, the intial portion of your ascent could also be a time of tissue loading...this doesn't really happen in rec diving.

 

[ianr33]

With ANY dive your 'bottom time' begins upon ascent. That applies from PADI OW course onwards...

You sure about that?

 

[DevonDiver]

You sure about that?

I was editing it as you posted It's 3am here... brain-fingers disconnect.

 

[AndyNZ]

You were taught that on a technical diving course?? :shocked2::shocked2::shocked2:

Not all agencies have the 'thou art alone' view.

My TDI courses have been very different from my GUE courses in that regard. Certainly, in a unified team having a team member be your backup is fine.

With a good team, if I am not running deco, I only really glance at my timer now and again to verify the run time. If the person running deco signals that their timer is kaput, I just take over and swap roles - typically that would mean giving up control of the SMB to the person who no longer has a working timer.

I'm quite OK with that, provided I know my team.

 

[syntaxerrorsix]

Redundancy is mandatory in my book.

We drifted our deco in 5 knot currents with about 10' vis this weekend. We traveled approximately 1 3/4 miles during deco and were separated by about 20' feet according to the boat captain. We "reconvened" at about our 100 foot stop but I was essentially alone from 200 to 100 due to our student's premature bag deployment.

I don't care how good your team is, it only takes one "issue" to get the snowball rolling downhill. I'd assume be prepared.

 

[AndyNZ]

I don't care how good your team is, it only takes one "issue" to get the snowball rolling downhill. I'd assume be prepared.

There is a line between "redundancy" and "too much", which for all of us is in a different place - so I'm not knocking your stance at all. And realistically an extra gauge is never going to be "too much" (whereas other items would be).

But I have to ask, what is the chances of both team separation and a gauge failure on the same dive?

In the past, I've worked as a risk analyst in various industries - just because a risk exists doesn't mean you have to address it. You have to think of impact and liklihood.

In your example, the cause of your issue was a poor team (someone deploying a bag too early). If you could mitigate that, you wouldn't have to worry about a gauge failure. I know which one is more predictable and more controllable a risk!

 

[syntaxerrorsix]

There is a line between "redundancy" and "too much", which for all of us is in a different place - so I'm not knocking your stance at all. And realistically an extra gauge is never going to be "too much" (whereas other items would be).


But I have to ask, what is the chances of both team separation and a gauge failure on the same dive?

Realistically? Very low in my opinion.

In the past, I've worked as a risk analyst in various industries - just because a risk exists doesn't mean you have to address it. You have to think of impact and liklihood.

In your example, the cause of your issue was a poor team (someone deploying a bag too early). If you could mitigate that, you wouldn't have to worry about a gauge failure.

Agreed, that was cause THIS TIME. Mitigating unwanted human behavior and reaction during stress sounds optimal but in reality having the capability to safely finish the dive if separated is far more important.

Despite the likelihood of being separated AND having one BT go south I will still be able to monitor my ascent/stops.

No matter how I "what if" the dive it is clear to me that as a team we will always have something to work on and that in general poop can and will happen and we as individuals need the redundancy to safely surface if separated.

If there is a line between redundancy and too much it surely fits between individual gas management capabilities and most everything else in my opinion.

I know which one is more predictable and more controllable a risk!

Me too! I will always have a second bottom timer. It will be predictable and I will be in control of it.


On a side note you should have seen how fast that bag pulled him away from us, it looked like a true Wiley E. Coyote moment