Hold that line boys! How precise should you hold depth?

[Gilldiver]

If you can't hold your depth at 100ft, you can't hold it at 10ft. ..

Actually I have seen many divers with dry suits that can hold depth well below 30 feet but in the 30-0 foot range have a lot of trouble keeping the bubble expansion under control. Lets face it, in a dry suit at 100-150 feet you can bounce up or down 10-15 feet without a problem on lung volume alone and never put into or dump any air from a dry suit. But above 30 feet just take a lot more control as the bubble and buoyancy changes happen a lot quicker.

But people are going over and over the precision of holding a stop, that +- thing when on many dives the accuracy of the stop is not so good for any number of reasons. On a anchor line deco from some of the larger boats you can have any number of divers trying to hit that magic 30/20/10 foot stop. This is where the jon line can be real helpful as you can string out a number of divers at the same depth or if you are in a deco traffic jam, just pull the stops lower by 5 or more feet.

 

[IDS_Bill]

Maybe, but it doesn't matter.

If you can't hold your depth at 100ft, you can't hold it at 10ft.

I understood what he was saying, but in *real* terms a depth is a depth. If you have to count on how deep the stop is in order to determine if your buoyancy control was good enough then something's wrong.... right?

R..

 

[DA Aquamaster]

Actually I have seen many divers with dry suits that can hold depth well below 30 feet but in the 30-0 foot range have a lot of trouble keeping the bubble expansion under control. Lets face it, in a dry suit at 100-150 feet you can bounce up or down 10-15 feet without a problem on lung volume alone and never put into or dump any air from a dry suit. But above 30 feet just take a lot more control as the bubble and buoyancy changes happen a lot quicker.

But people are going over and over the precision of holding a stop, that +- thing when on many dives the accuracy of the stop is not so good for any number of reasons. On a anchor line deco from some of the larger boats you can have any number of divers trying to hit that magic 30/20/10 foot stop. This is where the jon line can be real helpful as you can string out a number of divers at the same depth or if you are in a deco traffic jam, just pull the stops lower by 5 or more feet.

That's pretty much the point of having tight tolerances in training. If you cannot manage your buoyancy plus or minus a foot under good conditions with your configuration, you have no business doing deco diving in that configuration. Period.

I shoot for plus minus a foot on deco stops in the ocean with a dry suit. Even in poor conditions by shooting for +/- 1 ft you are far more likely to keep the total error under 2-3 feet. If you shoot for +/- 5 ft, you will be lucky to hold that and lord only knows how bad the extreme spread will be.

More importantly, if a dry suited diver is seeking perfection, they will do a much better job of managing the bubble in the first place and will have all ready ensured they are properly weighted, properly insulated and properly balance the gas between the suit and the wing. Once you get it all mastered, then you go deco dive - not before. It's a lot more productive than just excusing poor depth management at depthjs less than 30 ft.

Normally in the ocean, I plan on my last stop being at 20'. It lessens the impact of the ups and downs caused by swells, and it keeps you well beneath the ladder, props, rudders, etc. Once you finish you can do a nice slow gradual ascent to the ladder.

----

Someone mentioned O2. The PO2 is 1.6 at 21 ft so you can go -1 ft on the stop and still be within the 1.6 max. If you go to 27' the PO2 is now 1.8 and at 33' it is 2.0, so things get real high in a hurry. I would not want to be holding 25' on O2 after an hour at a PO2 of 1.2 to 1.4 and another half hour or so at PO2's near 1.6 as spiking it to close to 1.8 is just a really bad idea. In my opinion, +/- 5 ft is just no where near good enough.

 

[IDS_Bill]

Normally in the ocean, I plan on my last stop being at 20'. It lessens the impact of the ups and downs caused by swells, and it keeps you well beneath the ladder, props, rudders, etc. Once you finish you can do a nice slow gradual ascent to the ladder.

This is a very good practice in rough seas.

 

[The Chairman]

FWIW, today was a great day at Hudson Grotto. Both dives went off without a hitch and we found the 150' hole! Yay! (it's almost directly behind the 20 ft platform!)

The first dive was shallower than we had actually planned, but we kept to our schedule... arrived at the first stop a full minute ahead. Communication was good, even for us noobs, and we played around on the lines between all of the vehicles and even saw the Grotto Monster sign! :homealone:

The second dive was a bit more free lance as to navigation. We hit the wall just east of the twenty foot platform and went CCW until we found the hole! We did our max depths (140') with some water still under us and turned the dive. Our bubble traveling up the side caused a bit of silting, but nothing major! We traveled away from the side and at 70 ft I shot a bag. At 30 feet we saw that it was stuck under the twenty foot platform! During our twenty foot hang I moved the bag to the edge of the platform and then as we were ascending for our last stop (15 ft) I let the bag deploy the rest of the way! w00t! We then took pics with the TDI flag we took underwater with us!

Now, as I was sitting at my last stop (in between pics), I had a notion. If you are on pure O₂, as we were, what is the real difference between 10, 15 or 20 ft? The partial pressure of N₂ dissolved in your blood stream is the same at any of these depths, and so is partial pressure of N₂ in the gas you are breathing (0.000000 ata). Theoretically, there is no variation in tension (except through off gassing), so why risk the bubbles by going above 20 ft? Someone illuminate this one for me! What am I missing here?

 

[mikemill]

Someone illuminate this one for me! What am I missing here?

Oxygen toxicity?

 

[TSandM]

You're right, Pete -- you've maximized the gradient by going onto 100%, and you keep bubbles small by staying at 20 feet. The only advantage to going to 10 feet is that you're further from oxygen toxicity and you reduce the total O2 exposure. I was taught by two agencies that if surface conditions are at all dicey, just stay at 20 feet for the O2 deco portion of the dive, and do a very slow, controlled ascent from there at the end.

BTW, you're using almost enough exclamation points to make me think Papa Bear has borrowed your computer . . .

 

[The Chairman]

So shoot me!!! I'm psyched!!!

BTW, to top off an already wonderful day, we sautéed over to my favorite restaurant in all the world: Magerion Mykonos in Tarpon Springs. I had Cerrides Hellenika (Shrimp, Greek style), while JMW had the Gyro platter and Denise had the Greek Chicken. This was topped off with the best Baklava that I have ever had! I gave them a quick tour of the sponge docks and they bought some souvenirs. What a perfect day. Oh, I almost forgot!!! It took us a while to convince JMW that Gyro is pronounced "Hero".

!!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!! !!!

 

[Meng_Tze]

...just stay at 20 feet for the O2 deco portion of the dive, and do a very slow, controlled ascent from there at the end.

. .

I always make my last stop 20ft as well.

 

[BabyDuck]

pete, i think it's mainly the 'train as tight as possible' thing, like you parallel park over & over with cones so when you have to do it for your license it's no big deal.

when the heat's on, buoyancy control shouldn't take up much bandwidth. plus, precision can be fun itself!