CF17 Low ppO2 Warning rationale

[Goose_66]

Dear All,
during these holidays I have been studying my just received OSTC2 and something came up strange to me.

In the docs (I also do RTFM) at the CF 17 I get:
If the value is below 19 a warning simbol si displayed permantly during surface and dive mode. Do not dive with a OSTC configured that way!

Now it happens that in addition to being a diver I also fly and by doing that I am used to hipobaric environment. Currently regulation allow single pilot flying without oxygen in a range up to 12500 feet (roughly 4000 mt) in FAA world or 10000 feet (roughly 3000 mt) in the military and JAR world.

At 3000m, the standard barometric pressure is 72 kPa (537 mmHg). This means that there is 71% of the oxygen available at sea level. See Altitude.org|Altitude air pressure calculator

This also means that @15% O2 normobaric we should see no impairment in performance (either pilot or diver ....).

This is also shown in Oxygen levels where 100% Haemoglobin saturation is achieved with oxygen down to 12 kPa (roughly equivalent to 12% oxigen at sea level).

Now while I have no problem living with the limit set at 19% when I dive with my trimix 12/50 I will have a red ppO2 display in the initial few meters of the dive I feel that the permanent error display could be set somewhere lower than 19% like 15%.

Any toughts on this?

Thanks

Fabio

[heinrichsweikamp]

I think 19% is the common recommendation for CCR diving but maybe someone has newer informations? 12% is most likely suitable without any physical load...

Regards,
Matthias

[Goose_66]

Mathias,
my bad, it is .19 (or .15 or .12 ATA) that we are talking about.
I would reccomand setting the permanent error if the value in CF17 is below 15.
100% haemoglobine saturation becomes doubtful below 0.15 ATA.

Again not a definite requirement (especially if exposing manifacturer to legal issues) but a better description of phisiological limits.

Cheers

Fabio

[Anonymous User]

From what I learned it's 0,16bar: http://en.wikipedia.org/wiki/Breathing_gas

Jo

[heinrichsweikamp]

Hmm, I've heard 0.19bar is the "official" limit for CCR diving. At .19bar (And a problem with your O2 supply) a few breathes later you are below a suitable level.

Regards,
Matthias

[Goose_66]

Not going for a discussion for the sake of it but what you select in CF17 will depend on what kind of a dive you do. What I am discussing here is the opportunity of displaying a permanent error if CF17 is less than 19.

According to Wiki:
The minimum safe partial pressure of oxygen in a breathing gas is commonly held to be 16 kPa (0.16 bar). Below this partial pressure the diver may be at risk of unconsciousness and death due to hypoxia.

Now, I have flown an unpressurized aircraft (with my wife and kids on board) for 3.5 hours from Norfolk Virginia to Orlando Florida (you have guessed it was a Sea World, and Discovery Cove swim with dolphins trip) at 11.000 feet with no supplemental oxygen (ppO2 was 0.145 ATA). I can tell you I was not impaired and I was able to function as pilot (did not kill me or my family nor broke any of the few thousand rules (:D applying to an IFR flight in North America).

In the military I did receive training in altitude chamber and I was noticing impairment at about 18000 feet (unable to perform simple dexterity tasks and Math) with night vision impaired as low as 10-11 000 feet.

So I would set the permanent warning display if CF17 is less than 15 (or 16 to make Wiki happy) and leave the default value at 19. So who knows what he does can set it to 15 (16) without the permanent warning. I will keep diving with hypoxic mixtures even if my OSTC will display RED ppO2 in the few initial meters of my dive

Keep in mind that if you dive with 15% oxy at 3 Mt you have already .195 ATA ppO2 if you dive with 12% oxy at 6 mt you already have .195 ATA ppO2 ....

For CCR I believe is a different issue since you get the ppO2 from a sensor and the amount of gas is limited and you are depleting oxygen form the rebreathed mix: having a bit of early warning would be good so you set CF17 to 19 or maybe 21 without forcing those who can dive with .16 ATA (OC) to have a red error display or a permanent warning.

Cheers

Fabio

[Iglooz]

Just my 2 cents ...

The limit of 0.16b is also the minimum allowed by the French law if you want to dive in a French structure.

Article A322-91 :
La valeur de la pression partielle minimale d'oxygène inspiré par le plongeur est limitée à 160 hPa (0,16 bar). La valeur de la pression partielle maximale d'oxygène inspiré par le plongeur en immersion est limitée à 1 600 hPa (1,6 bar). La profondeur maximale d'utilisation du mélange est calculée en fonction de la pression partielle d'oxygène maximale admissible définie ci-dessus.

[sailor]

Hello,

i can tell you from own experience that it is not a good idea to rely on a loop po2 of 0,16.
Maybe there's a difference inbetween air and water like in a pressure chamber.
You will also become narked at a higher pressure in air compared to water.
I really like the limit of 0,19.
If you want another limit for yourself just adjust CF17 accordingly.

Reiner

[Goose_66]

Reiner,
that was exactly my point.

First I am a OC only guy so I have no loop but a tank and fixed amount of oxy in percentage varying in PP according to depth.

Second if I adjust CF17 to 0.16 ATA I get a permanent error (according to manual) in fact I get a red CF17 in surface mode on startup screen.

What I am debating here is the opportunity of lowering the permanent error threshold from 19 to 16 for CF17 but leaving the default value of CF17 to 19.

My understanding is that there is quite a difference between open circuit and closed circuit in Oxygen lower limit due to the different mechanics (depletion of finite volume vs one time usage of gas).

Cheers

[Iglooz]

Matthias has just pushed before the end of 2012 a patch in this sens :

Set CF17 (ppo2_warning_low) low level to 0.16bar

Before :
CF17 has to stay in [0.19b...0.21b] with 0.19b as default value

Now (2.59 beta) :
CF17 has to stay in [0.16b...0.21b] with 0.19b still the default value

[Goose_66]

Iglooz,

thanks, I did not notice: I am too much of a newbie of OSTC and I am not up to date with the betas.

I believe that should do it great!
Waiting for the next stable!

Cheers

Fabio

[Iglooz]

Goose_66 Wrote:
-------------------------------------------------------
> thanks, I did not notice: I am too much of a newbie of OSTC and I am not up to date with the betas.

This patch is just 3 days old and your original post is certainly its origin B)
And anyway, the 2.59 is not yet available for download.

[DiverM]

For CCR divers a minimum of 0.19Bar[sub]ppO2[/sub] makes perfect sense. There is an inherent delay in the measurement with chemical oxygen sensors, so levels can change quite a bit before the measurement becomes correct and stable.
So you really want a good safety margin when using a CCR, both with low and especially high ppO2 levels.

On open circuit you can get away with a smaller margin. But remember: The closer you walk to the edge, chances of falling off rise exponentially....