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

One of the most common tech support questions we get is along the lines of:

“My dive computer said the 20ft stop was supposed to take 4 minutes, but it actually took 10 minutes to clear. What went wrong”?

The answer is almost always that a more oxygen rich gas was enabled (turned on), but never actually used on the dive. This post will explain why leaving an unused gas turned on causes the stop time to be incorrect. This post only focuses on open circuit mode behaviour, and assumes that the reader has basic understandings of decompression theory including partial pressures of gases and inert gas tissue tension modeling.

You can program up to 5 gasses

Shearwater dive computers allow up to 5 gas mixes to be programed. Each gas can also be turned on or off. This is a convenience feature, since most divers do not typically carry 5 gases on a single dive. A standard set of mixes can be left programmed in at all times, with only the gases actually carried turned on.

If you do pre-program a set of standard gas mixes, it is important that only the gases actually being used on the dive are turned on. The reason for this has to do with decompression predications versus the current state calculations.

Turn off gases that will not be used on the dive

Turn off gases that will not be used on the dive

Many of the decompression values shown by a dive computer are predictions. They make assumptions about the future behaviour of the diver, so that predicted times can be calculated and displayed. If the diver’s behaviour deviates from these assumptions, then the predications will not be accurate.

Time To Surface

For example, Time-to-Surface (TTS) is the predicted time to ascend to the surface plus all deco stops. It assumes the diver will ascend at 33ft/min (10m/min) and use the best gas available at all depths. If the diver ascends at a different rate, or chooses to breathe different gases than what has been assumed, the actual TTS will not match the prediction. Values that are predictions are TTS, @+5, Stop Time, and NDL.

Other decompression values are based on the actual current state. These values include the inert gas tissue loading, CEIL, and GF99. The inert gas tissue loading is continuously updated in real-time based on the partial pressures of the inspired inert gases. For open circuit diving, this means that the tissue loadings are based on the current depth and the fraction of each inert gas in the breathing gas. The real-time current state is always updated based on the gas that the user has selected as the active gas. There is never any automatic switching of the active gas.

Stop Depth and Time

The Stop Depth is both a predication and a current state value, depending on how close you are to the Stop Depth. It is a predication when you are much deeper than the Stop Depth, since it factors in the assumed ascent rate while adjusting for on-gassing or off-gassing that occurs during the ascent. However, when you are near the Stop Depth, it is no longer a predication, but the current state that must be obeyed.

The Stop Time, however, is always a predication. It is predicting how long it will take to clear the stop, assuming that the diver remains at the stop depth and breathes the best available gas. If the best gas is not being used, the stop time will not decrease at the expected rate i.e. it won’t decrease by 1 minute for each real minute that passes. This is because the dive computer never forces the gas switch, so the real-time tissue tensions get updated by the currently selected gas but the predications still assume the diver is going to switch to the best available gas. The dive computer attempts to highlight that the best available gas is not being used by displaying the gas in an inverted yellow font.

The gas display will turn yellow when a better gas is available. This is a reminder to either switch to that gas, or turn off that gas so that predictions will become accurate. Gases can be turned off during the dive in the Dive Setup->Define Gas menu.

The gas display will turn yellow when a better gas is available. This is a reminder to either switch to that gas, or turn off that gas so that predictions will become accurate. Gases can be turned off during the dive in the Dive Setup->Define Gas menu.

How the best available gas is determined

The determination of the best available gas is determined as follows:
- Only applies when deco stops are needed (If no deco stops needed, then just assumes current active gas used for entire dive).
- The best gas for a given depth is the gas with the highest PPO2 at that depth, where the PPO2 is less than the “OC Deco PPO2” setting.
- The default “OC Deco PPO2” setting is 1.61 ata.

In the image above, air (21/00) is selected, but oxygen (99/00) is available. At this depth, the PPO2 of oxygen is below the “OC Deco PPO2” level of 1.61, so it is the best available gas. Therefore, the dive computer assumes oxygen will be used when calculating TTS and Stop Time. However, if the diver remains on air, the actual off-gassing will be slower than expected so the stop will last longer than the displayed prediction.

