Algorithm question (Perdix AI vs DSAT)

[hammet]

So are you saying you dive purely square profiles and push NDL to it’s limit? If so, I guess it could be considered significant. However, I don’t think most would notice any real impact for typical recreational, multilevel dive profiles.

The difference in some nitrox mix NDLs is often only about 20 percent over air.
Yet we choose that over air to eke out a few more mins of bottom time.

 

[bamafan]

Years ago I had an Oceanic Pro the bought a Predator and used the Oceanic as a backup. The Oceanics definitely are more liberal and don't go into deco as quick. However once into deco they give unreasonable stop times. I have finished many dives where I had 5 or 10 minutes of deco on my Predator but the Oceanic would show an hour or so of deco. Being more conservative is not a bad thing just like deco isn't something to be scared of either if you have the training and equipment.

 

[Joneill]

The difference in some nitrox mix NDLs is often only about 20 percent over air.
Yet we choose that over air to eke out a few more mins of bottom time.

The most common recreational mix I see in tropical diving destinations is EANx 32 and that has way more than a 20% benefit on NDL over Air:

Further, the real benefit to using Nitrox for NDL diving is not at depths below 80 feet where the absolute difference is only 9-10 minutes (and MOD is limited to 111 feet, anyhow) - it's in the intermediate depths (50-70) and on repetitive dives where the difference in NDL times can be VERY significant!

 

[gopbroek]

I have used Oceanic VEO’s for the last 5 years (about 600 dives) and recently purchased a Perdix. Prior to purchase I was concerned about the differences between the DSAT and GF algorithms and @scubadada recommended setting a Perdix to the low conservative setting of 45/95 to approximate the DSAT model. When I fell into a good deal on a Perdix, I figured I would try it and if I didn’t like it I could always sell it.

I test dove it in July in Cozumel and did a pretty mellow 2 tank per day schedule for 5 days with a VEO 3.0 and the Perdix strapped to my arm. Normally I do a little more aggressive schedule of adding an afternoon two tank trip every other day but during this trip there were no other folks for the afternoons. Next time I hope to get in more dives per day but this particular trip represents the average vacation diver trip of a week with 10 tanks over 5 dive days. I am using lots of nitrox (I watched a good friend do 11 chamber rides in May, I am using more Nitrox than I used to).

Initially the DSAT algorithm seemed definitely more liberal but as the repetitive dives were added it leveled out. I do not have enough data but it seems like the GF 45/95 reacts more liberal to repetitive diving and hopefully on my trips coming up over the next few months I can compare them on a more aggressive dive schedule.

Attached are screen shots of the 10 dives from Oceanic’s ‘Ocean log’ with text boxes showing the comparative logged NDL values for the two computers taken from Ocean log for the VEO and Shearwater Desktop for the Perdix. This is probably a more real world comparison as opposed to a dive planner. Personally I feel it is important to know how a computer handles repetitive diving.

FYI, yes I go up and down a lot while hunting Lion fish and no way will I give up my Perdix

 

[scubadada]

I have used Oceanic VEO’s for the last 5 years (about 600 dives) and recently purchased a Perdix. Prior to purchase I was concerned about the differences between the DSAT and GF algorithms and @scubadada recommended setting a Perdix to the low conservative setting of 45/95 to approximate the DSAT model. When I fell into a good deal on a Perdix, I figured I would try it and if I didn’t like it I could always sell it.

I test dove it in July in Cozumel and did a pretty mellow 2 tank per day schedule for 5 days with a VEO 3.0 and the Perdix strapped to my arm. Normally I do a little more aggressive schedule of adding an afternoon two tank trip every other day but during this trip there were no other folks for the afternoons. Next time I hope to get in more dives per day but this particular trip represents the average vacation diver trip of a week with 10 tanks over 5 dive days. I am using lots of nitrox (I watched a good friend do 11 chamber rides in May, I am using more Nitrox than I used to).

Initially the DSAT algorithm seemed definitely more liberal but as the repetitive dives were added it leveled out. I do not have enough data but it seems like the GF 45/95 reacts more liberal to repetitive diving and hopefully on my trips coming up over the next few months I can compare them on a more aggressive dive schedule.

Attached are screen shots of the 10 dives from Oceanic’s ‘Ocean log’ with text boxes showing the comparative logged NDL values for the two computers taken from Ocean log for the VEO and Shearwater Desktop for the Perdix. This is probably a more real world comparison as opposed to a dive planner. Personally I feel it is important to know how a computer handles repetitive diving.

