Limitations of TDI helitrox?

[LFMarm]

So now the agency which still teaches deep air to 180ft is concerned about divers getting in trouble by using too much helium? I'm not sure I follow the logic on that one, but then those guys were never known for logical thinking.

I look at this limitation on Helium similarly to the rule of sixth for intro to cave divers – you limit what / how much gas you can use to prevent situations that may be dangerous for that training level. Deep air is very bad practice.

 

[crofrog]

I look at this limitation on Helium similarly to the rule of sixth for intro to cave divers – you limit what / how much of your gas you can use to prevent situations that may be dangerous for that training level. Deep air is very bad practice.

Rules and procedures are designed to maintain operations within a safe and stable zone, incorporating sufficient safeguards to prevent the system from approaching or crossing the marginal boundary where risk becomes unacceptable.

Limiting the amount of gas you can use for inbound penetration insulates us from risk. Limiting the amount of He in your mix increases risk. There is in effect no downside to having more He in your mix increases risk.

If a rule exist that actively removes safety from a system it’s a dumb rule.

 

[inquis]

Limiting the amount of He in your mix increases risk

As does forcing the O2 content to be 21%. There are definitely some for whom a depth limit printed on a plastic card means zip, so I can believe the certifying agency might impose these limits in an attempt to help a newly minted deco diver make the right choice.

Sure, it's a "nanny" mindset, but do you think there would be fewer incidents if all the brand new deco divers had easy access to 15/55? More helium lowers risk, right up until human nature enters the picture.

 

[crofrog]

As does forcing the O2 content to be 21%. There are definitely some for whom a depth limit printed on a plastic card means zip, so I can believe the certifying agency might impose these limits in an attempt to help a newly minted deco diver make the right choice.

Sure, it's a "nanny" mindset, but do you think there would be fewer incidents if all the brand new deco divers had easy access to 15/55? More helium lowers risk, right up until human nature enters the picture.

I don’t follow your line of thought.

Limiting minimum o2 makes more sense than max He content. Once you’re in the 15% fO2 range you’re into hypoxic mixes and those have different training requirements and risk profiles.

For a 150ft dive 21/79 would have less risk from gas density and no narcosis (depending on if o2 is narcotic) than 21/35 or 21/20 and based on current research the increase in decompression time prescribed by the currently algorithms is likely incorrect.

The only meaningful reason to not dive Heliox is cost.

 

[NothingClever]

Reliance upon others is dangerous and leads to poorer skills.

Opinion, not fact.

The world is replete with examples of highly skilled individuals coming together to accomplish more as a team.

I don’t sense anybody’s asserting that team diving is the only way and that anybody who doesn’t choose it is _________ (insert negative adjective).

But it does seem like you keep asserting that people who choose team diving are negligent or unskilled. Perhaps that’s your experience but I don’t think projecting it on every other corner of the world holds any water.

It’s a favorite song you like to play but it doesn’t seem anybody’s tuning in.

Back to TDI and helitrox.

 

[inquis]

For a 150ft dive 21/79 would have less risk from gas density and no narcosis

Agreed, but you're assuming they stay above 150 ft as directed by the words on their cert card. By limiting He, there is more than those words telling them to behave.

In addition, is the risk reduction of 21/79 vs 21/35 actually meaningful for those who do stay above their cert limit? With an N2-only END of <nothing> vs 69 ft (or less)?

Again, this is just my speculation about how an agency might view things: good enough for some and a possible deterrent to others. From *their* view, there doesn't seem to be a downside.

 

[Tracy]

The only meaningful reason to not dive Heliox is cost.

And that annoying HPNS thing.
And logistics of moving that volume of bottled gases.

 

[crofrog]

Agreed, but you're assuming they stay above 150 ft as directed by the words on their cert card. By limiting He, there is more than those words telling them to behave.

In addition, is the risk reduction of 21/79 vs 21/35 actually meaningful for those who do stay above their cert limit? With an N2-only END of <nothing> vs 69 ft (or less)?

Again, this is just my speculation about how an agency might view things: good enough for some and a possible deterrent to others. From *their* view, there doesn't seem to be a downside.

By limiting helium, you're increasing their risk not telling them to behave.

If you instead focus on limiting the minimum oxygen content in a gas mix, the pO2 of the gas naturally creates a safeguard. Setting a maximum pO2 limit supported by research—such as 1.2 —establishes a meaningful margin of safety. For example, with a maximum pO2 of 1.2, the MOD for a 21/x mix becomes 155 feet (47 meters), ensuring a buffer against oxygen toxicity at depth.

This approach achieves the goal of increasing safety by creating rules that expand the margin between the operating state and the marginal boundary, particularly regarding maximum oxygen exposure and the risk of O2 toxicity.

Furthermore, there is anecdotal evidence to suggest that divers have a much greater respect for pO2 limits than they do for other factors like END, gas density/EADD. If someone is willing to disregard the pO2 limits, it's almost certain they will ignore END and gas density/EADD limits as well.

By crafting rules that reinforce adherence to scientifically backed, high-priority safety limits (like pO2), you reduce risks and foster safer diving behaviors. Arbitrary limitations on helium, by contrast, reduce the safety margin for those adhering to the rules and fail to meaningfully deter those who disregard them.

 

[rjack321]

And that annoying HPNS thing.
And logistics of moving that volume of bottled gases.

Well except you're double digit ATAs away from HPNS at normoxic O2 concentrations.

 

[inquis]

Arbitrary limitations on helium, by contrast, reduce the safety margin for those adhering to the rules and fail to meaningfully deter those who disregard them.

Preaching to the choir, man. It's the agencies that need convincing. Not something I'd hold my breath for, LOL.