Repetitive Dives w/ Trimix

[wedivebc]

I have done similar profiles to those described plenty of times. I have never heard of any hazards of multiple HE dives or any issues of following a trimix dive with a EAN dive.

 

[DuboisP]

trimix, ean40 and oxygen on morning, between 60 and 80m
double refilled with air, so triox as 30/20 or 30/15
triox and oxygen on afternoon, 40m max

 

[SWFTD]

This is an interesting question, mostly from the standpoint of how such an idea came to be.​

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Perhaps, some years ago, one possibility is that is isobaric counterdiffusion was recognized at some level, but without really integrating that with an accurate understanding of the offgassing characteristics of helium. Possibly one could then come up with the idea that EANx dives should not follow Trimix dives.

Isobaric counterdiffusion (resulting in inner ear bubbling) can indeed be precipitated by helium-nitrogen gas switching, usually at too deep a depth. ICD occurs when a tissue such as the inner ear is highly saturated with one gas, and then switching to a different breathing mix with a high concentration of a different gas, results in diffusion of that second gas into the confined space before the first has adequately made its way out. The result is a transient supersaturation of that compartment in terms of total gas concentration, which of course generates bubbling and hence ICD – DCS symptoms which can include incapacitating vertigo among others.​

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Switches both from high helium to high nitrogen and high nitrogen to high helium have been implicated. (This seemingly contradictory position is legitimate in that comes from the facts that whereas Helium does offgasses more quickly than nitrogen, Nitrogen is much more soluble than Helium. High solubility allows a substance to rapidly cross membranes and other tissues which can result in local transient spikes of dissolved gas even though the overall offgassing rate of Nitrogen is slower. Depending on which factor dominates, ICD may theoretically be precipitated by switching in either direction.) Depth is a major exacerbating factor.​

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The current theory, however, is exemplified by Weinke’s recommendation that switching from Trimix to Nitrox should be reserved for depths above 100 FSW. ​

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A rationale for this is that given the rate of offgassing of Helium, the total dissolved gas concentration (PTC) in the inner ear will subside a shallower depths using normal decompression profiles, such that a major increase in nitrogen associated with a switch, say, to EAN50 at 70 feet is no longer likely to generate ICD. ​

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This puts the perspective on the statement quoted by the OP. Before the FIRST dive is even completed, the ICD risk is already down to the point that using EANx is safe. By the time the next dive begins the risk is that much more remote.​

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In looking for other issues regarding using EAN or air on the second dive, the question is, does using a high nitrogen mix as opposed to Trimix pose any increased risk of nitrogen driven DCS, other than ICD? Well, there will be SOME residual Helium in the body compartments. Nitrogen loading on the second dive will be in proportion to the nitrogen concentrations and without regards to the presence of the residual helium. Hence the total dissolved gas pressure in the body compartments during the second dive will end up being higher than if that residual helium did not exist. However, if the first dive had been a nitrogen dive (air), the situation would be much worse. N2 off gasses more slowly than He and the residual total inert gas pressure at the start of the second dive will therefore be higher with a prior nitrogen dive than with a prior trimix dive. ​

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A prior dive with either will lengthen the decompression requirements of the second dive but to a lesser extent if the first dive is Trimix. However the conclusion that can be drawn is that beyond the lengthened decompression requirements associated with having made a first dive, there do not appear, based on current theory, to be any special risks associated with having the first dive be trimix and the second an O2/N2 mix.​

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BTW, although one individual’s experiences don’t settle issues like this, I have in fact done 175-200 FSW dives with bottom times around 40 minutes (on trimix), followed by EAN30, 120 FSW, 45 min BT dives on several instances with no problem. ​

 

[ajduplessis]

This weekend I will be doing a 300' dive and our bailout plan (yes, we use rebreathers) consists of 15/55, 21/35, 32%, 50%, and O2. This has been our standard gas usage for this dive for quite some time. I see absolutely no reason to be concerned with using Eanx after Trimix. As Doppler stated, helium on and off gasses much faster than nitrogen and should be out of your system with a long SI.

Correction: Nitrogen on gasses faster than He. He only off gasses faster than N2.

 

[SWFTD]

Correction: Nitrogen on gasses faster than He. He only off gasses faster than N2.

Unless I missed something (?) I don't believe I said anything about Helium ONgassing rates. The only mention of ongassing rates was in regards to ICD, and Nitrogen in particular. To clarify, (and I actually think we are on the same page here), the reason ICD is an issue with deep switches from Tri to high N2 mixes is that the high solubility of Nitrogen (as mentioned in my post), allows the absorption of N2 before He is adequately offgassed, which causes a temporarily higher (total gas) PTC in that anatomical compartment, generating the risk of bubbling. This is also the reason the computer algorythms call for lengthening the last stop or two before the switch to EAN.

PS did a great dive Sunday to 300 FSW on 14/55 and switched successfully to high N2 by waiting to 70FSW (EAN50); didn't bother with the 30/30 travel gas that dive. Also, I'd like to find a way to meet other OC tech divers diving the East Coast of Florida; I live in Naples and only meet them occasionally by chance on boats.

 

[ajduplessis]

My post refers to comment by Dsix36. Please read my post, all will be revealed.

I agree, we are on the same page.

 

[NAM001]

THIS IS MY FIRST POST IN THIS SECTION. The idea of trimix after trimix does make sence to me if I look at the general rule of deep dive first and progressivily shallower dives after that. Shallower dives in that ppn2's decrease on each repetitive dive. if the first dive was on lets say 10/50 as far as NITROGEN goes would be that same as nitrox 60 at 180' giving a ppN2 of 2.6 or same as 75 ft on air. So if the next dive was to 120 ft with 32% nitrox the ppn2 would be 4.0 or an EAD of 100 ft. So the second dive is a deeper one than the first dive from the aspect of nitrogen. The only way to do these dives would to be continue to reduce nitrogen levels with a helium mix or limit the dive to less than say 60' on 32% and shallower on air. . OK i am ready to be beat up. The math may be off a bit.

 

[PfcAJ]

THIS IS MY FIRST POST IN THIS SECTION. The idea of trimix after trimix does make sence to me if I look at the general rule of deep dive first and progressivily shallower dives after that. Shallower dives in that ppn2's decrease on each repetitive dive. if the first dive was on lets say 10/50 as far as NITROGEN goes would be that same as nitrox 60 at 180' giving a ppN2 of 2.6 or same as 75 ft on air. So if the next dive was to 120 ft with 32% nitrox the ppn2 would be 4.0 or an EAD of 100 ft. So the second dive is a deeper one than the first dive from the aspect of nitrogen. The only way to do these dives would to be continue to reduce nitrogen levels with a helium mix or limit the dive to less than say 60' on 32% and shallower on air. . OK i am ready to be beat up. The math may be off a bit.

But nitrogen isn't the controlling factor, total dissolved inert gas is. Helium and Nitrogen combined.

 

[NAM001]

My limited knowledge of helium is that it off gasses quickly and there for becomes moot long before the residual nitrogen does. A short surface intreval and helium is gone leaving the result of a regular air dive to carry on to the repeditive dive. Perhaps the halftime of helium is much longer than I think.