How much risk was I at for DCS?

[eelnoraa]

If I have to guess, you did a dive with max depth of 26.4m, total dive time of 40 min. You didn't spend entire 40 min at 26.4m. The key here is time average depth, but you will probably never find out. Next time pay attention to how much time you spend at max depth. You should have a depth and time device with you for every dive.

 

[Peter Guy]

I would expect a DM who would do the things this one did to lie.

B.S.

J.L., please provide your experience which would lead you to so expect.

 

[Kevrumbo]

A Quick Contingency "Rock Bottom" Calculation and Gas Plan Estimate for Open Water. . .

For a single 11 litre tank (AL80), a total of 11 litres/bar metric tank rating and a volume Surface Consumption Rate (SCR) of 22 litres/min -same as a pressure SCR of 2 bar/min*ATA (divide 22 litres/min by 11 litres/bar)- using an example NDL air dive to 30m (4 ATA) depth in Open Water.

Emergency Reserve/Rock Bottom pressure calculation, from 30 meters with one minute stops every 3 meters to the surface,
-->Just "tally the ATA's":

4.0
3.7
3.4
3.1
2.8
2.5
2.2
1.9
1.6
1.3

Sum Total: 26.5

Multiplied by 2 bar/min*ATA equals 53 bar Rock Bottom absolute reading remaining on your SPG. --this also happens to be the pressure in bar needed for one person in an emergency contingency to reach the surface with the above minimum decompression ascent profile.

So ideally for a two person buddy team, multiply 53 by 2 which is 106 bar for both to reach the surface (sharing in a buddy Out-ot-Gas contingency).

But realistically, for two experienced divers stressed: 106 bar plus 30% of 106 bar equals 138 bar Rock Bottom SPG reading.

For two novice divers stressed: 106 bar plus 100% of 106 bar equals 212 bar (!!!)
--->obviously then, two novice divers on single 11L tanks should not be diving to 30m for any significant length of time. . .

This illustrates why you should not do "Trust Me" dives to these depths beyond 18m for the beginning/novice Open Water Diver!

 

[boulderjohn]

I would expect a DM who would do the things this one did to lie.

I have no idea why you would say something like this.

Something like this is happening somewhere in the world several thousand times per day. If you go to Cozumel, you will find about 100 DMs per day leading dives in which they follow a multilevel profile using their computers to guide them. Every one of those dives will be off the charts on the PADI RDP. In many and perhaps most cases, the divers in the group are also using computers, but quite a few divers are trusting the DM to lead them. I have had several cases in which students told me that their experienced diving friends told them that once they got out of class, they could forget all that dive planning stuff because "in the real world" the DM plans the dive and leads them safely. For many people, that is indeed the world of diving. I am pretty darn sure that those DMs are indeed leading a multilevel dive within the computer's NDL. Why would they do otherwise? Why would they feel compelled to do otherwise and lie about it?

That does not make it right for the diver to follow the DM blindly. If you are going to use the RDP, then you simply should not be doing dives like that. If you are going to follow a computer-based multi-level dive, then you need to have your own computer. You should not be relying on someone else.

 

[chrpai]

That does not make it right for the diver to follow the DM blindly. If you are going to use the RDP, then you simply should not be doing dives like that. If you are going to follow a computer-based multi-level dive, then you need to have your own computer. You should not be relying on someone else.

I agree with you. That said, I've flown the DM's profiles before. My wife got cancer, I retired and gave every piece of gear that I owned away except my Suunto Mosquito. A few years later she was in remission and we went to Jamaica together. I really, really wasn't planning on diving down there. I know this to be true because I didn't even bother to take my Suunto with me.

Then I ran into a George. He was a DM that I had dove with many times over previous trips. I knew him and he knew me. He just looked at me like "No diving mon? Ya's gots to be ****tin me mon." We would discuss the profile before each dive while I agreed to stay at or above him at all times and let him worry about the time limits.

Ya, I know it's not the right thing to do and I'm not saying it's ok for a newbie to do it. But I don't feel I really put myself in any considerable danger. Oh and a fun week of diving it was.

