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

Discussion in 'Computers, Gauges, Watches and Analyzers' started by Jay, Jun 3, 2019.

  1. Jay

    Jay Need to dive more!

    # of Dives: 100 - 199
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    Please see Post 47 for a summary.

    --------------------------


    Please see Posts 2+ below first. We're up to page.4 but have't yet been able to resolve the questions in this text below <24hrs so I'll ask a Mod later to help by adding text above when answers are known. Parts of my OP below could be more nuanced than what I've represented, but with Teric FW16 it does change MOD behaviour (correctly for Deco diving, possibly an unintended aspect for Single Tank divers changing inter dive to Nitrox).



    -----------------------------

    This caught me out(*) re what MOD the Teric was using. It might catch out some divers like me who don't dive Deco and/or ignored the OC Deco PPO2 setting, and thought that switching to/from Air --> Nitrox was correctly done via the quicker Select Gas menu option.

    Short version; after entering a new gas, say Nitrox 32%, if you switch gases from Air to Nitrox via the (easier/quicker) Select Gas (Select Gas header) menu item, you'll be using OC Deco PPO2 (default of 1.6) for your Nitrox MOD calc. If you select Nitrox via the Settings / Gases (Edit Gases header), you'll be using OC MOD PPO2 (default 1.40) for your Nitrox MOD calc.

    FWIW, either method of switching from Nitrox to Air will use OC MOD PPO2.

    Here's why: The Select Gas menu leaves Air turn on. The Settings / Gases route, forces you to turn off Air. And 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."

    As a non Deco Diver the way I'm getting around this is by making OC Deco PPO2 = OC MOD PPO2 (short of doing it the proper / long way (and turning unused gases off))

    * Please don't think I didn't know and dive my MOD pre-dive, just that I haven't used that as a display option on the Teric / noticed it on the Info menus.
    • I've read the manual (Rec parts) ... perhaps I missed something but IMO p.27 (5.1) doesn't quite spell it out like it has been above (re MOD implications).
    • I'm aware from a Tec diver's perspective whose familiar with correct gas switching etc this might seem a rookie error, and I would have typed this in the Basic forum ... but please avoid any flames as this post is to ensure other Rec divers don't err as well.
    •[EDIT to add] IDK re how the Perdix works in this regard.
     
    Tournesol2000 likes this.
  2. Griffo

    Griffo DIR Practitioner

    # of Dives: 200 - 499
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    Are you suggesting that if you have multiple gases configured, and "quick switch" to a gas with a high ppO2, it ignores that for MOD ?
    Are you talking about actual diving or in the planner mode? Because I struggle to believe that it would during the dive. In fact the manual suggests that it should use the OC Mod ppO2 value - which defaults to 1.4.
     
  3. Jay

    Jay Need to dive more!

    # of Dives: 100 - 199
    Location: Melbourne, OZ.
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    It will treat it as a Deco gas and accordingly use Deco PPO2 for its MOD calc.

    On land [edit: more on this - see below]

    Which part of the manual is that found in?
     
    KWS likes this.
  4. Griffo

    Griffo DIR Practitioner

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    Page 68 - ppO2 limits

    OC MOD. PPO2 PPO2 displays in flashing red when greater than this value and when more than 7.6m/25ft deeper than the next deco stop. (Default 1.4) This is the maximum allowable PPO2 during the bottom phase of the dive.
     
  5. Jay

    Jay Need to dive more!

    # of Dives: 100 - 199
    Location: Melbourne, OZ.
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    Your second question above now makes me wonder if this is more nuanced than in my OP:

    Also, p.27 (5.1 Rev C) "On ascent, it is assumed that the diver will perform decompression stops using the gas with the highest PPO2 below the OC Deco PPO2 value (default 1.61)."

    During an actual dive, it does seem to look like that if Air is on, Nitrox is Active, and like that for the entire dive, that on the 'bottom phase' OC MOD PPO2 is used, and on the ascent phase, OC Deco PPO2 is used. (Meaning my OP above is more nuanced and needs altering).

    Given it's not the proper way to handle a single gas dive (leaving 2 gases on), and the bottom phase isn't breaching the MOD of the higher O2 gas, and Air is the leanest mix ... IDK. I'll ask SW.
     
  6. Griffo

    Griffo DIR Practitioner

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    Without trying to sound like an ass, i'd suggest reading the manual a few more times.

    I've just got a Perdix myself so this is based on my understanding from limited dives and reading the manual.
    When you have several gases configured, the planner assumes that you will use the gas with the highest available ppO2 as a deco gas.
    MOD should be limited by the gas in use + the MOD setting which defaults to 1.4 - when you are below 7.6m (or something). Above that it allows for higher ppO2 such as 1.6 for use with O2 at 6m.

    You've got me thinking though. Maybe they need a "single gas diver" mode as well that only lets you use a single gas.
     
    Jay likes this.
  7. KenGordon

    KenGordon Rebreather Pilot

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    A special ‘SB first computer recommendation mode’?

    An annoyance with these computers is when switching the gas you are carrying you must always have one active. So if the one you were previously using is before the required one you have to turn the new one off, cycle past the irrelevant ones and then disable the old one.

    This is a classic UI problem.

    The really annoying part of all that is that I know all that and still get it wrong. Unless it is wildly different trimix I just change the percentage of one gas, it results in less swearing.

    A good thing is that the setting for ‘tech’ and ‘rec’ modes are completely independent. So if the gas is air I just set rec mode.
     
  8. JohnnyC

    JohnnyC Divemaster

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    I leave 3 gases active when O/C diving. I switch them accordingly based on tank contents. Since the only thing it would effect is when the number turns red and tells you that you've been a very bad boy, I don't worry about it in the slightest because it's entirely irrelevant to my diving. It calculates my PO2 based on the programmed FO2 and depth. When it changes color is irrelevant.

    I also understand that people are uncomfortable with their understanding of PO2 and the effect of high PO2 on the body. Ultimately, either leave one gas active and use the computer the way it was designed, or take responsibility for using the computer outside of its intended design parameters. You can't abdicate your grey matter if you're not going to use the hardware as intended.

    It's not a UI problem in the computer. The computer has to know what you're breathing to properly calculate your inert gas load. Making you turn on what you ARE breathing before you turn off what you WERE breathing is its way of making sure it's able to do its job properly. Trying to break its logic out for your own convenience is a pretty slippery slope. I can think of a couple scenarios that could turn out pretty poorly for those involved.
     
  9. KenGordon

    KenGordon Rebreather Pilot

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    To be clear I am talking about the system setup menu where you set the gases prior to the dive.

    They could allow me to select no gases and then not leave the menu until one was selected. Then to change from gas one (eg air) to gas two (eg 32%) would save cycling the menu an extra time.

    There is a choice of two invalid/wrong states. They have chosen (if they thought about it at all) the state of two gases selected as less wrong than the state of no gases selected. In my opinion that is a worse choice because in some circumstances that is a valid state, whereas no gases is certainly invalid and so can be detected and treated as such.

    It is the classic radio button vs check box question.
     
  10. JohnnyC

    JohnnyC Divemaster

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    Thanks for the clarification. Having two gases is an easily rectified situation were you to hop in the water with the wrong gas selected. Having no gases selected is a different animal, and you would have to select and define a new gas as opposed to just switching. I'm not sure which is preferable.

    I agree, button vs. check box.
     

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