Repetitive Dives w/ Trimix

[Shcubasteve]

Hi All,

What’s your SOP for making repetitive dives after using trimix? It was explained to me that after a dive with TMx, all subsequent dives should include helium- Ie: no air/EANx dives after trimix.

Here is the rational for the question: Im planning to dive the Jupiter Helldiver this Friday- profile should be 185' for 20-30 on 20/25. The charter I will use consistently offers three tankers even on tec trips- not everyone on board will make a tec dive for the first drop. My practice is to sit out the second dive to get a nice long SI and dive the third drop of the day. Last dive of the day will be around 80-90'.

I’ve done the above tec dive on air (I know, I know..) and single tank EANx for subsequent dives. Now that Ill be diving TMx, I am debating on using whatever I have left in my LP95s for the second dive, or blending a single tank with some lean trimix, say 32/15.

What would be your preferred option for the example given above?

thanks for the input
steve

 

[SuPrBuGmAn]

Hi All,

What’s your SOP for making repetitive dives after using trimix? It was explained to me that after a dive with TMx, all subsequent dives should include helium- Ie: no air/EANx dives after trimix.

I'm trying to get my head around the reasoning for this... when it was explained to you, did they give any sort of legitimate reason?



If you're keeping your next dive in the 80-90' range, I wouldn't bother with helium at all.

A long SI is probably a great idea.

 

[Superlyte27]

Got a Helium analyzer and a transfill whip? If so, just top off your first tanks with air, analyze and go again.

 

[Shcubasteve]

Ill say that I am not a decompression expert, just a diver looking to better understand repetitive dives with trimix, so I maybe wrong- here is how I understand it: Just like nitrogen offgasses while we are on the surface, so too does He. When a repetitive dive is made, He is still present and offgassing until a diver drops under. High oxygen and nitrogen partial pressures draw out the disolved He creating an increased risk of DCS.


Avoiding air/eanx was covered in the course and was on the test.

From the tec trimix diver manual: Page 63:
"Helium content should be appropriate for the depth. Many decompression experts recommend that you do not follow a trimix dive with a repetitive dive that uses air or enriched air as the bottom gas. Although the concerns are theoretical and not universally agreed upon, this is a conservative approach."

That manual is from 2005 and maybe outdated since decompression technology seem to be accelerating quickly nowadays. Hoping to get others opinions

 

[Doppler]

Ill say that I am not a decompression expert, just a diver looking to better understand repetitive dives with trimix, so I maybe wrong- here is how I understand it: Just like nitrogen offgasses while we are on the surface, so too does He. When a repetitive dive is made, He is still present and offgassing until a diver drops under. High oxygen and nitrogen partial pressures draw out the disolved He creating an increased risk of DCS.


Avoiding air/eanx was covered in the course and was on the test.

From the tec trimix diver manual: Page 63:
"Helium content should be appropriate for the depth. Many decompression experts recommend that you do not follow a trimix dive with a repetitive dive that uses air or enriched air as the bottom gas. Although the concerns are theoretical and not universally agreed upon, this is a conservative approach."

That manual is from 2005 and maybe outdated since decompression technology seem to be accelerating quickly nowadays. Hoping to get others opinions

Interesting theory and the first I've heard of it -- at least for recreational exposures (non-sat tech dives included).

One point worth thinking about is that the transit times for helium are pretty rapid and during your SI, my bets would be that with a proviso that your original deco was good, you will have off-gassed the helium and essentially be starting again.

Also, not too clear on the logic behind the theory you quote (sound like someone's interpretation of isobaric counter diffusion) since for around 20 years of trimix diving, I always follow spells breathing mix with time breathing nitrox... it's called decompression.

 

[SuPrBuGmAn]

Ill say that I am not a decompression expert, just a diver looking to better understand repetitive dives with trimix, so I maybe wrong- here is how I understand it: Just like nitrogen offgasses while we are on the surface, so too does He. When a repetitive dive is made, He is still present and offgassing until a diver drops under. High oxygen and nitrogen partial pressures draw out the disolved He creating an increased risk of DCS.

Not that it needs to be said again, Doppler is kinda the man for this kinda stuff, but He will offgas quicker than eanx. So if you're OK with doing repetitive eanx dives, I don't see why you wouldn't do a repetitive eanx dive after an He dive.

Keep your ascent slow and rock out.

 

[Rhone Man]

Good thread. Added to the things I didn't know that I didn't know.

 

[LiteHedded]

still a lot of voodoo out there with regards to mixed gas diving...

 

[Slamfire]

What’s your SOP for making repetitive dives after using trimix? It was explained to me that after a dive with TMx, all subsequent dives should include helium- Ie: no air/EANx dives after trimix.

Here is the rational for the question: Im planning to dive the Jupiter Helldiver this Friday- profile should be 185' for 20-30 on 20/25. The charter I will use consistently offers three tankers even on tec trips- not everyone on board will make a tec dive for the first drop. My practice is to sit out the second dive to get a nice long SI and dive the third drop of the day. Last dive of the day will be around 80-90'.

I’ve done the above tec dive on air (I know, I know..) and single tank EANx for subsequent dives. Now that Ill be diving TMx, I am debating on using whatever I have left in my LP95s for the second dive, or blending a single tank with some lean trimix, say 32/15.

What would be your preferred option for the example given above?

thanks for the input
steve

Like many participating here, that's the first I hear about this kind of logic. I've done eanx and air dives after tmx dives and never had a problem or an itch or a skin bends. I've actually done a very similar set of dives to the ones that you're asking about. In another thread, I posted a dive I did on 19/24 to a max depth of 182' for 25 mins bottom time. What I did not post was that 2 hrs and 6 mins later I was starting a 2nd dive on EAN 32% to a max depth of 83' with a total run time of 55 mins. These are ocean dives on the Canadian west coast with water temps ranging from 44 F to 50 F (dives started at the mouth of a cold water creek). By comparison, similar FL warm water dives should be more conservative just based on temp. As I mentioned before, there were no symptoms and I felt fine.

I'm attaching the profiles for both dives below. As you can see, the predator did not even consider that the 2nd one went beyond NDL.

Click thumbnails to see full size images

 

[Dsix36]

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.