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Thread: Measuring VO2 on a SCR

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    Measuring VO2 on a SCR

    I had my Dolphin out for the first time where I spent most of my time recording my pO2 at various depths and exertion levels to calculate my VO2.

    For the most part the numbers made sense, with increasing (but a little lower than expected) VO2 with higher exertion levels.

    The portion of the dive that has got me scratching my head is the last 15 minutes or so. I was just about drifting in, so expected a low VO2, but an average VO2 of ZERO doesn't make any sense. The grouping of 7 readings was pretty tight with no "flyers" that were wildly out of range from the rest of the numbers.

    I was ascending at a VERY low rate as the bottom was nearly level, depths were in the 20ft range. I am using an oxygauge to measure pO2, it was calibrated before the dive and at 1 atm it displayed the correct pO2 of the supply gas. I did not check pO2 of the gas after the dive (I will next time) but later I turned it on again and it displayed .21 in air.

    Was this normal?

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    Re: Measuring VO2 on a SCR

    So I guess what you're saying is that your readings were whatever the ambient pressure was times the percentage of O2 in the tank.

    You can't, obviously, have a VO2 of zero unless you're dead. Even if you're paralyzed and anaesthetised then it's about 250 ml/minute, so there must be an error somewhere.

    Having said that, at very low VO2's then there is not that much difference between the tank FO2 and the steady state loop FO2 - maybe a small enough difference to be accounted for by errors in the oxygauge, tank percentage etc. Steady state loop concentration takes a fair old while to reach as well so it's a bit difficult to get precise numbers.

    I have an Excel SCR sim at http://www.davidteubner.com/Research.htm (Loop O2 percentage calculator for semiclosed rebreathers) which lets you look at time to steady state and what the steady state is with different jet flows and tank percentages.

    I would be interested to see the actual numbers i.e. oxyguage reading, depth, tank percentage and jet flow rates.

    Dave T

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    Re: Measuring VO2 on a SCR

    Quote Originally Posted by dteubner
    So I guess what you're saying is that your readings were whatever the ambient pressure was times the percentage of O2 in the tank.
    No, the readings taken are depth and pO2. Along with Qs and percent O2 in the supply gas these are used to caluclate VO2.

    Quote Originally Posted by dteubner
    You can't, obviously, have a VO2 of zero unless you're dead. Even if you're paralyzed and anaesthetised then it's about 250 ml/minute, so there must be an error somewhere.
    Yup.

    Quote Originally Posted by dteubner
    Having said that, at very low VO2's then there is not that much difference between the tank FO2 and the steady state loop FO2 - maybe a small enough difference to be accounted for by errors in the oxygauge, tank percentage etc. Steady state loop concentration takes a fair old while to reach as well so it's a bit difficult to get precise numbers.
    I would think ten minutes of a sustained activity level would show a trend to a steady state in the breathing loop. That is why I am scratching my head, trying to figure out what went wrong.

    Quote Originally Posted by dteubner
    I have an Excel SCR sim at http://www.davidteubner.com/Research.htm (Loop O2 percentage calculator for semiclosed rebreathers) which lets you look at time to steady state and what the steady state is with different jet flows and tank percentages.
    I'll take a look at this, thanks.

    Quote Originally Posted by dteubner
    I would be interested to see the actual numbers i.e. oxyguage reading, depth, tank percentage and jet flow rates.

    Dave T
    Here you go:

    FsO2=.63 Qs=5.7
    cross current swim out at moderate pace
    D (feet) pO2 calc VO2 (lpm)
    20 .94 .61
    25 1.02 .67
    30 1.11 .66

    with current at lower pace
    32 1.18 .43
    41 1.36 .34
    36 1.26 .39

    against current at highest pace
    34 1.18 .66
    34 1.16 .77
    33 1.11 .96
    33 1.10 1.01

    slow pace shore bound with current
    26 1.12 .05
    21 1.11 -0.8
    22 1.05 0
    23 1.09 -0.1
    20 1.02 0
    20 1.02 0
    18 .97 .03

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    Re: Measuring VO2 on a SCR

    My calculations in brackets after yours.

