Luna o2 clock question

[Rgiggi127]

Hi all. I didn't have much luck in the general "Dive computer" forum so I'll try it here. I'm a newly certified diver (appx 1.5 yrs). I bought a Galileo Luna from my LDS and I absolutely love it. It's easy to use and understand and it's AI by way of wireless unit (which I'm aware a lot of you don't like). My question stems from the "CNSo2 clock" that is displayed as a bar graph on the left side. Although I am a new diver, it's my nature to know as much as I can about what I am participating in. That leads to ALOT of reading. As I have read tons and tons of literature over the last couple of years, I understand that oxygen pp can become a problem in two ways. The CNSo2 and pulmonary issues due to sudden ppo2 levels above 1.4 bar and prolonged exposure above .5 bar for extended periods of time respectively. My Luna tracks both. According to the manual, it give an "MOD EXCEEDED" warning for CNSo2 protection if you exceed your maximum depth for the gas you're breathing. It also tracks prolonged exposure above .5 bar by way of the bar graph. My issue is that in the manual Scubapro calls that graph a "CNSo2 Clock". Is it just me misunderstanding something or would that be more accurately referred to simply an "o2 clock" or even a "pulmonary toxicity clock". Thanks for any input. I just want to make sure I understand what I'm reading from someone with more experience than me

 

[scubadada]

Hi Rgiggi127,

Because of the limited duration and intermittent exposure, pulmonary oxygen toxicity is not a practical problem in most diving.

On the other hand, CNS oxygen toxicity may result in seizure and almost certain drowning. CNS oxygen toxicity can be acute and unpredictable, generally with exposures in excess of 1.4 bar, or can be related to more prolonged exposure. It is the latter that is tracked by the CNS clock and these values are generally taken from the NOAA oxygen exposure table included below. Most dive computers, and your Luna, alert you to either type of potentially dangerous exposure by telling you when you have exceeded the maximum operating depth for your gas (this can often be set anywhere in the range of 1.2-1.6 bar) or when you are approaching or have exceeded 100% of your CNS clock.

Some dive computers (like my Oceanic) track CNS oxygen exposure on a rolling 24 hour basis and this is how the NOAA table was designed to be used. Other computers (like your SP Luna) include an oxygen elimination half life when oxygen exposure is low, below 0.5 bar with the Luna. The oxygen elimination half life for the Luna is not stated in the owner's manual, it would be interesting to hear from Scubapro what value is used. The half life is most commonly said to be 90 minutes. I was interested in this topic and posted here on SB. It turns out that the topic is somewhat controversial and there is no consensus http://www.scubaboard.com/forums/advanced-scuba-discussions/471123-cns-o2-exposure.html

Hope this helps, good diving, Craig

[TABLE="width: 144"]
[TR]
[TD="class: xl69, width: 192, bgcolor: transparent, colspan: 3"]O2 Exposure (h)
[/TD]
[/TR]
[TR]
[TD="class: xl64, bgcolor: transparent"]pO2
[/TD]
[TD="class: xl65, bgcolor: transparent"]Single
[/TD]
[TD="class: xl66, bgcolor: transparent"]24 hours
[/TD]
[/TR]
[TR]
[TD="class: xl67, bgcolor: transparent"]0.6
[/TD]
[TD="class: xl68, bgcolor: transparent"]12
[/TD]
[TD="class: xl63, bgcolor: transparent"]12
[/TD]
[/TR]
[TR]
[TD="class: xl67, bgcolor: transparent"]0.7
[/TD]
[TD="class: xl68, bgcolor: transparent"]9.5
[/TD]
[TD="class: xl63, bgcolor: transparent"]9.5
[/TD]
[/TR]
[TR]
[TD="class: xl67, bgcolor: transparent"]0.8
[/TD]
[TD="class: xl68, bgcolor: transparent"]7.5
[/TD]
[TD="class: xl63, bgcolor: transparent"]7.5
[/TD]
[/TR]
[TR]
[TD="class: xl67, bgcolor: transparent"]0.9
[/TD]
[TD="class: xl68, bgcolor: transparent"]6
[/TD]
[TD="class: xl63, bgcolor: transparent"]6
[/TD]
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[TR]
[TD="class: xl67, bgcolor: transparent"]1
[/TD]
[TD="class: xl68, bgcolor: transparent"]5
[/TD]
[TD="class: xl63, bgcolor: transparent"]5
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[TR]
[TD="class: xl67, bgcolor: transparent"]1.1
[/TD]
[TD="class: xl68, bgcolor: transparent"]4
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[TD="class: xl63, bgcolor: transparent"]4.5
[/TD]
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[TR]
[TD="class: xl67, bgcolor: transparent"]1.2
[/TD]
[TD="class: xl68, bgcolor: transparent"]3.5
[/TD]
[TD="class: xl63, bgcolor: transparent"]4
[/TD]
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[TR]
[TD="class: xl67, bgcolor: transparent"]1.3
[/TD]
[TD="class: xl68, bgcolor: transparent"]3
[/TD]
[TD="class: xl63, bgcolor: transparent"]3.5
[/TD]
[/TR]
[TR]
[TD="class: xl67, bgcolor: transparent"]1.4
[/TD]
[TD="class: xl68, bgcolor: transparent"]2.5
[/TD]
[TD="class: xl63, bgcolor: transparent"]3
[/TD]
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[TR]
[TD="class: xl67, bgcolor: transparent"]1.5
[/TD]
[TD="class: xl68, bgcolor: transparent"]2
[/TD]
[TD="class: xl63, bgcolor: transparent"]3
[/TD]
[/TR]
[TR]
[TD="class: xl67, bgcolor: transparent"]1.6
[/TD]
[TD="class: xl68, bgcolor: transparent"]0.75
[/TD]
[TD="class: xl63, bgcolor: transparent"]2
[/TD]
[/TR]
[/TABLE]

 

[Rgiggi127]

Thanks Craig. That helps a lot. From what you said, am I correct in thinking that the CNS02 clock is effected by a combination of both the prolonged exposure AND sudden high exposure? I guess that's the part I was missing.

