Stop at 15'-Why that depth?

[jeckyll]

The responses thus far neglect the effect of ascent rate. I was instructed to use 60 ft/min, but the computer I bought tells me to obey 30 ft/min above 60 fsw. This slower rate of ascent may accomplish the same thing as the 1 min stop at 20 ft. The slower ascent may be more important than an additional safety stop in normal recreational diving.

Actually, if you do some research on DAN site, they found that doing a faster ascent with actual stops produced better results than doing the same ascent without stops over the same time. I used to have the link in my signature and will try to find it

I'm _not_ saying that you should have a fast ascent from 20 feet, but to get there you might want to ascend a bit faster and then do some actual stops along the way.

Just my $0.02

Bjorn

 

[herman]

The responses thus far neglect the effect of ascent rate. I was instructed to use 60 ft/min, but the computer I bought tells me to obey 30 ft/min above 60 fsw. This slower rate of ascent may accomplish the same thing as the 1 min stop at 20 ft. The slower ascent may be more important than an additional safety stop in normal recreational diving.

I agree, while safety stops are a real good idea a SLOW accent from the final stop is also very important. Open a 2L soda bottle slowly and another fast. You will quickly see why to go slow -both while diving and opening soda bottles.... I hate mopping the floor.

 

[jeckyll]

Found the link and quote from the Dan article:

What is interesting, and not necessarily intuitive, is that an in-water stop with a relatively rapid ascent rate appears to be more effective at eliminating inert gas than a very slow ascent rate........

Link: http://www.diversalertnetwork.org/news/article.asp?newsid=514

Again, this is about deep stops, not the stuff above 20 feet, but I thought it might be interesting to some based on the topic.

 

[Blackwood]

This calculation can be performed easily in the diver's head while underwater.

... but should be performed before the diver gets wet.

 

[MookieMoose]

I don't mean to be difficult Aquamaniac, but in the interests of other people that may read this thread and take your explanation to heart, there are a few things that I need to correct in your section under 'Why?'

Bubbling does not interfere with gas elimination becasue of diminished driving forces. If a small amount of bubbling has occured, as usually does after most dives, the lung will trap those bubbles, and the gas in the bubbles will move across the alveolar wall and be exhaled the same as if it were disolved in the blood. The problem is that the bubble will occlude the capillary so no other blood can move through it to release it's nitorgen, CO2, etc. OR pick up oxygen until the bubble clears. This is what can hamper off-gassing - the reduction in blood flow through the lung. Bubbles can also cause other problems, like DCS and AGE.

Like everyone else I would recomend a safety stop after every dive, regardless of depth - the fewer bubbles you form, the fewer you have to remove, and the less likely you are to develop DCS or AGE.

Cam

 

[DandyDon]

My first 100 dives (save my AOW dives) were at 60' or a less. I was diving with people who just had the attitude that at that depth, it was almost impossible to get bent, given the gas supply of an 80. The plan was always, dive until you have 700 psi, take a couple of minutes to surface and then figure out how long we needed in the boat before repeating that plan.

You should have received a copy of DAN's Alert Diver this week. Interesting article about a couple of newbies getting bent doing that kind diving - treatment without insurance. I guess bankruptcy was their next option. :11:

If you did not receive that magazine, you need to. Comes with a DAN membership, even if you insure elsewhere.

 

[Tortuga James]

thanks guys, from what I have read so far, the way we approached it in the past, taking an 80 down to 700 psi, which is usually about 40 minutes because we hunt and exert ourselves, is within the NDL's with a margin. Then taking 120 seconds to surface, even without a stop is probably enough time to off gas, since we really aren't diving perfectly square profiles, the sand is at 60', but we are constantly up an over the structure which usually has relief of at least 20'.

The guy that got me to start diving that way is no newbie, he is well over 1000 dives. But like I said in my original post, I am working on refining my planning techniques, not necessarily for those types of inshore dives (not that it wouldn't be a good idea there too), but because of my desire to move further offshore to the deeper ledges and wrecks.

 

[neil]

Ok, none of that is the question, I can't seem to find a suitable explanation (or probably don't need it in practice) for why is 15' is the magic depth for a safety stop?

The original studies on safety stops, done in the 70's by Andy Pilmanis were done at depths of 10 and 20 feet. Other calculations were done by others at 15' and that was shown to have the most benefit.

Suggested reading: "Proceedings of Biomechanics of Safe Ascents Workshop" from the AAUS. There are some good explanations of why safety stops work the way they do.

There's no "magic" to the 15' stop, it's science.

 

[Gilldiver]

The old US Navy tables gave you 60 minutes at 60 feet and most new divers could not make a bottle last that long at 60. So that is where the take it down to 700 psi and go up came from.

Well actualy, it came from the use of J-valves in the 50's to 70's. The J valve worked by shutting down the air at about 500psi (well 300-700 or somewhere in that range.). When your air shut off. you pulled the rod that opened the tank full and you got the last 500 psi to go to the surface.

At this time there were no computers, timers, etc. If you were lucky you had a SPG (they came into use in the early 60's or so) and had a glass tube that ran around the outside of the gage. You looked for a bubble in the glass and that told you how deep you were.

Some had dive watches but the first real good bottom timers were the Princton Techtronics which were mechanical stopwatches activated by a pressure switch. You just had to remember to set them to zero time and wind them.

Now the 10/15 foot stops came in to use in the late 80's early 90's and there is no real reason not to use them. As for the accent rate - it used to be "go as fast as your slowest bubbles." Which would put you around 50/60 feet/minute. To control your accent to 30 without a computer, or for practice. Do this:

If you have an up line.

Start at the bottom and reach up and arm length, grab the line and pull up your other hand slowly till they meet - repete 3 times (you should have moved about 10 feet up the line).

Stop and count slowly to 10.

Repeat etc. etc.

You will find real quick that 30/minute is much slower then you think. Most new divers that I see will be heading up at 60-90+ feet and think it is slow. That is unless they have a computer that beeps at them all the time.

Pete

 

[Bismark]

One other item of importance that was considered in calculating safety stop depth was finding a balance between being shallow enough to create a "useful" pressure gradient for off gassing while at the same time not being too deep to start building up nitrogen again at a rate that would offset the value of the offgassing. Also, I think it is useful to note that the safetystop depth is best considered as a "range" of depths between 10-20 ft to allow for such things as surface swell, etc. If the depth was a fixed 15' we would have a lot of anxious divers who noticed they strayed within a couple of feet of that depth and then they were worried they were going to get bent because of that. New divers in particular who have not completely mastered buoyancy control may find it difficult to hold 15' in less than perfect sea conditions or without a line or the bottom to hold onto or rest on.

just my thoughts here.