FYI, yes I go up and down a lot while hunting Lion fish and no way will I give up my Perdix

Repetitive dive info is very important and rarely available. I have not systematically collected repetitive dive data while diving DSAT and Buhlmann, however, the equalization over time seems evident to me too. Buhlmann seems more liberal on repetitive dives.This seems obvious to me when I choose the "correct" GF to correspond to my dives and then notice that I have considerably more NDL time on my Nitek Q than I would have expected.

 

[tbone1004]

@hammet what I was about to say is what was confirmed by @gopbroek in his chart.
What you are seeing is a more liberal algorithm for the first dive, and one that favors NDL's at shallow depths *odds are you aren't able to hit NDL on dives shallower than 60ft.
If you look at his chart, pay attention to the comparison on the second dive at deeper depths when the Perdix is showing a LOT more NDL time based on the slow tissue calculations vs. DSAT.

I personally don't agree with diving the Perdix in low conservatism, but that's just me and plenty are doing it all the time.

 

[scubadada]

@hammet what I was about to say is what was confirmed by @gopbroek in his chart.
What you are seeing is a more liberal algorithm for the first dive, and one that favors NDL's at shallow depths *odds are you aren't able to hit NDL on dives shallower than 60ft.
If you look at his chart, pay attention to the comparison on the second dive at deeper depths when the Perdix is showing a LOT more NDL time based on the slow tissue calculations vs. DSAT.

I personally don't agree with diving the Perdix in low conservatism, but that's just me and plenty are doing it all the time.

Anecdotally only, some of the NDLs at relatively shallow depths, say 60 feet, at GF hi of 95, frighten me as being very long. This is only my impression after about 220 dives with an Oceanic VT3 primary and a Dive Rite Nitik Q backup.

 

[tbone1004]

Anecdotally only, some of the NDLs at relatively shallow depths, say 60 feet, at GF hi of 95, frighten me as being very long. This is only my impression after about 220 dives with an Oceanic VT3 primary and a Dive Rite Nitik Q backup.

are you on nitrox for that? ongas is pretty slow for the slow tissues and at 60ft on nitrox you are at 50ft EAD which is near unlimited for all intents and purposes *read an impractical amount of gas required to actually hit that ndl

 

[scubadada]

are you on nitrox for that? ongas is pretty slow for the slow tissues and at 60ft on nitrox you are at 50ft EAD which is near unlimited for all intents and purposes *read an impractical amount of gas required to actually hit that ndl

Hi @tbone1004

In SE FL, I often do 4 dives per day. Avg depths are often around 60 ft, or deeper. Dives are generally 70-75 minutes total. Many of the dives end up butting up on NDL or occasionally light deco. The Buhlmann NDLs are frequently longer than the DSAT NDLs for these dives, thus, my concern. After 15 years of diving DSAT successfully, I am hesitant to change. I will continue to dive DSAT and Buhlmann in parallel for now, continuing to accumulate experience with both.

Good diving, Craig

 

[tbone1004]

Hi @tbone1004

In SE FL, I often do 4 dives per day. Avg depths are often around 60 ft, or deeper. Dives are generally 70-75 minutes total. Many of the dives end up butting up on NDL or occasionally light deco. The Buhlmann NDLs are frequently longer than the DSAT NDLs for these dives, thus, my concern. After 15 years of diving DSAT successfully, I am hesitant to change. I will continue to dive DSAT and Buhlmann in parallel for now, continuing to accumulate experience with both.

Good diving, Craig

why not cut the gf's back if you think they're too aggressive?
If you read up on GF's and how they work as a model, you'll see that when set at say 95, they won't let you up if any tissues are over that theoretical loading percentage regardless of what happens on the repetitive dives. If you aren't comfortable with that level, back the GF down so you get up with less total nitrogen time.

Just ran some numbers. 60ft for 70 minutes based on what you said, and an hour surface interval because any longer makes the day too long, used EAN32, so not sure what you're diving, but this is for ean32.
At GF's of 50/95, it's true NDL diving.
At 50/85 which is imo a bit more reasonable, dive 1 is NDL, dive 2 is 2 minutes at 10ft *which is accounted for in your safety stop*, dive 3 is 6 minutes *so extended safety stop*, and dive 4 is 8 minutes.
Reasonable decompression stops for those dives, towards the end. If you go to 1:15 surface intervals, it goes back to NDL diving, or real close to it.