Diving using min-deco - questions about depth averaging, repetitive dives

[elgoog]

Hi all -

The last few dives I had my Tec2G in gauge mode and was using the the min-deco rule I learned in Fundies to plan and dive the dives. I like this way better than relying on the computer for ascents and NDL information and am going to use this method for future dives as much as I can. I had some questions which I couldn't find very good answers/information for.

1. For average depth, do you start depth averaging once you've completed your descent or count the descent in the average?
In terms of calculating remaining bottom time, this does not make a difference for me because I'm limited by gas rather than MDL. My interest here is in best practices as well as getting a more accurate SAC calculation. I make note of my tank pressure and dive time (I guess technically this is bottom time) right when I start the ascent and use those values in the SAC formula instead of the total dive time and tank pressure when I'm back on the boat/shore.
Right now, I'm using the Tec2G to do the averaging for me (it has a resettable average depth and stop watch) but am also doing this in my head to see how close I get to it.

2. Is there a recommended number of repetitive dives that should/can be done using this rule?
This table specifically mentions up to 4 dives a day but uses an ascent longer than the "normal" min-deco ascent - DIR-diver.com - Using the min deco table
For normal dive days, this is not a factor as there's never a case where we do more than 3 dives per day. This would become something to consider on a trip to, say, Bonaire since there's unlimited shore dives per day or even on a liveaboard. Even with a 2hr SI between dives, it seems like it would be possible to do 5-6 dives a day with 50-60ft average depth and 45min bottom times. Admittedly, this is pushing the MDLs quite a bit.

I realized that there may be different variants of the min-deco rule, so here's the one I'm using (all values are for dives on EAN32)
Max bottom time = 130 - average depth
Ascent at 30fpm to 50% of average depth, then 1min stops every 10ft to the surface
For SI of 1-2hr, bottom time is 50% of max bottom time calculated above
For SI >2hr, bottom time is 75% of max bottom time

Thanks in advance,
elgoog

 

[SanDiegoSidemount]

If you are diving a relatively square profile (say up/down an anchor line) then you can exclude descent from the average depth. If you are following bottom contours down on a longer descent, you can average that in. In practice, either way should not change your depth average much, since your descent will typically represent a pretty small percentage of the overall dive time. If you want to be more conservative (safe), leave out the descent, and round your averaging downward.

I have done up to 6 repetitive dives per day using Min Deco, and felt great. However, you should be more conservative on your later dives by extending your shallow stops longer, and ideally adding 10 minutes of O2 at 20 feet if you are trained to do so.

Remember that there is nothing hard-and-fast about Min Deco, or any decompression strategy for that matter . There's NEVER a case where you're going to be 100% safe from DCS, whether you are following a computer, dive tables, or Min Deco.

What Min Deco really does is start you on the path to thinking about your OWN personal deco profiles and limits. The more you dive, and evaluate how you feel afterward (post-dive fatigue is a sign of asymptomatic DCS) the more you'll learn what kinds of profiles make YOU feel the best during a day of repetitive diving.

 

[rjack321]

Even with a 2hr SI between dives, it seems like it would be possible to do 5-6 dives a day with 50-60ft average depth and 45min bottom times. Admittedly, this is pushing the MDLs quite a bit.

On 32% that's not pushing MDLs at all, although personally I would be making the last dives slightly shallower and stretching out the shallow stops.

Remember not to sawtooth the profiles too drastically (>20ft), depth averaging starts to break down if you have a big long dip back to depth at the end of the dive. On these 5 dive Bonaire days its not an issue, just poke around <20ft for longer and longer if you can. And use EAN32

 

[elgoog]

If you are diving a relatively square profile (say up/down an anchor line) then you can exclude descent from the average depth.

This scenario was exactly what I was thinking of when I thought about excluding descent.

However, you should be more conservative on your later dives by extending your shallow stops longer.

just poke around <20ft for longer and longer if you can.

Good ideas - I'll add this to my repetitive dives.

ideally adding 10 minutes of O2 at 20 feet if you are trained to do so.

Not an option, at the moment.

