Nitrox limits

[Sandie7]

I am new here, and I apologise in advance if this doesn't make any sense.

I heard that, according to the GUE standards, Nitrox limits are more conservative than the "regular ones".

Is this correct, or have I understood wrong ?

Thank you.

 

[decompression]

Kinda, depends on what you mean by conservative. The depth limits are similar but the dive times can be longer due to the use of min deco vs a dive computer or tables. It's been awhile since I've used nitrox tables so I'd have to do a side by side comparison. The max PO of 1.6 could be considered more conservative but is really accepted as best practice amongst divers.

The idea with standardized gases are many but one is to keep an Air equivalent depth at 100', so you'll use the gas at the depth you need to keep an equivalent depth as if that dive was to 100' on air (reduce narcosis). So a dive some might do on nitrox GUE would do on heliox or mix, just to stay within limits.

Does that make sense?


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[Jack Hammer]

I am new here, and I apologise in advance if this doesn't make any sense.

I heard that, according to the GUE standards, Nitrox limits are more conservative than the "regular ones".

Is this correct, or have I understood wrong ?

Thank you.

GUE standard gasses typically try to keep a maximum PO2 of 1.2 for the working portion of the dive and 1.6 for deco. Many other agencies teach a max PO2 of 1.4 for the working portion and 1.6 for deco. GUE's approach does not make as dramatic a difference in maximum usable depth as one might imagine. e.g., 32% has a max depth of 100' vs 111'.

 

[Freezerbrn]

GUE PO2 working is 1.3 for all depths. the gas used is adjusted for depths that keep the working PO2 at or below 1.3 Standard gas is 30/30 or 32% for 0 to 100' Basic idea is to keep the narcosis effect at depth low by adjusting the gas mix for no higher then a 1.3 PPO2 for the deepest part of the dive. YMMV

 

[Cyprian]

GUE PO2 limits are 1.4 ATA during the working phase.

GUE Standards 1.4.1

1.4 General Diving Standards The following standards apply to all GUE diving activities. Additional course-specific standards are listed under the requirements of the specific class.

1. PO2 Limits Individuals should not plan dives which exceed 1.4 ATA/bar during the “working”phase of the dive; during the “resting” phase the planned PO2 may be elevated to1.6 ATA/bar.
​

The older standards mentioned 1.2, but the working phase has been 1.4 for as long as I remember. Earlier standards might have been 1.2

This is Standards version 6.2 where adjusting down to 1.2 as a target is mentioned, but note 1.4 is still the max during working phase.

1.4.1 PO2 Limits

During the working phase of all dives, PO2 should not exceed 1.4ATA. This figure should be adjusted
downwards to reflect diving conditions, and an overall target PO2 of 1.2ATA should be employed. During
the resting phase of the dive, the PO2 may be elevated to 1.6ATA.


The “working phase” of the dive is defined as the portion of the dive where the divers are swimming,
traveling or generally conducting tasks. The “resting phase” is defined as dives during which it is not
reasonably expected that a diver will have to expend any unusual amount of energy, for example during
decompression.
-----

32% and a max depth of 100 doesn't come from the PO2 restriction, but the END. GUE restricts the END to no more than 100 feet.

1.4.2. END Limits END (Equivalent Narcotic Depth) during dives should not exceed 100 feet/30 meters.

 

[WhiteSands]

If you're referring to the MOD, yes it is probably more conservative, but only very slightly. I suspect GUE does that for consistency and practical reasons more than for conservatism.

GUE sets the MOD for Nitrox 32 to 30m. As long as you do not have to reject the gas for being too far off the mark, it's always 30m.

There are a number of advantages to doing this. First, it's easy to remember, and doesn't change regardless of whether your fill is 31.5% or 33.2%. Realistically, few people can remember the exact MOD from dive to dive down to the decimal point. You can't see the sticker on the tank when you're diving, and you probably set your dive comp to 32% and leave it there. I don't know of any dive comps that let you set gas mix down to the decimal point anyway. So setting it at 30m every single dive makes things simple.

Secondly, it is more conservative, and gives some leeway to the analyzers' degree of error. E.g. if your fill is exactly 32%, that makes the PPO2 (0.32 x 4) 1.28. If GUE standards say max working PPO2 of 1.4, that gives you a lot of wriggle room. Fills could be off, instruments could be off, a level of conservatism is good.