Helium content in Trimix

I'm planning a trip with 2 dives in a day with dive depths of approximately 55m and bottom times of 30 minutes for each dive.

Since I have one set of twins to take with me and only have access to air top offs on the boat, I am thinking of having the tanks filled with 20/50.

For the first dive that gives me a max PO2 of 1.3 and an END of 15m.

Assuming I use 2/3 of the gas then top off with air, I get a mixture of 20/16 for the second dive which will give me an END of 42m - not brilliant, but better than nothing.

Neither of these mixtures is covered by the IANTD tables I am familiar with and that I usually use to calculate deco schedules. For this type of dive where I had access to trimix fills or multiple tanks, I would normally use the 20/25 table.

If I use Zplanner or similar to generate my schedules the only significant difference for this dive profile is a lengthening of the deco for the first dive (on 20/50) and slight shortening of the deco for the second dive (on 20/16) as compared to 20/25.

Is there anything else that I should consider when planning this schedule ?

I would particularly welcome any comments about the difference made by the relatively high helium content in the first dive.

Thanks

It sounds like you are using back gas to deco with as well?

In my opinion there is no way to do 2 dives safely with only one set of doubles. I base this on the fact that any initial mix topped off with air will not have an appropriate helium content for the second dive. If you use a stage and some of the back gas for the first dive you could make this work.

If I only had one set of doubles for this dive I would use a AL80 for a stage and breath off of it for the first dive. The back gas and stage mix would be 18/45. I would also reduce the bottom time to about 20-25 minutes.

I would also have an AL40 bottle of 50% for deco for the first dive and a AL40 of 50% and 100% for dive 2.

If you are going to be doing this you need to match your gas to your buddy as well.

omar

Some good reading...

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Common Gas Diving Topics

I definitely agree with Omar here that deco with 50% and O2 would be much preferred and stages would definitely help you do multiple dives with the same backgas.

If you don't do the dives right are they really worth doing?

Don't know where your diving but here is what I do on the great lakes. One set of double 104's for a two dive day. We might do a 160 - 180 (maybe a little deeper) with a 20 - 25 min bottom time. The second dive is on a different wreck or a shallower part of the same wreck in say the 120 -140 range. One set of doubles is enough.
if I was doing longer or deeper dives and could only bring one set of doubles I would pack a stage of bottom mix. If you must blow air on top go high on the He and low on the O2 and don't use 2/3 on a single dive. Depending on the second dive depth you might get an exceptable mix for the second dive.

It's hard to cheat, too much O2 is bad and too little He is bad.

aamalcolm once bubbled...
I would particularly welcome any comments about the difference made by the relatively high helium content in the first dive.

Thanks

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Don't be afraid of the helium. I would fill a set of 104s or 108s to 3600PSI of 18/60. If you breathe it to 1600, you will get a nice 19/32 if you blow it back to 3000 with air. That is way more than enough gas for the dives.

If you use GAP or Decoplan or Decoweenie, you will get a little more deco for the high helium. If you have a good understanding of decompression, you can adjust it a bit. You will feel better after the dive. The downside is that once you dive with higher helium, you will like it so much that there will be no such thing as enough helium.

I do a perticular dive plan quite often.
dive one 300 feet on a wall, dive two 140 feet on a wreck

I use one set of dbl 112's filled with 10/50, then topped off with air for the second dive. I use two staged deco bottles.
1 - 36% 2 - 80%

I also use a helium analyzer, O2 analyzer, two VR3's, Pro planner, and Voyager to plan my second dive once I have topped the tanks up again. this provides true gas analasis and true dive planning

because both my deco bottles are 80 cu ft i usualy have plenty of deco gas for the second dive. if I am going to do a long second dive I will bring additional deco bottles.

Thanks for the replies.

I should have mentioned that the twins (2 x 12L) are for bottom mix and deep stops only. For deco I will have a stage of EAN50 or EAN70.