    FsO2=.63 Qs=5.7
    cross current swim out at moderate pace
    D (feet) pO2 calc VO2 (lpm)
    20 .94 .61 [0.63]
    25 1.02 .67 [0.69]
    30 1.11 .66 [0.67]

    with current at lower pace
    32 1.18 .43 [0.45]
    41 1.36 .34 [0.35]
    36 1.26 .39 [0.40]

    against current at highest pace
    34 1.18 .66 [0.68]
    34 1.16 .77 [0.79]
    33 1.11 .96 [0.98]
    33 1.10 1.01 [1.03]

    slow pace shore bound with current
    26 1.12 .05 [all roughly the same as yours-in error]
    21 1.11 -0.8
    22 1.05 0
    23 1.09 -0.1
    20 1.02 0
    20 1.02 0
    18 .97 .03

    On the descent phase of the dive, you may have been activating the bypass valve, which would add gas at a greater amount than you would metabolise, so this may be the reason for the lower readings at the start of the dive.

    I have had the odd anonolous reading before, were there any other reasons, such as loop flushing that could have given rise to your readings? You were diving a higher mix than wuld be normal for the 60% jet, try a 50% mix and the 50% jet. How old is the O2 cell? Are the batteries in the handset OK?

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    Re: Measuring VO2 on a SCR

    Quote Originally Posted by Freef
    On the descent phase of the dive, you may have been activating the bypass valve, which would add gas at a greater amount than you would metabolise, so this may be the reason for the lower readings at the start of the dive.

    I have had the odd anonolous reading before, were there any other reasons, such as loop flushing that could have given rise to your readings? You were diving a higher mix than wuld be normal for the 60% jet, try a 50% mix and the 50% jet. How old is the O2 cell? Are the batteries in the handset OK?
    David,

    Thanks for running the numbers. Your answers are all slightly higher, how are you calculating VO2? The VO2 formula I am using was not in my manual, my instructor talked about it in the classroom and I copied the formula and then reconstructed it from my notes to a spread sheet.

    VO2=Qs*(FsO2-FiO2)/(1-FiO2)
    FiO2=PO2/(D/33+1)
    where
    Qs is the sonic valve flow in lpm
    FsO2 is the fraction of O2 in the cylinder
    FiO2 is the fraction of O2 in the breathing loop
    PO2 is the partial pressure of O2 as measured by an O2 sensor
    D is the depth (in this case feet because I live in the metricly challanged USA)

    I don't think I was loop flushing, as I was on this dive mainly to record numbers to start to establish my VO2 under different conditions. So I was very consious of my technique. I don't plan on using the mix/jet combo often, I only have the 4 liter cylinders and wanted to do an extended dive in shallow water.

    As you suggest, the O2 cell is old and probably at the end of it's useful life. As stated in my original post it does calibrate and does give answers in the correct range in known conditions (on shore). The Oxygauge reports that the battery is OK on startup. I purchased the unit used and it is about 3 years old, with "about 25" dives prior. I do have a new sensor I purchased when I bought the Drager. It is in an unopened package and I hesitate to open it if the one I am using now is OK. Note to self: don't be cheap, open the package!;)

    I am learning about O2 cell technology and don't have much of an experience base yet. Thanks again for taking the time to run the numbers.

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    Re: Measuring VO2 on a SCR

    Quote Originally Posted by 4ster
    Note to self: don't be cheap, open the package!;)
    I'd go with that. Someone may know the mV readings of the Drager cell [calling Narked@90?] and which terminals to read them from so if you have a voltmeter you can check your one.

    The Drager manual gives the flow rate of the 60% jet at 5.8 L/min, not 5.7, but the difference in our calculations is very small.

    The formula is:



    For a more detailed discussion on flow rates and vO2 have a look at:

    http://www.rebreatherworld.com/semi-...culations.html

    I have a spreadsheet that I use for my vO2 calcs, and it has other useful stuff on it as well, PM me with your email address for a copy. I'll even convert it to feet for you.

    I can also email you my lengthy experiences so far while Dolphin diving, again PM me.

    David.

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    Steve Sprague 4ster is a glorious beacon of light 4ster is a glorious beacon of light 4ster is a glorious beacon of light 4ster is a glorious beacon of light 4ster is a glorious beacon of light 4ster is a glorious beacon of light 4ster is a glorious beacon of light 4ster is a glorious beacon of light 4ster is a glorious beacon of light 4ster is a glorious beacon of light 4ster is a glorious beacon of light 4ster's Avatar
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    Re: Measuring VO2 on a SCR

    After some reading on other threads here I think the reason for the low numbers at the end of the dive was condensation in the loop. I gather that moisture makes O2 cells read low. The cell mounted in the inhalation bag on the Drager is not in the best location and there is no way to dry it out with a loop flush as with ccrs. At about an hour and forty minutes this was the longest dive I had done on the Dolphin and the condensation (and spit) accumulation was the highest I have experienced. When I do my VO2 tests from now on I will do them at the begining of the dive when possible.

