Factors affecting deep scuba diving

[ajduplessis]

I am not aware of any FFM rated to 1000ft? I might be wrong. The ocean reef is rated to short of 300ft if my memory serve me correct. These masks have surface vents and comms ports that is the limiting factor. I am by no means an expert on FFM, this is just how understood the design.

 

[archer1960]

The more I have been thinking about this nonsense, the more I wonder why such dives are not done with FFM. Some of the risks like seizures from HPNS or OxTox would require a FFM to survive. And I think WOB issues resulting in CO2 retention could be reduced by a using positive pressure reg in a FFM instead of requiring demand inspiration. If hardhat divers have gone deeper, why not borrow some of the benefits while still staying in the realm of scuba? I'm sure that the gas management of multiple tanks is the reason for not using a FFM. But having used one for many years with a gas switch block and QDs, I think it could be done with no more effort than managing multiple second stages.

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A FFM isn't necessary for this particular use (though there may well be other reasons to use it). Even something as simple as a retention strap on the regulator will keep it in your mouth during a seizure.

 

[kwinter]

But what about the positive pressure aspect to reduce WOB? Gas density is the big factor. Doc Deep was expecting to have some permanent lung damage from the dive. Helium only goes so far in reducing density. At some point you have to push the gas into the lungs. Ajduplessis might be correct about the depth limit on the FFM. But I think it would be more a matter of a limit on the positive pressure second stage than the mask itself.


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[Rhone Man]

Brilliant article, Steve. Thank you.

The other strange thing about these "records" is that they are all phony. We all know that commercial divers and military divers go deeper. But we create an artificial category of "recreational" records so that people can die trying to prove something.

 

[Pinecube]

Thank you for the write-up Steve. It is very informative and easy to understand even for us newbies!

The more I have been thinking about this nonsense, the more I wonder why such dives are not done with FFM.

I believe I see a problem. I've never used a FFM so I'm not sure how quickly the gas cycles through... However, the mask itself will be holding a certain amount of gas will it not? With a normal regulator and mask the airspace in the mask doesn't matter, but in a FFM you're actively breathing that additional airspace. I believe that some of the higher O2 percentage gas would still be in the mask and mix with the next gas you switch to. On "normal" dives this may not be an issue, but if the gas required at 1200' has to be so precise that you need specialized equipment and testing just to check your mix I would not want gas from the previous mix messing up the proportions even a little.

 

[archer1960]

Thank you for the write-up Steve. It is very informative and easy to understand even for us newbies!


I believe I see a problem. I've never used a FFM so I'm not sure how quickly the gas cycles through... However, the mask itself will be holding a certain amount of gas will it not? With a normal regulator and mask the airspace in the mask doesn't matter, but in a FFM you're actively breathing that additional airspace. I believe that some of the higher O2 percentage gas would still be in the mask and mix with the next gas you switch to. On "normal" dives this may not be an issue, but if the gas required at 1200' has to be so precise that you need specialized equipment and testing just to check your mix I would not want gas from the previous mix messing up the proportions even a little.

I'm no expert on this, but I don't think the dead space is a problem for the oxygen concentration, but it might be for CO2 retention.

 

[ajduplessis]

My understanding is that the FFM uses mushroom valves placed in specific areas within the mask to eliminate CO2 retention and "control" airflow behavior.

 

[kwinter]

It all depends on the particular FFM. Some have the gas flow into the visor space and then through mushroom valves into the oronasal cup. These could potentially have some residual O2 from the previous gas, but I think the amount would be inconsequential compared to tidal volume. There should be no CO2 because of the one-way flow through the mushrooms. Other masks incorporate a mouthpiece, either with or without an oronasal cup. And some have completely separate spaces for the mask and breathing portion.

Again, I'm really referring to the positive pressure aspect as a way of reducing WOB.


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[mikefunk]

Please forgive my ignorance, I've been a divemaster for only a couple of years, but I have a question (that will reveal my limited knowledge). I calculate that the ambient pressure at 800' is 3500 psi, near the maximum a scuba cylinder can be filled to. Our regulators are designed to provide gas at the ambient pressure, but if that pressure equals or exceeds the pressure in the tank then would it not be impossible to suck the air out of the tank? At 1200' the pressure would be 5300psi, far beyond what a scuba cylinder could provide. without an air supply to counter the ambient pressure a persons chest cavity would be crushed. Please correct me if I'm wrong in following this line of reasoning. Kindest regards, and keep it safe.

 

[Redshift]

Please forgive my ignorance, I've been a divemaster for only a couple of years, but I have a question (that will reveal my limited knowledge). I calculate that the ambient pressure at 800' is 3500 psi, near the maximum a scuba cylinder can be filled to. Our regulators are designed to provide gas at the ambient pressure, but if that pressure equals or exceeds the pressure in the tank then would it not be impossible to suck the air out of the tank? At 1200' the pressure would be 5300psi, far beyond what a scuba cylinder could provide. without an air supply to counter the ambient pressure a persons chest cavity would be crushed. Please correct me if I'm wrong in following this line of reasoning. Kindest regards, and keep it safe.

Doing the maths in metric, 1200' are 366m. This means 37.6 bar. Much lower than a cylinder fill. And google tells me that this is approximately 547 psi.