Hopefully this post has clarified why it is important to only turn on gasses that will actually be used on a dive. We recommend a quick check of the gas list before each dive, and turning off any gases that will not be used on the dive.

14 Responses

  1. Quick question on this as I seem to be missing something. How do I turn off a gas in OC – I’ve a Predator V63 and can’t see where I do that and it would be really useful. I’ve been in this situation before and realized what was going on so good to see a note.
    Sorry if I’m missing something basic here

    • Tyler Coen

      Hi Robin, On the Predator you need to turn-on the option to be able to turn on or off gases. On the “Deco Setup” menu page, there is an option called “Gas On/Off”. Set this to “On” and then in the gas setup menus you should get the ability to turn on and off gases.

      On newer models like the Perdix, Petrel, and NERD this setting is always enabled.

  2. You are doing great, Tyler. Keep on!

  3. Thanks Tyler

    Users leaving a gas enabled when they don’t actually have it is possibly because you cannot switch off a gas if it’s the only one that’s ‘On’. You need to scroll past and enable others before going back to switch it off.

    A better solution would be to remove that constraint and put a check on the ‘Exit’ routine.

  4. Hi Thanks for publishing the answer to Tyler’s question. I now understand how to turn on and how to turn OFF gases.

  5. joe

    Hi there,
    Quick question ,
    is there anyway that the computer can switch back to 21%AIR automatically upon surfacing, after been used with different gases for deco stops.
    for example if i enabled all the gases that will be used for the dive 21 / 50 and 99, and the last gas was used is 99%, i have to manually go to the setting and enable 21% before the next dive , other wise the last used gas will be enabled.

    • Hi I have used my new perdix for a weeks diving and have found that when I have looked down at it the corpus has turned no without me setting it. I don’t think I have knocked it either. Can you help

      • Shearwater Research

        Please send an email to our address and we will assist you.

    • Shearwater Research

      Hello Joe,
      This is by design. We believe it’s up to the diver to be in total control of their dives and selected gases. That means making sure they are on the correct gas for their next dive.

  6. A question about this, I used to dive a lot of mines with multilevel and multigas dives on OC and would liked to have had a END adjustment on the best choice of gas. For instance at 40 metres I do not want the Petrel to suggest a 32% (or 30%) gas when one needs to swim for long periods, but compute the obligations on, say a 25/25.

    Thanks for very good products, I use a standalone Petrel 1 and a diveCAN JJ with Petrel 2.
    Jonas B

    • Shearwater Research

      Hello Jonas,

      We recommend that you only enter the gases (or have turned on)into your computer that you will actually be carrying on your dive. If you have 32 entered and are carrying 25/25, turn the 32% Off or delete it entirely and enter 25/25 in its place. Only the gases that are entered and turned on are used in TTS and deco calculations.

      • The case I’m considering is a multistage multilevel cave/mine dive with, for instance 25/25, 30% and 100%, which includes swims at, for instance, 20meters and 40metres. For the 40meter swim I do not want to use a 30%, instead use a 25/25 and the petrel suggest the 30% gas, which I think is to narcotic for the enviroment. I would suggest a max END for the gas in the deco computations.

        Yours sincerely,
        Jonas Björnsson

  7. Thank you for making things clear!
    I know that other dive computers (Suunto’s EON Steel for example) work in the same way, but I also know that some tec divers regard this behaviour as undesirable as they would like to see the actual TTS when only using the CURRENT gas without having to deactivate all other gases first.
    Wouldn’t it be possible to add an (advanced) option like “Calculate TTS on [actual | planned] gas”? Wouldn’t this also contribute to “We believe it’s up to the diver to be in total control of their dives (…)”? 😉


    • Tyler Coen

      Hi Peter, thank you for the interesting suggestion. We will give it some thought. The biggest drawback I see is that it will be a difficult setting to understand, and may be prone to misuse or misunderstanding. On the menu screens we only have space for a few characters per setting, so we cannot call it “Calculate TTS on [actual | planned] gas”. It’s definitely worth some thinking though for future firmware release cycles.


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