Oxygen partial (ppO2) setting in dive comp

[BLD_THRST]

Guys,

Sorry if this has been asked before. Jus got myself the Aladin Prime and there is a manual setting for ppO2. Default setting is at 1.4 bar.

If i understand it correctly, oxygen concentration when diving air is 21%. Based on ppO2 of 1.4 bar, that means that i would be at a max depth of 6.7ata or equivalent to about 67m?

If i am diving recreationally, and at a max depth of 40m, does it not mean that i should set the ppO2 to 1.05bar i.e. 5 ata times by 21%?

I am confused... default setting is more conservative?

 

[Rick Inman]

I don't know that computer, but I believe that usually the PPO2 setting is a MAXIMUM setting for setting when the warnings and sirens and flashing lights start telling you that your gonna die.

IOW, for the computer to calculate MOD.

Recreational agencies use 1.4, so it is the default on must computers.

Is that what you're asking?

 

[DevonDiver]

A ppO2 of 1.4 is generally the maximum allowed partial pressure of oxygen to ensure relative safety from the risk of CNS oxygen toxicity.

Obviously, the maximum operating depth (MOD) for Air is considerably deeper than the maximum depth allowed for recreational diving. This maximum depth (40m) is recommended for several reasons.

Firstly, at this (and deeper) depths, you will encounter very short No-Decompression/Stop limits....and a small distraction could easily lead you into an emergency decompression scenario.

Secondly, your air consumption deeper than this depth warrants greater capacity cylinders...and possibly some considerable redundancy in the event of regulator/valve malfunction (your air will free-flow to nothing very quick at 5ata and below).

Thirdly, below 40m the effects of nitrogen narcosis are much more severe and much more likely.

If diving air, there is no need to alter the ppO2 settings of your computer.

However, if you were diving nitrox (and coming much closer to your MOD)...and wished to add a greater safety margin to the risk of oxygen toxicity, then you could decrease the ppO2.

You should never increase the ppO2 setting. The only time that a higher ppO2 is used is for accelerated decompression, using oxygen rich mixes....but, this calls for proper technical training (and even technical divers do not use over 1.4 ppO2 during the bottom time of their dives).

 

[DevonDiver]

If i understand it correctly, oxygen concentration when diving air is 21%. Based on ppO2 of 1.4 bar, that means that i would be at a max depth of 6.7ata or equivalent to about 67m?

Nitpicking....

ata = atmosphere Absolute.

6.7ata = 57m

 

[LiteHedded]

it's just gonna beep at you when you hit a 1.4
no worries. you can set that warning to whatever ppo2 you're comfortable with

 

[Cave Diver]

I am confused... default setting is more conservative?

Rick pretty well sums up my thoughts as well. Although I'm not familiar with that computer either, the ppo2 setting is generally to keep you from exceeding the maximum operating depth of a given gas.

A ppo2 of 1.4 is generally the maximum for the working portion of a dive and 1.6 for resting deco. Some people advocate slightly more conservative ppo2 settings for working portions of dive, depending on conditions.

 

[MXGratefulDiver]

If you turn off the alarms (another manual setting) it won't matter what you set it to ...

... Bob (Grateful Diver)

 

[BLD_THRST]

thanks guys.... makes it all clearer...

thanks devondiver... forgot about sea level 1 ata... heheh

 

[bleeb]

Guys,

Sorry if this has been asked before. Jus got myself the Aladin Prime and there is a manual setting for ppO2. Default setting is at 1.4 bar.

If i understand it correctly, oxygen concentration when diving air is 21%. Based on ppO2 of 1.4 bar, that means that i would be at a max depth of 6.7ata or equivalent to about 67m?

If i am diving recreationally, and at a max depth of 40m, does it not mean that i should set the ppO2 to 1.05bar i.e. 5 ata times by 21%?

I am confused... default setting is more conservative?

Lots of good answers regarding the technical reasoning. I'd just like to emphasize one point, which may help the OP or other readers understand why the function is in the computer and how the manufacturer intended it to be used.

As DevonDiver and others explained, this setting is normally used primarily by divers using Nitrox (breathing gas with a larger fraction of oxygen and a lower fraction of nitrogen than air). Using Nitrox involves certain additional risks such as the Maximum Operating Depth, so a principle feature of computers claiming Nitrox support is a MOD-based alarm. For divers simply wishing to set a max depth alarm, most computers that have a separate and easier to set up function.

As an aside, the OP may want to consider a Nitrox course. Even if you don't plan on using it often, the benefits can be significant when you do (such as increased bottom time, if that's how you want to use it), and it's pretty good at increasing your understanding of how breathing gasses work and how they affect your diving. They're also generally not very expensive.

 

[Doc Harry]

If i understand it correctly, oxygen concentration when diving air is 21%. Based on ppO2 of 1.4 bar, that means that i would be at a max depth of 6.7ata or equivalent to about 67m?

Yes, you got it, for a max pO2=1.4.

If i am diving recreationally, and at a max depth of 40m, does it not mean that i should set the ppO2 to 1.05bar i.e. 5 ata times by 21%?

No.

As other have mentioned, oxygen is toxic at higher partial pressures. There have been numerous studies regarding oxygen toxicity. The results of those studies demonstrate that there is significant individual variation regarding tolerance to higher oxygen pressures before a seizure occurs. Some generalities can be made based on these studies.

Very few people have a seizure when pO2<1.4, so that is the generally accepted maximum allowable pO2 that you will see quoted. Keep in mind that keeping your pO2<1.4 doesn't absolutely guarantee that you won't have a seizure. There are no absolutes in diving. If you are diving is very cold water or exerting yourself or have a hangover, etc., then it would be wise to be more conservative by keeping pO2<1.2 for instance.

Tech divers will often accept pO2 up to 1.6 so that they can use rich mixtures (e.g., 100% oxygen) to shorten decompression time. When breathing gasses where pO2=1.6 they must limit their activity and ensure they stay warm to lower their risk of seizure at that high pO2.

When the pO2>1.6 the risk of seizure is very high, although it is clear that many tech divers will expose themselves to these higher partial pressures. I think the record for an air dive is over 400 feet, which means pO2>13 if my math is correct. So clearly people can tolerate very high pO2 and survive but not without great risk.

So back to your question.

You computer will calculate a maximum operating depth (MOD) for your gas based upon your two inputs: (1) the type of gas that you are breathing (e.g., air, Nitrox 32%, etc.) and (2) your desired maximum pO2.

Your computer will then alarm when you reach that MOD (or pO2), depending on the specifics of your computer and whether or not you have the alarm turned on.

A Nitrox course will make all of this clear to you.

Read your computer's instructions carefully.

If you don't fully understand the effects of pO2 then I suggest you shouldn't be diving deep. Dive within the limits of your training. What you don't know can kill you.