Decompression safety explained really well, great article

[SADS 669]

Ok you can,

I was trying to start a Christmas fight but you've obviously been caught with that old chestnut before.....

 

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[Jason in Enid]

What did you say about my nuts?

 

[Boatlode]

Hi Mike,

As you can see from the 32 table you get a Max of 20 mins but that's at a partial pressure above the recommended 1.4 ( shaded areas,) at that depth if it were me I would go on air and use the US navy table and do my obligation of 6 minutes at 10 ft deco (ensure you have a hang tank or take a 40 cu ft with you) and you then get 25 mins. 30 minutes at the 120 depth earns you an obligation of 14 at 10 which is probably what I would choose but you have to consider weather, and many more factors as you know.

Clearly I would add a few more minutes on as a safety stop too because I am old and fat. Having enough air is the main issue doing stuff like this so make sure you know your surface consumption by the minute and work out your use at every 10 ft on the way upas well as on the bottom.



View attachment 1393233

Old and fat - thats me, Kev. At 115 feet I would probably suck a tank empty before I reached the NDL. The reason I asked is because I'm thinking about taking the Nitrox course before I do the U-352 next summer.

 

[Jason in Enid]

Old and fat - thats me, Kev. At 115 feet I would probably suck a tank empty before I reached the NDL. The reason I asked is because I'm thinking about taking the Nitrox course before I do the U-352 next summer.

Taking a nitrox course (assuming its a good instructor) Will benefit your diving knowledge and open new opportunities (especially for safety) for the rest of your diving days.

As for Nitrox to dive the U-352, that would be a god choice for a 32% mix. You could either use the mix to extend your time at depth, or you could dive an air profile to increase your safety

 

[SADS 669]

Jason, now we are gonna fight.......whereas I agree with every word you say above. Doing a dive like the 352 for someone in Mike's position (only gonna do it once) is the time to really spend as much time as possible there. I don't think a 32 mix does that for him 115 is too near the edge for me. It's more for the 100 ft area in my opinion.

If I were doing it I would buy/ rent a garbage can sized tank ( that way you don't suck it empty on your once in a lifetime dive) go down on a US navy table do a bonking great safety stop and tell Jason to Get Stuffed, he's got funny shaped nuts anyhow.......

All that said I think I would feel pretty safe if I could get some 30% mixed and go on a nitrox computer ( rent a back up from somewhere) or look at the software tables for 30% nitrox.



Ps do a nitrox course you cheapskate dive instructors have to eat too.. Ha ha. Oh and by the way isn't it " we" do the 352 next summer?

 

[Jason in Enid]

Yeah SADS, well your beer is warm and gross!


I'm not a slave to the 1.4 PPO. It's one of those hyper-safety things the dive agencies came up with as a CYA, back in the days when rec nitrox was considered a bad thing. So if my plan has my bottom gas go to 1.41, 1.42, etc I dont care. I'll deco on ~1.6

U-352 is one of my bucket list dives as well.

 

[Salvor6]

Don't forget 115 is the bottom depth of the U-352. You will spend most of your time around the conning tower at 90 feet.

 

[agflit]

I'll agree with Kevin here...a 30% mix..even a 31% would be just about the sweet spot for a MOD of 115...you'd get basically 21 minutes of BT ...with a PPO2 of just around 1.4.

I'd get the biggest tank you can swing, and make it a max limit dive...hang a hi % O2 pony bottle, say...70% or so..at your safety stop and REALLY flush yourself for about 10 minutes. THATS a nice conservative and rational plan without a huge expense... IF..this is a single tank, non repetitive dive day. I think the secret here is carrying enough bottom mix to actually max the BT... and remember...you need to carry enough gas for "contingency planning". Heck, I'd maybe even sling an extra 80 of 21% as a descent bottle and switch to the 31% when ya hit the wreck.

Just my .2 cuft

( don't rule out a small set of doubles if your a doubles diver)

ag

 

[SADS 669]

Pete, for my money you are bang on, so is Jason and the Ag.....looks like my attempt to stir the pot failed like my attempt to impersonate Elvis... Ha ha

 

[Jason in Enid]

I'll agree with Kevin here...a 30% mix..even a 31% would be just about the sweet spot for a MOD of 115...you'd get basically 21 minutes of BT ...with a PPO2 of just around 1.4.

I'd get the biggest tank you can swing, and make it a max limit dive...hang a hi % O2 pony bottle, say...70% or so..at your safety stop and REALLY flush yourself for about 10 minutes. THATS a nice conservative and rational plan without a huge expense... IF..this is a single tank, non repetitive dive day. I think the secret here is carrying enough bottom mix to actually max the BT... and remember...you need to carry enough gas for "contingency planning". Heck, I'd maybe even sling an extra 80 of 21% as a descent bottle and switch to the 31% when ya hit the wreck.

Just my .2 cuft

( don't rule out a small set of doubles if your a doubles diver)

ag

Everything you said is correct. The only problem I have ever run into with travel diving is that some shops only offer set O2 mixes as they keep those banked and don't have time to mix custom for each diver. Thats why I have my own fleet of tanks and O2 PP blending whips.

Damnit I miss diving, I have gone far too long without any serious dives. The tourist reefs are fun, but strapping on a set of doubles and slinging a couple ponies gets you to a lot more interesting places!

