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Thread: Why not nitrogen and helium sensors too?

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    RBW Member RC is an unknown quantity at this point RC's Avatar
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    Why not nitrogen and helium sensors too?

    I'm not a rebreather diver yet but have read a lot about them and do plan on getting one down the line.

    One thing I was wondering about is what type of nitrogen and helium sensors are available and how is their reliability? Are there reliable solid state ones available and if so, why aren't they incorporated?

    I know one of the main weaknesses is O2 sensors and that they need constant calibration, potential for current limiting, etc. What I was thinking is if there are very reliable nitrogen and helium sensors available, couldn't they be used to validate the O2 sensors?

    It would be similar to the constant validation done on the Discovery but instead, all you need to do is (1-fraction of helium-fraction of nitrogen)=O2 percentage. It wouldn't replace the oxygen sensors but could be used as a constant way to validate the output of the oxygen sensors.

    Would it work? If not, why not?


    Robert

  2. #2
    Supporting Member Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36 has a reputation beyond repute Dsix36's Avatar
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    Re: Why not nitrogen and helium sensors too?

    You would need a way to measure all the other possible inert gasses also, such and CO2 and maybe even CO.
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    Re: Why not nitrogen and helium sensors too?

    Quote Originally Posted by Dsix36  View Original Post
    You would need a way to measure all the other possible inert gasses also, such and CO2 and maybe even CO.
    CO2 is absorbed by the scrubber and CO if present will be absobed by your lungs.

    For the acuracy required only O2 He and N need to be measured. All others will be under 1%.
    Of course a CO2 and CO sensor would be nice, but they are warning you of an issue as opposed to being used to calculate deco.

  4. #4
    Dave Tomblin wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc's Avatar
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    Re: Why not nitrogen and helium sensors too?

    The partial pressure of oxygen in important to monitor because it is essential for life support and it is metabolized by the body. Inert gas is not metabolized and the partial pressures are very predictable if you know the PO2 the rest is easy to figure out.
    Bottom line, not worth the effort.
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    RBW Member RC is an unknown quantity at this point RC's Avatar
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    Re: Why not nitrogen and helium sensors too?

    The point of the nitrogen and helium sensors wouldn't be for deco calculations but to have a way of being able to constantly verify the output of the oxygen sensors.

    What if you have current limited sensors reading only to 1.2 but are running a 1.3 setpoint? With the extra sensors, you could easily set up a alarm to verify the problem is with the oxygen sensor instead of having the controller pumping in more and more oxygen and spiking the PPO2.

    The way I was thinking of it is as another check that could be done seamlessly without needing to do a diluent flush---it'd be automatic and done constantly to give early warning.

    Quote Originally Posted by wedivebc  View Original Post
    The partial pressure of oxygen in important to monitor because it is essential for life support and it is metabolized by the body. Inert gas is not metabolized and the partial pressures are very predictable if you know the PO2 the rest is easy to figure out.
    Bottom line, not worth the effort.

  6. #6
    Dave Tomblin wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc has a reputation beyond repute wedivebc's Avatar
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    Re: Why not nitrogen and helium sensors too?

    Quote Originally Posted by RC  View Original Post
    The point of the nitrogen and helium sensors wouldn't be for deco calculations but to have a way of being able to constantly verify the output of the oxygen sensors.

    What if you have current limited sensors reading only to 1.2 but are running a 1.3 setpoint? With the extra sensors, you could easily set up a alarm to verify the problem is with the oxygen sensor instead of having the controller pumping in more and more oxygen and spiking the PPO2.

    The way I was thinking of it is as another check that could be done seamlessly without needing to do a diluent flush---it'd be automatic and done constantly to give early warning.
    Got it. That strategy would also have to be able to measure CO2 and water vapor in order to subtract it from the total gas pressure.
    Cheers,

    Dave....

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    Re: Why not nitrogen and helium sensors too?

    Quote Originally Posted by RC  View Original Post
    The point of the nitrogen and helium sensors wouldn't be for deco calculations but to have a way of being able to constantly verify the output of the oxygen sensors.

    Take a look at Poseidons solution (well, it's not theirs originally) regarding validation of cells. I believe that's a good approach to keep the risks associated with diving as low as possible. You have sensor validation on Surface, at 6 meters and then throughout during the dive.

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    Re: Why not nitrogen and helium sensors too?

    Quote Originally Posted by RC  View Original Post
    I'm not a rebreather diver yet but have read a lot about them and do plan on getting one down the line.

    One thing I was wondering about is what type of nitrogen and helium sensors are available and how is their reliability? Are there reliable solid state ones available and if so, why aren't they incorporated?

    I know one of the main weaknesses is O2 sensors and that they need constant calibration, potential for current limiting, etc. What I was thinking is if there are very reliable nitrogen and helium sensors available, couldn't they be used to validate the O2 sensors?

    It would be similar to the constant validation done on the Discovery but instead, all you need to do is (1-fraction of helium-fraction of nitrogen)=O2 percentage. It wouldn't replace the oxygen sensors but could be used as a constant way to validate the output of the oxygen sensors.

    Would it work? If not, why not?


    Robert
    the complexity/cost will (in my opinion) far exceed the benefit

    it is not so that O2 sensors are the most critical thing in a breather: there are good sensors available, and good methodology on using and validating sensors before/during diving: we have learned to work safely with 'non-perfect' sensors
    www.rEvo-rebreathers.com
    ...."Yes you have to pre-breathe to activate the scrubber sorb, anyone who says different doesn't know what they are talking about!"...
    .... to get more accurate CO2 injection in the breathing machine we put 2 mass flow controllers in series ...
    .... The noise is a few tens of nano-volts, so DL were able to reduce the output voltage ...
    .... radial scrubbers give longer dwell time than axials...
    .... the earth is flat and ...

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    Re: Why not nitrogen and helium sensors too?

    Quote Originally Posted by wedivebc  View Original Post
    Got it. That strategy would also have to be able to measure CO2 and water vapor in order to subtract it from the total gas pressure.
    Just the water vapor pressure remains constant, also be partial pressure of CO2. Which further stimulates ventilation.

    rc greet

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    RBW Member abohmann is an unknown quantity at this point abohmann's Avatar
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    Re: Why not nitrogen and helium sensors too?

    Quote Originally Posted by RC  View Original Post
    The point of the nitrogen and helium sensors wouldn't be for deco calculations but to have a way of being able to constantly verify the output of the oxygen sensors.

    What if you have current limited sensors reading only to 1.2 but are running a 1.3 setpoint? With the extra sensors, you could easily set up a alarm to verify the problem is with the oxygen sensor instead of having the controller pumping in more and more oxygen and spiking the PPO2.

    The way I was thinking of it is as another check that could be done seamlessly without needing to do a diluent flush---it'd be automatic and done constantly to give early warning.

    That idea was propably behind the coming new "Liberty Rebreather" from Divesoft.

    It is going to be equipped with both, HE and O2 sensors and they claim that the HE sensors may take over(with limited accuracy) when O2 sensors fail. Nitrogen will be estimated during calibration.
    Electronically controlled CCR Liberty - Detail - DIVESOFT.CZ
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