Two tech dives per day?

[gsk3]

Over the last few months I've been occasionally asking divers I respect whether they do two dives per day. Most do (with conservative profiles), but a few try to avoid it. Then recently Tobin bravely put his story out there, and one of the commenters mentioned that every time he had been bent it was after a second dive.

So, do you do two dives per day? Assume we're talking dives with less than 30-45 minutes decompression per dive. If you do, how do you make your profile more conservative? Any rules for when you won't do two dives (e.g. you'll do a deeper dive for the first but your second dive has to be less than 130 feet, only after an extended surface interval, etc.)?

 

[PfcAJ]

I *have* done more than one sub 150' dive in a day (like 2 or 3 times ever), however, I don't like to do that. Not worth it, imo.

 

[DevonDiver]

It was pretty standard back when I was living in Thailand - tech liveaboard trips with an early am dive and one late in the afternoon.

 

[Doppler]

I will plan two staged decompression dives in a day, and have executed three within a 24-hour period. However, kept a very, very close watch on NOAA daily CNS limits.

I dive conservatively anyway, but make no special concession on the ascent profile. I do though hydrate extremely well between dives (not sure how much difference that makes but whatever) and watch what I eat.

 

[68raggtop]

My dive buddy and I started changing our approach last year after he had mild skin bends on two occasions. We had been doing two 200'+ dives with 2-3 hour si (once or twice a month) the last couple summers. We now keep the second dive to around 130' or so and minimal deco on the second dive. It seems to have worked, as there have been no other issues. For the record, I'm upper 50's and Al is older still :shocked2:, and I would say that we are both in better physical condition than average for our age.

 

[laurin]

We've been doing two dives a day for a while now, with no issues this far (not that it proves anything!). The rationale for doing so has quite a few considerations:
-the good weather windows are few and far between, and the spots are relatively far (on Baltic scale, at least)
-we plan to get stuff done on the dives (ie. scouting/checking vis, video, stills, preferably multiple takes), generally being able to have a bit of SI to plan things for second dives tends to get better results
-we're not able to do really long BTs anyway, as majority of the team is OC, so one long dive is not an option

We do perceive longer runtimes as more risky, not necessarily from deco perspective (though personally I prefer 2 'more dived' profiles to 1 more experimental). The more important reason is that the weather may change considerably during a multi-hour exposure, and the dives are mostly on extremely busy shipping lanes - we're not so worried about heavy seas (though they are bad too), but if the surface visibility is reduced we're in deep doo-doo because it gets really hard to spot (both visual/radar) the cruise ships, container ships and tankers early enough to give them a warning and coax them off the divers...

This with 3 minimum SI, preferably more, with good hydration and food in between. Not particularly hard to achieve as we're not limited by tide schedules etc, and usually dive 1 team at time, so things proceed at a fairly easy pace, apart from having to wake up early to get it all done, and the blending circus afterwards.

During the last summer our range was mostly 200'-250', with total runtimes limited to about 90min, 100min in a pinch -> 15-25min bottom times. I'm aware there are teams doing similar wreck documentation work as us, who are more content doing one long dive a day (total rt 3hrs or so on RBs), but that's their choice. Given the local (Baltic) conditions I'm happier doing 2 dives.

If this is really worth it is of course up to debate. Virgin, intact U-boat wrecks and such make worth it for me.

 

[SuPrBuGmAn]

Its not really worth it to me to do two deep tech dives in a day. I have done a deeper tech dive and turned around and done a shallower second dive afterwards.




Honestly, nowadays, I'm pretty damned content with just getting a single good dive in and going to find some good food to eat, and have a couple beers.

 

[PfcAJ]

Yanno, my perspective might be off. All my gas dives are in caves, and typically have rather long bottom times. If I were ocean diving, I might do 2x a day, now that I think about it.

 

[Rainer]

Pretty much standard procedure for our group. We're in the ocean, BTs around 30 minutes, deco about the same. SIT of 90-120 minutes. We make no adjustments to our schedules for the second dive. It's worked so far. We do try to bring O2 for the second dive now and take no deco credit for it (i.e. do the same profile as if we only had 50%).

 

[gsk3]

Thanks for the replies so far. Seems like most of the responses so far are from folks who do far bigger dives than I do (which makes sense, given that the folks with the most deco experience are likely to be doing bigger dives). So it brings up some aspects I hadn't considered.

So far, my impression is that most everyone feels it's a little riskier than one dive per day, but that the risk may be small enough to be offset by other factors (convenience, surface risks, limited time, etc.), and that how much riskier seems to vary by person.

One question to come out of this is related to laurin's: Given that you want a given amount of bottom time, should you break it up into one dive or two? Ignoring logistical considerations (which is somewhat silly, since something like very heavy seas can offer risks greater than deco risks), would you rather do two 20 minute 170ft dives or a single 40 minute 170ft dive?