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

[stuartv]

I have Air and Nitrox in the gas list on my Perdix in Rec mode. Switching from one to the other in the Select Gas menu always displays the appropriate MOD. I was surprised to find out that it doesn't work that way on the Teric.

How do you define "appropriate"?

If I have air and EAN32 in my gas list, that means I am going to be using air as my bottom gas and EAN32 for deco. In which case, the MOD for air should be the MOD at a ppO2 of 1.4 ATA, and the MOD for EAN32 should be the MOD at a ppO2 of 1.6. In that case, if the MOD of my EAN32 is shown as 111', I would say it is not showing me the appropriate MOD.

Given that the documentation makes it clear that the "correct" usage of the computer is to only have gases enabled which you are carrying, I believe the way the new Teric firmware works is the way they should all work.

 

[rongoodman]

It shows the MOD for the PPO2 which I have set in the Nitrox Gases pane in System Setup, in this case 1.4. I have no real issue with the way the Teric is doing it, but am surprised to see that the Perdix and Teric aren't consistent. I hope that Shearwater resolves this soon.

(It actually shows the Max Depth Setting if that's shallower than the MOD selected, e.g., for Air.)

 

[stuartv]

It shows the MOD for the PPO2 which I have set in the Nitrox Gases pane in System Setup, in this case 1.4. I have no real issue with the way the Teric is doing it, but am surprised to see that the Perdix and Teric aren't consistent. I hope that Shearwater resolves this soon.

(It actually shows the Max Depth Setting if that's shallower than the MOD selected, e.g., for Air in my case.)

It doesn't have 2 MOD settings? One for diving and one for deco?

As for the difference between how the Perdix and Teric work, since the Teric just got a firmware update that made this change, it would not surprise me if the next firmware update for the Perdix contained the same change.

 

[rongoodman]

It doesn't have 2 MOD settings? One for diving and one for deco?

Not in Rec mode, which isn't surprising. There are some other differences between Rec and Tec modes which didn't get carried forward to the Teric, such as Maximum Dive Depth. It will be interesting to see in which direction Rec mode will converge on the two.

 

[rongoodman]

Removed.

 

[Jay]

An update on the question if changing gases the wrong way (between dives) via Select Gas (and having two gases On, a higher % one Active, a lower % one On) affects NDL:

SW said "NDL will not be affected by having more than one gas turned on. Predictive gas switching is only applicable while performing decompression [I had queried that I never saw a gas in yellow suggesting a switch].

However, if you enter decompression, on purpose or by accident, the computer will immediately assume you are carrying and plan to use all gasses that are turned on. As a result, if you have gasses turned on that you are not carrying, the decompression information shown by the computer will be inaccurate. For this reason, it is important to turn off all gasses you are not carrying and plan to use on every dive.

If you encounter a situation where you must perform emergency decompression and you have more than one gas turned on. You can turn gasses off underwater in Main Menu > Edit Gasses. Once you have turned off all the gasses you are not carrying, your decompression information will be accurate. "

 

[Jay]

A summary if there's any confusion or you're new to the thread:

Short story is to know if you’ve inadvertently left a gas on (that you won’t be breathing), and how to correctly change gases between dives i.e. using Edit Gases only.

If you’ve previously setup Nitrox on your Teric, and change to it from Air using the wrong method (i.e. via Select Gas) it will have MOD implications. This only applies from FW16 onwards and is due to an improvement in MOD behaviour for Deco divers.

The MOD implications are that, for the entirety of the dive, the Teric will classify Nitrox as a Deco gas, which will use Deco PPO2 to calculate its MOD (instead of OC MOD). To quote SW: "You should only have gasses you are carrying and plan to use on the current dive turned on. Only one of these gasses are usually used on the working (bottom) part of the dive, the leanest mix. So that mix will use MOD PPO2 while all other mixes are assumed to be decompression gasses that will use the Deco PPO2 limit.”. In the Air / Nitrox example above, Air (On) would use MOD PPO2 and Nitrox (Active) would use Deco PPO2 for their respective MODs.

Select Gas - does not turn off the gas you moved from (via 2nd item down). It make Active a gas that was Off, but the other (old) gas is left On <— don’t use this for single tank dives (when changing gas types during dives).

Edit Gases - forces you to turn off the old gas. (either via 4th item down, or Settings / Gases / …)

[I've used Nitrox and Air as an example, but naturally it applies to any gases, where a higher % one is Active, a lower % one is On]

FW16 relevant detail:

 

[KenGordon]

Oh dear.

Other sorts of dive computer differentiate between gases which you carry and expect to use and so used for TTS calcs, gases which you don’t expect to use but might (for example a buddy’s gas) and gases used another day.

Here it seems you can always select a gas even if you are not carrying it unless you set it to zero o2.

This is all rather complicated for the simple mode.

Does anyone actually use Rec mode for multi gas deco diving? Isn’t it a bit silly to have it be multi gas and suffer from all the complications and user error risks given the Tec mode exists?

 

[Jay]

Does anyone actually use Rec mode for multi gas deco diving? Isn’t it a bit silly to have it be multi gas and suffer from all the complications and user error risks given the Tec mode exists?

I was wondering the same. An easy refinement would be to remove the Select Gas menu from OC Rec.

 

[Chidiver1]

If you encounter a situation where you must perform emergency decompression and you have more than one gas turned on. You can turn gasses off underwater in Main Menu > Edit Gasses. Once you have turned off all the gasses you are not carrying, your decompression information will be accurate. "

So you're saying it would still be accurate deco information even if you were to switch it off after the dive has started?