    This is a great forum, lots of great folks with practical experience. I view it as continuing education.

    Steve

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    New Member Taz is an unknown quantity at this point Taz's Avatar
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    Question Re: Measuring VO2 on a SCR

    Hey guys

    Just been reading your calculations.

    I've just been on a weeks diving on the Dolphin in Egypt, driving it with the VR2 and O2 analyser.

    I had similar calculations to you guys, but then noted that the calculations were, at times, wildly different on the VR2.

    I came to the conclusion that although the calculations can be useful, as they are not accurate I would not drive the Dolphin without the O2 analyser and computer. Comparing the MOD on the O2% in the breathing bag via calculation and the actual O2% according to the calibrated computer, the results were often wildly different.

    This struck me as a dangerous calculation which could quite easily lead to an O2 CNS situation (not good!).

    No matter how good the calculations are, there are too many quickly changing variables on a dive for any calculation to be accurate.

  9. #9
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    Re: Measuring VO2 on a SCR

    Quote Originally Posted by Taz
    Hey guys

    Just been reading your calculations.

    I've just been on a weeks diving on the Dolphin in Egypt, driving it with the VR2 and O2 analyser.

    I had similar calculations to you guys, but then noted that the calculations were, at times, wildly different on the VR2.

    I came to the conclusion that although the calculations can be useful, as they are not accurate I would not drive the Dolphin without the O2 analyser and computer. Comparing the MOD on the O2% in the breathing bag via calculation and the actual O2% according to the calibrated computer, the results were often wildly different.

    This struck me as a dangerous calculation which could quite easily lead to an O2 CNS situation (not good!).

    No matter how good the calculations are, there are too many quickly changing variables on a dive for any calculation to be accurate.
    So, I am not sure I understand what you are saying. Are you saying that post dive calculations are inaccurate? Or are you saying that the calculations done by the VR3 computer during the dive are inaccurate? A properly functioning and calibrated Oxygauge should give about the same pO2 as a correctly functioning VR3.

    My point on a different thread is that if moisture is affecting the O2 cell reading, then the calculations the computer is doing to report your deco obligation and your O2 exposure are going to be wrong. At no time durring my diving on the Oxygauge did I enter an alarm condition where the PO2 was too high or too low for the current depth. It was only after the dive that I noticed a trend in the readings that indicated that the gauge was recording the loop O2 incorrectly in the latter part of the dives. Can the same thing be happening with your VR3? If that is the case, then wouldn't it be better to dive the Dolphin without deco calculations using realtime measurement of the loop O2?

    Dave (Freef) and I are on similar paths going slightly different directions. I am logging data to calculate VO2 for the same reason I calculate my surface air consumtion on every open circuit dive - dive planning. It is a good idea to know your own personal VO2 under various conditions. How one uses that information for dive planning depends on goals and personal conservatisim. But it is still essential (in my opinion) to know how you personally perform, not pick a number from a range of numbers in a training manual because it says that "average" VO2 in average conditions is X.

    I don't dive deeper than the MOD (1.4ata) of my supply gas. This is partly because I don't trust the Oxygauge, also because the consequences of getting an O2 hit if your technique is poor past the MOD are so bad. Sure, the MOD of the inspired gas from the loop is probably greater, but for me, right now, I'll stick with the supply gas MOD. If I want to go deeper I need a different tool than a Dolphin.

    BTW, Egypt sounds like a blast, sounds like you had a good time!

    Steve

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    New Member Taz is an unknown quantity at this point Taz's Avatar
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    Re: Measuring VO2 on a SCR

    Sounds like I may have gotten the wrong end of the stick.

    I tried using calculations putting a 36% Nitrox blend through the 60% jet.

    The problemn with that was the O2% in the brathing bag was calculated on a normal breathing rate. Once that was determined I was calculating the MOD for the dive at a normal breathing rate.

    For instance a 40% blend through the 60% jet at a breathing rate of 1.25 litres per minute gives an o2% in bag of 23.5%, giving a MOD of 49.5m. However when diving on this mix at a normal breathing rate, the O2% according to my VR2 was approx 34%, fluctuating by a few % either way.

    My point was along the lines of the dangers of using the calculation as a given for the MOD and trying to hit that 49m.

    My apologies for any confusion caused by my initial confusion.

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