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

[JohnnyC]

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

The deco information is always accurate based on the gas you are breathing. If you have selected an incorrect gas, your deco information would be incorrect. That's true of any computer, it only knows what you tell it.

The only issue having a different PO2 set for bottom and deco mixes is the point at which it will alarm, and the depth at which it will alarm, based on the PO2 you tell it you want it to alarm at. If you tell it your bottom mix wants to be 1.4, and you blow your MOD and the PO2 is 1.5, it will yell at you. If you have your deco gases set at 1.6, it will simply allow you to breathe your deco gases with a higher PO2 before telling you that you're doing something wrong.

You could put 100 gases in and have them all active and it will still provide the correct decompression obligation as long as you have the actual gas you are breathing selected. The TTS may not be correct, and the "stop time" will not decrease at the actual rate, but that's the only place it's making assumptions. It will tell you there's a better gas, and if you use a "bottom" gas at a PO2 higher than what you have set in the settings, it will alarm, but it will not change anything that actually has an effect on your dive, your decompression obligation, or your safety.

 

[Jay]

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

if you’re in Deco and you correct your gas by turning the one you don’t carry to Off then yes, your deco info will then be corrected.

 

[JohnnyC]

@Chidiver1 read this post on the Shearwater blog:

Only Enable (Turn On) Gases That You Will Actually Use on a Dive - Shearwater Research

 

[stuartv]

The deco information is always accurate based on the gas you are breathing. If you have selected an incorrect gas, your deco information would be incorrect. That's true of any computer, it only knows what you tell it.

[snip]

You could put 100 gases in and have them all active and it will still provide the correct decompression obligation as long as you have the actual gas you are breathing selected. The TTS may not be correct, and the "stop time" will not decrease at the actual rate, but that's the only place it's making assumptions. It will tell you there's a better gas, and if you use a "bottom" gas at a PO2 higher than what you have set in the settings, it will alarm, but it will not change anything that actually has an effect on your dive, your decompression obligation, or your safety.

This is not correct.

If you have exceeded your NDL and have a mandatory deco stop, a Shearwater computer will display the deco obligation based on the gases it thinks you are carrying - not just the one you are breathing.

E.g. if you tell a Perdix or Teric that you are carrying Air and O2 and you go into deco, while you are still breathing Air, the displayed deco info will be based on an assumption that you will switch to O2 at 20'. If you are still breathing air and the computer says you have 2min at 20' and 10 min at 10', it's not suddenly going to drop a bunch of that deco time when you switch to O2.

As indicated on the previous page, in the quote from Shearwater Support that was posted by Jay, it is called Predictive Gas Switching.

If it worked the way you are saying, the displayed deco obligation(s) would change every time you performed a gas switch on the computer.

The TTS and the deco obligations go together. It would make no sense whatsoever to display a TTS of, for example, 10 minutes when it is telling you that you have a 2 min stop at 20' and a 10 min stop at 10'.

 

[JohnnyC]

@stuartv read my last paragraph again. I specifically stated that the TTS may not be correct and the stop time will not decrease at the actual rate. Obviously it will depend on what gasses you are carrying. If my first paragraph was unclear, I apologize.

You started typing before reading my whole paragraph.

 

[stuartv]

@stuartv read my last paragraph again. I specifically states that the TTS may not be correct and the stop time will not decrease at the actual rate. Obviously it will depend on what gasses you are carrying.

You started typing before reading my whole paragraph.

I did read it all. The first sentence of your last paragraph is specifically wrong. That, and the similar statements you made are what I was responding to.

You could put 100 gases in and have them all active and it will still provide the correct decompression obligation as long as you have the actual gas you are breathing selected.

This is not correct. It will only provide the correct deco obligation if you have programmed exactly and only the gases you are carrying and going to use.

If I am going to dive to 100' and enter a new, optimum gas for every 10' of my ascent, when I'm sitting there at 100', breathing Air, and watching my deco go up, it's going to say I have a short deco obligation. Much shorter than what I will actually have if I really only am carrying Air.

 

[JohnnyC]

I did read it all. The first sentence of your last paragraph is specifically wrong. That, and the similar statements you made are what I was responding to.



This is not correct. It will only provide the correct deco obligation if you have programmed exactly and only the gases you are carrying and going to use.

If I am going to dive to 100' and enter a new, optimum gas for every 10' of my ascent, when I'm sitting there at 100', breathing Air, and watching my deco go up, it's going to say I have a short deco obligation. Much shorter than what I will actually have if I really only am carrying Air.

It will absolutely provide the correct decompression obligation regardless of what gasses you have selected. It will not bend you because you didn't select the gas it thinks you should select. However, the information displayed in TTS and stop time will not be accurate. If you don't switch, it will tell you 10 minutes, but it will not tick down 10 minutes, it will take however long to clear based on the gas you have selected. It's straight out of their website I linked above. See the image below copied directly from the article I linked.

What you suggest is that it auto-switches without telling you. It absolutely does not. The predictive gas switching is solely a display of information. It does not affect how the algorithm is running.

 

[stuartv]

It will absolutely provide the correct decompression obligation regardless of what gasses you have selected. It will not bend you because you didn't select the gas it thinks you should select. However, the information displayed in TTS and stop time will not be accurate. If you don't switch, it will tell you 10 minutes, but it will not tick down 10 minutes, it will take however long to clear based on the gas you have selected. It's straight out of their website I linked above. See the image at the link below copied directly from the article I linked.

Imgur

The bolded part here is a quote directly from Shearwater:

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. "

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.

 

[Joneill]

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

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.

Then I either switch which is one is on or adjust the Nitrox gas setting before or between dives if the mix is changing. I've never had an issue in 120 dives.

It’s quick and easy - somewhat more so on the Teric versus the Perdix AI (where it is deeply buried in the System Setup menu and easier to click past your desired change and have to re-enter the menus). However, even on the Perdix, it only takes ~30 seconds to make changes versus ~15-20 seconds on the Teric (just timed myself to turn on and adjust Nitrox level and turn air off).

One thing I do note is that the MOD for AIR on the Teric shows 186 ft while the Perdix AI shows 130 feet - both units in OC Rec and PPO2 @1.4.

Also, it seem you can set different PPO2 for OC diving versus OC deco when computer is in OC Rec Mode on the Teric while you cannot do that on the Perdix AI - at least in the settings menu.

 

[stuartv]

One thing I do note is that the MOD for AIR on the Teric shows 186 ft while the Perdix AI shows 130 feet - both units in OC Rec and PPO2 @1.4.

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..)