Teric and Nitrox diving - Rec divers take care re method of changing gas (affects MOD).

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Is that possibly because you have a Maximum Depth setting on the Perdix that is set to 130 and don't on the Teric?

I apologize for asking the "is it plugged in" question... (unless that really is what it is..) :)
I’ll check - but I don’t recall ever entering that kind of info! And no offense taken!
 
The bolded part here is a quote directly from Shearwater:



If it says 10 minutes, but doesn't tick down 10 minutes, then the info that said 10 minutes was not accurate. The reason it's not accurate is because the diver is not going to do what the computer is assuming the diver will do.

The computer is saying 10 minutes based on an assumption that the diver will switch to some gas that the computer was told the diver is carrying. The diver is not carrying that gas, will not switch to it, and the assumption the computer made is wrong - so the deco information displayed is wrong.

I see now. We're arguing over two different things. The computer calculates your continual decompression obligation based on the gas you are breathing, and will continue to do so until you switch. The predictive information is not tied to the actual decompression obligation, it's tied to the "ascent plan," hence the discrepancy between the stop time display, and its actual movement. The computer won't switch gas for you, which is why that 10 minute stop time might take 15 minutes. Imagine it like running a lost gas plan. The only difference is that it's continually calculating based on what you're breathing. The exact same thing would happen if you got to your 50 bottle and found it empty and didn't switch.

What it absolutely will not do, is count down the stop timer based on the gas that you do not have selected. It KNOWS that you haven't switched, so it's not going to start changing its off-gassing math. You're calling the decompression obligation the stop time, not the actual running algorithm. I'm calling the decompression obligation what the algorithm is actually doing, not what the stop time is displaying.

I don't think I'm articulating it well. The computer isn't going to bend you because you have an active gas that isn't selected. And remember, we're talking about recreational diving here, not 6 bottles of hypoxic wreck diving. And ultimately, if a diver can't be bothered to use their computer correctly, maybe they shouldn't be using a computer that gives them the option. Do we remove features that are useful to other divers because a small subset are unable to use it correctly?
 
I see now. We're arguing over two different things. The computer calculates your continual decompression obligation based on the gas you are breathing, and will continue to do so until you switch. The predictive information is not tied to the actual decompression obligation, it's tied to the "ascent plan," hence the discrepancy between the stop time display, and its actual movement. The computer won't switch gas for you, which is why that 10 minute stop time might take 15 minutes. Imagine it like running a lost gas plan. The only difference is that it's continually calculating based on what you're breathing. The exact same thing would happen if you got to your 50 bottle and found it empty and didn't switch.

What it absolutely will not do, is count down the stop timer based on the gas that you do not have selected. It KNOWS that you haven't switched, so it's not going to start changing its off-gassing math. You're calling the decompression obligation the stop time, not the actual running algorithm. I'm calling the decompression obligation what the algorithm is actually doing, not what the stop time is displaying.

I don't think I'm articulating it well. The computer isn't going to bend you because you have an active gas that isn't selected. And remember, we're talking about recreational diving here, not 6 bottles of hypoxic wreck diving. And ultimately, if a diver can't be bothered to use their computer correctly, maybe they shouldn't be using a computer that gives them the option. Do we remove features that are useful to other divers because a small subset are unable to use it correctly?

NOW we're on the same page. :D
 
I’ll check - but I don’t recall ever entering that kind of info! And no offense taken!
I think it's the default setting in Rec mode on the Perdix, which makes sense when referring to recreational dives. I ran across the same thing at the beginning of this discussion and it was making me crazy until I found it buried in the Advanced Config pane of the Rec mode System Settings. The max you can set it to is 165'. As far as I can tell, there is no equivalent on the Teric.
 
I’ll check - but I don’t recall ever entering that kind of info! And no offense taken!
I checked on mine and, as Ron indicated above:
  • Perdix AI - you can change this in the Advanced Config menu - but max selectable is 165 ft, regardless of what you enter for PPO2.
  • Teric - don't see anywhere that you enter or change this info.
 
... And remember, we're talking about recreational diving here, not 6 bottles of hypoxic wreck diving. And ultimately, if a diver can't be bothered to use their computer correctly, maybe they shouldn't be using a computer that gives them the option. Do we remove features that are useful to other divers because a small subset are unable to use it correctly?

The correct use of the Select Gas menu option is solely for switching gas (intra-dive). The number of Rec divers that carry more than one gas in relatively minuscule to those that just carry one gas. Given that, and given OC Rec is for for Rec divers, for OC Rec, I don't see why the Select Gas menu should be there at all. If you carry more than one gas OC Tec seems like the more appropriate place.

If SW don't remove Select Gas from OC Rec, then I guess they'll find a way to make things work for Rec divers in the Rec areas of the Teric.

On the issue about using Select Gas correctly, the Teric's manual (Rev C) doesn't talk about the behaviour of Select Gas (what it does to the gas switched from). In two parts of the manual (one under Deco Info, the other in OC Tec example IIRC) it talks about not having a gas On that you don't carry. In other parts of the manual (namely the Tec parts) it talks about switching gas. If you approach using the Teric from a Rec perspective, focus on the Rec parts of the manual, chances are when you come to change gas (inter-dive) chances are you'll do it with the first menu option that seems to be correct, it appears to work, and the only way you would know it's incorrect is to then going into Edit Gases and looking. So it's not about being "unable" or "can't be bothered"; it's more about being made aware.

With the changes in FW16, I would expect RevD of the manual to be released at some point and for some explicit comments in the manual about Select Gas assuming it remains.

Or just do what I've done for 5 years now as an NDL Rec diver (no gas switches/deco). I only use the Edit Gases screen and use only 2 gases in the list - top entry for Nitrox and bottom entry for Air.
.

TBH I'm perplexed how I managed to live in bliss of not doing it the right way. OK, so it didn't have any implications (pre FW16) but I'm typically more 'particular' than that :)

-----

The other thing I noticed when reading some of the prior posts was terminology.

In SW speak we have:

Active Gas <-- the gas you are breathing at that time. (and only one Gas can be Active at any time).
On or Enabled Gas <-- a gas you intend to use on a dive and it's listed where gases appear (either via Select Gas or Edit Gases).
Off <-- it's listed, not intended for use. It might be carried (emergency gas, a buddy's gas etc - see @KenGordon post #48 above), although the manual does say it's not carried (6.2.1 Teric).
 
I would think this would be common sense. Am I missing something?

The OP (and I've read others talk about this as a "feature" of the Teric) was using a shortcut to switch between Nitrox and Air, not realising that this affected the MOD calculations as the Teric assumed that all gases would be used on the dive, not just the "active" gas.

Personally while I get that it's great to be able to store your common gases and turn them on/off at will, I wonder if for 99% of people diving in "Rec" mode, whether allowing more than 1 gas is just an accident waiting to happen. They should at least clearly call out in the manual that leaving other gases "on" has an effect on the calculations. If you are carrying more than 1 gas surely you should have it in Tec mode.
 
The OP (and I've read others talk about this as a "feature" of the Teric) was using a shortcut to switch between Nitrox and Air, not realising that this affected the MOD calculations as the Teric assumed that all gases would be used on the dive, not just the "active" gas.

Personally while I get that it's great to be able to store your common gases and turn them on/off at will, I wonder if for 99% of people diving in "Rec" mode, whether allowing more than 1 gas is just an accident waiting to happen. They should at least clearly call out in the manual that leaving other gases "on" has an effect on the calculations. If you are carrying more than 1 gas surely you should have it in Tec mode.


You summed up my thoughts on it.
 
https://www.shearwater.com/products/perdix-ai/

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