 

[SADS 669]

Jason, tomorrow I am doing 200 for 20 mins on the wall. 2 X 80's 1 x 40 with 50/50 ...plus 1,X 40 with 100% plus a 20 with air to ensure I have a third left at the end

 

[Boatlode]

I don't own a tank. Never saw a need to buy one when its just as easy to rent. So I'll have to go with whatever tanks Olympus Dive Center in Morehead City has.

Also, I haven't had tech diving training. So I'm pretty much limited to no-deco.

 

[SADS 669]

We'll figure it all out, don't worry and I know they will have a huge tank for you, if not I'll organise one.....

 

[Boatlode]

Speaking of rental tanks: I thought all steel single tanks were the same size. When I went diving out of Tavernier, I had to tighten the tank strap on my BCD, which was adjusted for the rental tanks here at my local dive shop. Evidently the tanks the dive boat in the Keys rented were smaller diameter, even though they were 3300 psi full, like the tanks here.

 

[ScubaDetector]

Speaking of rental tanks: I thought all steel single tanks were the same size. When I went diving out of Tavernier, I had to tighten the tank strap on my BCD, which was adjusted for the rental tanks here at my local dive shop. Evidently the tanks the dive boat in the Keys rented were smaller diameter, even though they were 3300 psi full, like the tanks here.

Not all tanks are the same size at all. Even aluminum 100's and 120's are bigger around than standard 80's. Nope, you have many different size tanks.

 

[agflit]

Pete, for my money you are bang on, so is Jason and the Ag.....looks like my attempt to stir the pot failed like my attempt to impersonate Elvis... Ha ha

Elvis??...you have NO sideburns and ya can't sing a lick !! geeeeshhhhhhhhhh

 

[seekerGH]

I have a few wrapped 40's that go to 5000! Not that I have to fill them myself!

I like that the options and profiles have evolved, especially with the dynamic response of the dive computers...that stated,
A few notes:
Be careful as the dive tables assume 0 for the altitude/elevation.

What is 0 based on you ask, well, good question.
It is based on baro pressure standard, 1013.25 mbar (101.325 kPa; 29.921 inHg; 760.00 mmHg) at 15 degrees C (60 degrees F) and Rel Humidity of 0%. (Currently, Orlando Airport is at 102.4 kPa at 59 degreesF (100 feet vs 292 feet)
We all agree that diving and decomp is about pressure, but what is your starting point of 0?

Rel humidity in the equation is minor, as that increased percentage has very little effect on altitude.

Temperature is a much different issue, as a few degrees affects the calculations quite a bit. This is why airports give the temperature and current baro reading, as that is what the aircraft need to calculate altitude.

Does 20 feet difference in depth mean a difference of bottom time or comp? That is where the importance comes in.

Diving a high altitude location, or planning on flying out right after a dive? That is where the calcs become important, especially the latter case. Just for weird science, and in reality some applications, you are picked up by a helo from the dive to an offshore platform. What is your 0 for the calcs?

We were diving on some high altitude dams, over 3000 feet, and were transported in and out by helo. Diving depth at the dams was 100 feet below surface. (no problem, right?) What is bottom time and decomp? (oh yes, add with Nitrox, because of the parameters)

Okay, that is a bit over the top, but far more relevant, you have a dive with decomp, and you fly out that day..is the aircraft pressurized or not?, outside air temp and baro pressure, etc....how long do you need to wait after the dive before you fly? (well, I have to admit this was far more relevant before 911 and the security checks at airports ) I guess perhaps TSA has been saving divers, even though they were not aware of that!
For those of you who fly internationally, please consider these issues.

Long story short, a good understanding of the terminology and calculations is golden, especially when pushing the limits, or perhaps better, playing with the myriad of conservative factors within the calcs.

 

[seekerGH]

Found this:

Altitude.org | Diving at Altitude

Even if you do not do high alt dives, it does explain some of the mechanics.

 

[SADS 669]

Generally as a rule any dive ( at sea level) to 33 ft or less it's ok to fly straight away.

If deeper no stop dives are completed then Flying the next day is ok.

If you are " kicking the backside " out of your trip i.e. Lots of repetitive dives on repetitive days then leaving 48 hrs before flying makes sense

I managed 68 dives in two weeks at Truk lagoon many very deep and off gassed for two and a half days before flying.

 

[Jason in Enid]

I managed 68 dives in two weeks at Truk lagoon many very deep and off gassed for two and a half days before flying.

Geeze, that makes me tired just reading it! Most diving I have done was somewhere around 25 in week during a live-aboard. No nitrox, just air, and by the end I didn't even want to dive any more because I was so exhausted (aka sub-clinical DCS).

Back to DCS and altitude, there's no need to worry about a few feet of change in elevation, or changes in baro pressure because they just aren't going to have a measurable impact on dcs or standard off-gassing. I don't even worry about changing dive plans for 1000 foot elevation. Now, you start talking about 5000 or more foot then you should be knowledgeable of changes or hopefully using a computer that adjusts for it.

Mountain range diving can get tricky. You can start out diving at high elevations, and then have to drive over even much higher mountain passes when leaving the site. Those kinds of dives really need some special consideration and planning ahead of time.