Recreational limits...and why?

[Christi]

Here is one that I am sure will spark up some solid discussion.

This question was posted on another board not too long ago.

"Why is recreational diving limited to 130 feet is it because of NDL or nitrogen narcosis?"

I was SHOCKED to see an instructor respond like this:

"You're breathing 100% oxygen. Deeper, and you risk convulsions. Cheers y'all.

20% X 5 atmospheres = 100% O2.

DSAO

XXXXX (name deleted to safe her some dignity)

She never came back to correct her response, after myself and several others gave her the benefit of the doubt of being a typo. This answer truly frightens me coming from an instructor particularly and I PRAY that she doesn't teach her students this. It is clear that she has no understanding of the very basics and pp.

Two questions here:
1. What is the real answer for the original question?
2. What is wrong with the instructors response?

Go for it

 

[jonnythan]

At 130 feet, I guess you're breathing a partial pressure of oxygen equivalent to breathing 100% at the surface.. but even a ppO2 of 1 is way lower than even the most conservative limits of 1.4 and 1.2. Assuming an O2 percentage of 21, 130 feet give you a ppO2 of between 1.0 and 1.1. You'll need to reach 190 feet before you violate the 1.4 ppO2 limit.

The reasons, IMO, are because of NDL's.

 

[Paco]

I believe the reason for the limit is nitrogen narcosis and what jonnythan said seems right on to me.

 

[paolov]

"Why is recreational diving limited to 130 feet is it because of NDL or nitrogen narcosis?"

check the link

http://www.mtsinai.org/pulmonary/books/scuba/sectionb.htm

and look for the heading "WHY IS 130 FEET THE MAXIMUM DEPTH IN RECREATIONAL DIVING?"

TRUE STORY :

a group of relatively new scuba divers, four divers plus a DM

Anchor descent line as guide, 1st buddy group proceeded to go down with dive breifing to wait for all to assemble before going around.

2nd buddy group, one buddy had trouble equalizing, cause the 2nd buddy group to descend much slower than 1st buddy group.

after around two minutes of delay , 2nd buddy group reaches depth of 1st buddy group, 130ft. DM checks on all divers, then signals ALL UP (ABORT) after making the check.

upon surfacing, buddy groups ask why he signalled abort and not go around.

DM asks divers to check their air. two of the divers are less than 500psi on air with one nearly 200 psi left. Total dive time 11 minutes.

awareness of new divers ....

 

[Jason B]

To be honest, I was told the same thing, many, many years ago when I got my DM. Once I got nitrox certified (through a different instructor) I realized that that was incorrect.

Jason

 

[Mako Mark]

Jonythan and Paco are right, it is theoretically possible to get CNS Oxygen toxicity at appO2 of 1.05 (40 m) but you would have to stay there between 250 and 300 minutes (about five hours) which is pretty unrealistic for most recreational divers.

So..

No decompression limits become unreasonably short for most recreational divers at this depth, and Narcosis becomes a very significant factor.. These are the reasons for the 40 m limit, although there is nothing magical about this number.

 

[FatCat]

Yup,

Narcosis and NDL/Bottom time. However, it's also wise to separate NDL-diving from decodiving from an educational point of view. The amount of knowledge a new diver has to assimilate is quite enough already.

You might argue that from an advanced level on it could be beneficial to teach decodiving. Still, from my own experience I must say that once you pass the NDL's regular scuba gear as prescribed by the rec-agencies is not enough. You need at least two separate regs (and of course two valves) to go decodiving in cold water.

I'm quite happy with the 40m limit. I'll refer anyone who wants to go beyond that to a good tec-instructor.

 

[Christi]

check the link

http://www.mtsinai.org/pulmonary/books/scuba/sectionb.htm

and look for the heading "WHY IS 130 FEET THE MAXIMUM DEPTH IN RECREATIONAL DIVING?"

Paolov,

I know the answers and the theory behind them. I was posing this question as was posed on another board, and to allow people to think about why...

As Mark said, 130 ft is not a magical number...making sure people understand the pp and WHY this depth is recommended is really more of my point.

Your link explains that 130 is more or less the recommended depth limit, bt it doesn't explain WHY.

 

[Keysdrifter454]

To be honest, I was told the same thing, many, many years ago when I got my DM. Once I got nitrox certified (through a different instructor) I realized that that was incorrect.

Jason

I've heard it was an arbitrary limit set due to bottom time and regulator performance, but I don't know if anyone really knows.

 

[Rick Murchison]

The author in the article cited is correct that the 130' limit was a Navy limit, and had to do with useful work on air. But the Navy wasn't so averse to deco stops in those days as to let a 10 minute NDL be the determinant, so I think the story I read in the '60's is probably more accurate - it goes something like this...
The Navy's DMO (Diving Medical Officer) was charged with writing the SOP (Standard Operating Procedures) for SCUBA. He asked one of the divers what the maximum useful working depth was for the equipment the divers were using, and the diver said "anywhere below about 130' breathin' that reg's like tryin' to suck-start a Harley."
The DMO said "sounds good to me" and wrote 130' into the SOP as the max working depth for SCUBA.
As it turns out, 130' is a very convenient depth with reference to normal air supply, no-stop bottom time and narcosis for the typical recreational diver, so we sort of "backed into" a good number for other reasons than the ones we use now to get to the same limit.
Rick