 

[Rogersea]

I have made that dive, I do not believe that you were at much risk, I base this on the fact that DM's have a fairly standard route and they have to look out for their own safety as well. I do not believe your DM lied to you about your profile. but without having your own computer it would be difficult to determine what your Nitrogen load was. If you are going to continue diving you really need to have and understand your own computer.... Just curious why would you ask 4 months after the fact???

 

[Quero]

I would expect a DM who would do the things this one did to lie.

Oh good grief, Jim! It's a huge leap from the DM leading a trust-me dive to the DM lying about the profile.

The divemaster took me down to 26.4 metres depth; twice my certification. At the end of the dive, I determined that my pressure group on the PADI RDP (30m 40 min) was way past the NDL (the nearest thing we did to a decompression stop was 3 min at 5 m.)
Was I at much risk of DCS? His computer said we were in a safe pressure group (he claimed.)

To the OP: It's unlikely that you were at an elevated risk of DCS on this particular dive.

However, you very definitely should be at least participating in the planning of any dive you will be doing and monitoring its execution as you do it. In other words, during the dive briefing, if the DM outlines the plan of a dive he intends to lead, ask questions and make suggestions so that the dive is within the parameters you find reasonable given your experience and training. During the dive, keep track of depths and time as they relate to the plan you have developed.

 

[TSandM]

To the OP: Tables assume what's called a "square profile". That means they assume you spend the entire dive at the maximum depth, which can be true for dives like wreck dives, where you go down to the wreck, wander around, and come back up. But most terrain-based dives have a very different profile. Instead of going down and staying there, you reach your maximum depth and slowly make your way shallower along the structure. If you think about it, you aren't loading nitrogen the whole time the way you would be at your maximum depth. This is the typical "tourist" dive, but tables weren't designed to handle this kind of profile.

When I first got certified, I fretted a great deal over the fact that the terrain-based shore diving I was doing didn't fit in the tables at all. For most of my dives, the tables said I should have been dead Slowly, I learned more about decompression theory, and how the tables and computers work, and realized that of all approaches to diving, tables are the most conservative. Computers run calculations continuously, and don't "charge" you at the bottom time rate, if you have moved shallower.

That said, I agree with everybody above who has said that you abdicated responsibility for your dive to the DM, and you shouldn't have done that. You should have known the planned maximum depth and dive time before you splashed (that's the absolute MINIMUM you should know about any dive) and if it didn't fit your tables, you should have asked questions about why the DM thought that profile was a safe one to dive. Do not become a passive diver! You may have had an excellent and knowledgeable and prudent DM, but they are not all like that. Peruse the Near Misses threads, and you will see a good many incidents that occurred because somebody did a "trust me" dive behind a DM who either wasn't trustworthy (hadn't planned well enough) or who wasn't available when something went pear-shaped.

Good lesson. I'm glad you came here and asked. I think next time, you'll be far better prepared to ask the right questions.

 

[supercritical]

I do realize that the dive was multilevel, but the fact that I'm not trained in multilevel diving worried me. What worried me most however, was the depth.
@Rogersea: I'm asking now because I recently registered on ScubaBoard, and this question burning, I decided to post it.
@Quero and TSandM: I'll take a note of that.

For those of you requesting the data I had on the dive:
Starting pressure: 210 bar
Ending pressure: 100 bar
Bottom time: 40 minutes
Maximum depth: 26.4 metres
Safety stop: 5 metres for 3 minutes
Surface temperature: 26 C
Underwater temperature: 30 C
Conditions: Freshwater, Lake.
Beginning pressure group: A
Ending pressure group: ?

 

[chrpai]

I do realize that the dive was multilevel, but the fact that I'm not trained in multilevel diving worried me.

I suppose you could be trained if you really wanted to: Multilevel Diver

The reality is very, very few are. It's good stuff to know but in the end computers are a very effective tool and not that hard to use. You don't see people planning and executing multilevel dives on tables until you get into technical diving. Within rec limits, there really isn't much point vs using a computer.