Thanks again
Rob

 

[kelemvor]

This is covered in page 42 of the Luna manual:

3. D iving with Galileo 3.1 Nitrox Nitrox is the term used to describe breathing gases made of oxygen-nitrogen mixes with oxygen percentage higher than 21% (air). Because Nitrox contains less nitrogen than air, there is less nitrogen loading on the diver’s body at the same depth as compared to breathing air. However, the increase in oxygen concentration in Nitrox implies an increase in oxygen partial pressure in the breathing mix at the same depth. At higher than atmospheric partial pressures, oxygen can have toxic effects on the human body. These can be lumped into two categories: - Sudden effects due to oxygen partial pressure over 1.4bar. These are not related to the length of the exposure to high partial pressure oxygen, and can vary in terms of the exact level of partial pressure they happen at. It is commonly accepted that partial pressures up to 1.4bar are tolerable, and several training agencies advocate maximum oxygen partial pressures up to 1.6bar. - Long-exposure effects to oxygen partial pressures over 0.5bar due to repeated and/or long dives. These can affect the Central Nervous System, cause damage to lungs or to other vital organs. Galileo keeps you safe with respect to these two effects in the following ways: 1) Against sudden effects: Galileo has an MOD alarm set for a user-defined ppO2max. As you enter the oxygen concentration for the dive, Galileo shows you the corresponding MOD for the defined ppO2max. The default value of ppO2max from the factory is 1.4bar. This can be adjusted to your preference between 1.2 and 1.6bar. It can also be turned OFF. Please refer to section 2.3.4 for more information on how to change this setting. 2) Against long exposure effects: Galileo “tracks” the exposure by means of the CNS O2 clock. At levels of 100% and higher there is risk of long-exposure effects, and consequently Galileo will activate an alarm when this level of CNS O2 is reached. Galileo can also warn you when the CNS O2 level reaches 75% (see section 2.6.2). Note that the CNS O2 clock is independent of the value of ppO2max set by the user. The CNS O2 clock increases when the oxygen partial pressure is higher than 0.5bar, and decreases when the oxygen partial pressure is lower than 0.5bar. Hence, while on the surface breathing air you will always be decreasing the CNS O2 clock. During the dive, the depth at which 0.5bar is reached for various mixes is as follows:

(emphasis mine)

Notice that the o2 clock ticks below 43' even on air since your pp02 is going to be over 0.5bar.
Hope this helps.

 

[scubadada]

Thanks Craig. That helps a lot. From what you said, am I correct in thinking that the CNS02 clock is effected by a combination of both the prolonged exposure AND sudden high exposure? I guess that's the part I was missing.

Thanks again
Rob

Hi Rob,

The CNS oxygen clock is affected by pO2 and duration of exposure as is illustrated in the NOAA table. Exceeding the MOD simply contributes to this overall exposure. For example, pO2s of 1.5 and 1.6 are listed in the table, these exceed a pO2 of 1.4, generally considered the maximum for the working portion of a dive. CNS oxygen toxicity can occur due to this exposure OR it can occur more idiosyncratically due to high pO2 exposures, generally exceeding MOD

Your computer has two independent alarms reflecting this risk of O2 toxicity. This is covered on pages 42-44 of your owner's manual. The CNS clock alarm displays "CNS O2=100%". If you exceed the MOD, the computer will display "MOD Exceeded"

Best, Craig

---------- Post added April 2nd, 2014 at 10:54 AM ----------

This is covered in page 42 of the Luna manual:


(emphasis mine)

Notice that the o2 clock ticks below 43' even on air since your pp02 is going to be over 0.5bar.
Hope this helps.

Hi kelemvor,

Any idea what oxygen elimination half life Scubapro uses?

Thanks, Craig

---------- Post added April 2nd, 2014 at 11:30 AM ----------

You might enjoy this OXTOX: If You Dive Nitrox You Should Know About OXTOX ? DAN | Divers Alert Network ? Medical Dive Article

 

[kelemvor]

Any idea what oxygen elimination half life Scubapro uses?

I believe that it's a simple timer which increments if you're above 0.5bar and decrements if you're below 0.5 until it hits zero. The manual doesn't seem to indicate if the rate of change on that timer is affected by how far from that "tipping point" you are in either direction. Maybe someone from scubapro could comment. I'd sure hope that it incremented faster at 1.5bar than it does at 0.6, and mathematically it should be extremely simple.

 

[jazzcat]

@Rgiggi127: Diving Galileo SOL, I've used a Nitrox 50 deco gas and used it near MOD.
I've noticed that, near MOD, my CNSO2 clock increased by 2% or even 3% for every 4 seconds.
Finished the dive with 134%CNSO2.
My impression is that the Galileo increase in CNSO2, as you approach MOD, is almost exponential.