What Min Deco really does is start you on the path to thinking about your OWN personal deco profiles and limits. The more you dive, and evaluate how you feel afterward (post-dive fatigue is a sign of asymptomatic DCS) the more you'll learn what kinds of profiles make YOU feel the best during a day of repetitive diving.

This is something I'm trying to be more and more mindful of as I get more experience.

On 32% that's not pushing MDLs at all

I always thought "pushing MDL/NDL" meant getting within 5min or so of the number. So, a repetitive dive to 60ft would have an MDL of 52min and doing a 45min dive would be close-ish to the number. This obviously varies from person to person but what's generally considered pushing the limit? Maxing out and starting ascent with 0 additional time?

Follow up question based on some of the feedback before -
I understand the reason to increase shallow stops for repetitive dives. Do you also hold longer stops at 20ft and/or 10ft for the deeper dive in the beginning of a dive day? For me, deep is anything in the 75-100ft range done on a single HP100 at home or AL80 in warmer waters.

 

[rjack321]

I always thought "pushing MDL/NDL" meant getting within 5min or so of the number. So, a repetitive dive to 60ft would have an MDL of 52min and doing a 45min dive would be close-ish to the number. This obviously varies from person to person but what's generally considered pushing the limit? Maxing out and starting ascent with 0 additional time?

How did you arrive at 52 mins?
The NDL for 1 dive at 60ft is >90mins (depending on your model, e.g. using VPM+2 you could do ~103 mins @ 60ft on EAN32 as a no-stop dive).
So your formula of 130-60 = MDL of 70 is already massively conservative, your second layer (75%) for repetitive dive conservatism is just a double layer.

Hence I don't think even your fifth 45min @ 50-60ft dive of the day using 90-120mins SIs is even remotely aggressive on 32%. I would totally do that and I'm a deco weenie.

Do you also hold longer stops at 20ft and/or 10ft for the deeper dive in the beginning of a dive day? For me, deep is anything in the 75-100ft range done on a single HP100 at home or AL80 in warmer waters.

minimum is just minimum, if you have the gas and you aren't cold, bored, getting beat up by the current, have something to look at on the reef etc. why wouldn't you extend the stops?
e.g. 1,1,4,5,5 instead of 1,1,1,1,1

 

[elgoog]

How did you arrive at 52 mins?

For 2nd dive to 60ft after 2hr SI, 75% of (130-60) = 52.5min

So your formula of 130-60 = MDL of 70 is already massively conservative, your second layer (75%) for repetitive dive conservatism is just a double layer.

Yeah, I'm getting that now. I ran some sample profiles through Multideco with VPM+2 as well as GF 35/75 (high conservatism) and was getting longer bottom times with similar-ish stop durations.
For now, I'm still going to use the formula just because it's simpler but my perception of what's considered "pushing the MDL" has definitely changed.

minimum is just minimum, if you have the gas and you aren't cold, bored, getting beat up by the current, have something to look at on the reef etc. why wouldn't you extend the stops? e.g. 1,1,4,5,5 instead of 1,1,1,1,1

I have no issues hanging out for some extra time. Mostly, I was trying to get an idea of which stops were being extended. I was thinking of more time at 20 and 10ft but was curious to see what the common practice was and if extending 30ft was also done. Of course, if there's a reason to be at a depth in that range to look at something purty, I would do that.

 

[rjack321]

I have no issues hanging out for some extra time. Mostly, I was trying to get an idea of which stops were being extended. I was thinking of more time at 20 and 10ft but was curious to see what the common practice was and if extending 30ft was also done. Of course, if there's a reason to be at a depth in that range to look at something purty, I would do that.

You can extend the 30ft stop but its basically "neutral" for the diving you are talking about. No ongassing but not as much offgassing as 20 and 10ft (and the surface where you offgas the most). You can hang out indefinitely at 30ft on EAN32. Although if I did a 90ft dive for say 30mins (technically a no stop dive), which then had 20mins at 30ft I wouldn't stress about trying to add time at 20 and 10. Just do a 2 min ascent and be done with it.

Obviously if you have a problem and for min gas calculations you use "minimum deco". But if the problem is severe, to hell with the min deco just get to the surface, you can fix bent - drowning is permanent.