Why do people add a few minutes to their last deco stop?

[boulderjohn]

I am always dehydrated on three hour dives because I don't use a p-valve

It sounds like you are qualified to have a big-money podcast.

 

[Nick_Radov]

I thought it was normal to have twice as much gas to complete the deco in your bottle, in case your buddy lost his bottle.

Not really. For ocean diving we plan to be able to deco out if one team member loses one deco stage. That may include trading a working deco stage back and forth between team members at alternating stops or extending some stops and doing them on back gas. For bigger dives it just isn't practical to carry twice as much deco gas.

 

[MyBuddy]

Sorry, but I just have to comment on this one statement.

This is simply not the case, but it gets repeated over and over (and over) and carried forward from generation to generation as though it were Holy Writ. I have heard it from any number of divers, instructors, and dive shop owners; but, no matter how many people say it, it's still wrong. You can actually do the math; PV=nRT. This is not a matter of opinion, in other words.

The *pressure* gradient is uniform, about 1ATA per 10m (33FSW) regardless of depth. The pressure differential between 300m and 299m is equal to the pressure differential between 1m and the surface.

It is *volume* that varies more with depth in shallower water. The volume of a given amount of gas in a ballon would be halved by taking it from 0m to 10m, and not halved again until it's taken form 10m to 30m. Or, doubled from 30m to 10m and doubled again from 10m to 0m. For the metrically declined, that's from the surface to 33FSW, from 33FSW to 99FSW, and the reverse.

This distinction matters. The larger change of volume with changes in depth when shallow is what makes pressure-related lung injuries more common in shallower water, makes buoyancy control easier as you go deeper, and so on.

More pertinently, it means any inert gas bubbles grow more during the final stages of ascent than they grow during the initial phases of ascent. This is one reason some people pad their last stops a bit and ascend that last few feet at a speed that would allow a motivated snail to go faster. It's probably the riskiest part of the ascent from a bubble growth point of view.

You could have saved yourself a lot of typing by just inserting the word or two he left out. Which would have then made it true and brought his statement in line with your points about gas volume.

The biggest difference in pressure is found in the last few meters to surface.

The biggest difference in pressure by percentage is found in the last few meters to surface.

Which is what people are taught and know to be true but articulate the concept somewhat clumsily on dive boats and internet forums.

33’ (10m) to 0’ (-1 bar/ATM) is a 50% decrease in pressure / 100% increase in volume.
1cf at 10m would be 2cf at 0’

984.25’ (300m) to 951.44’ (290m) (-1 bar/ATM) is a 3.22% decrease in pressure. 3.33% increase in volume.
1cf at 300m would be 1.033cf at 290m

As @ofg-1 pointed out, Boyle’s Law is much more useful for these calculations than the Idea Gas Law. Not sure what kind of diving you’re doing, but I don’t know or need to know how many moles of a gas I have.

 

[MyBuddy]

My instructor says he'll often add a few minutes to his last deco stop, and I've seen a few people on these forums mention that they do a similar thing.

It doesn't make a lot of sense to me, when I hear that in my brain I hear "I don't think my deco plan / GFHi is conservative enough so I'm going to add extra deco on top of it". Why not just decrease the GFHi instead of arbitrarily adding a few minutes on the end (Note: I'm not talking about doing the slow ascent from last stop to surface, I'm talking specifically about adding extra time to the last stop before starting the ascent to surface).

It makes sense to me. It’s prudent for NDL divers add a 3 to 5 minute safety stop to their dives and they don’t even reach the GF they’ve selected. Tech divers head for the surface when they get back down to the GF they’ve selected. In theory, they have given their GF much more consideration than the average NDL diver, but what’s wrong with adding in a little more conservatism to a deco profile if there are no external pressures (weather, antsy boat captains, aggressive sharks, low on gas, etc) to get out of the water. Whether that be adding a couple of minutes to the last stop or creeping up to the surface from the last stop or both. The set GF might be good for 200 dives but on the 201st dive the tolerances stacked the wrong way and caused a hit. To quote my AN/DP instructor, “Deco sucks,” but it’s better and cheaper than a chamber ride.

 

[lostsheep]

…but a dive computer won't let you update your plan in the middle of a dive and tell you whether you're at risk of hitting minimum gas.

I’m straight up calling BS on this.

I could make my own lengthy argument but I think Mark Powell is more articulate and comprehensive:

Evolution of Dive Planning

'Plan the dive' and 'dive the plan' has long been the mantra employed in all areas of diving. For many recreational divers, dive planning has become a lost art, but technical divers still place a large emphasis on the value of dive planning.

www.shearwater.com

ETA: To answer the OP, my AN/DP instructor quipped, "if you don't want to get bent, do more deco."

 

[Germie]

The biggest difference in pressure by percentage is found in the last few meters to surface.

This is what I meant, but wrote it wrong. Oops

 

[bakodiver391]

Why do we do it? Because it works. I have more than 1500 deco dives, most fo them at Bikini Atoll. My work mate, the late great Edward Maddison, and I decided early on that we were going to do an extra 10 minutes on high O2, more if we were hanging with other divers waiting to finish their deco. Having seen dozens of DCS hits, (ageing customers, repeatative diving), I never felt less than invigorated after a dive. Was it the extra 10 minutes? Maybe, but it sure didn't hurt. There is literally no down side to doing extra deco in those conditions (warm, calm water), and you might see a shark or two. There is also merit in knowing when you COULD surface, according to your gradient factors, should you need to.

 

[elmo]

To quote my AN/DP instructor, “Deco sucks,”

Not always. Last week I stayed an extra 15 min after clearing deco because we were enjoying the company of several thresher sharks.
(I agree with your sentiment by the way)

 

[dmaziuk]

One consequence of having surfacing M-values is that there is depth at which tissues can never on-gas to the point where you can hit your M0's upon surfacing: the no-limit depth. DSC-wise there is no downside to hanging at or above that: you may get cold, bored, OOG, pee yourself, etc., but you won't get bent (as far the model is concerned).

 

[MyBuddy]

Not always. Last week I stayed an extra 15 min after clearing deco because we were enjoying the company of several thresher sharks.
(I agree with your sentiment by the way)

That’s not deco… that’s what people go diving to see! Lol

Where was this?

If I’m lucky, the highlights of my deco are some tiny translucent critters floating by. More often it is brown snot looking water.