Buddy Dive's Using Different Algorithms (computers)

[GameChanger]

I am probably way overthinking this....
Setup:
I just bought the Perdix AI. It will replace my Oceanic OC1. I plan on carrying the OC1 as a backup.
My dive buddy (18 yr old son) is a new diver and he has the Oceanic Veo 3.0.
100% of our rec diving is in warm Caribbean waters (Cozumel, Grand Cayman, etc). We will be in Cozumel next week for 8 days and have 6 2-tank dives planned. Maybe a night dive as well.
We do not push NDL's.
We use EAN32/PPO 1.4.

Question: The Perdix AI uses the Buhlmann ZH-L16C with 3 adjustable GFs and the Oceanic's are set to Pelagic DSAT. From what I have read, the Oceanic's Pelagic DSAT and the Perdix AI- set to GF of "LOW 45/95" - should be just about the same, give or take 3-7 min's NDL (Perdix AI with less NDL time) depending upon depth. While I am not too concerned with a 3 minute variance in NDL, 5-7 does give me pause for concern. Again, we do not push NDL's. What options or suggestion are available besides buying my son a Perdix DC? What other concerns are there using different algorithms with your buddy diver? (aside from always being the one to have to come up first).

Am I overthinking this??

 

[Jim Lapenta]

You should have been told in your OW class when diving as a team and using different computers to use the more conservative computer. When it says it's time to go, you go.

If you're not pushing the NDL's at all, you can also plan the dive using both computers and see which one is more conservative before getting in the water.

I don't use an Oceanic computer any longer except as a backup bottom timer on occasion. I do own two perdixes and a predator. Are you able to go and do a couple of dives before your trip? Make them very conservative and use the Perdix in rec mode. It has 3.

Choose the most conservative and compare it with the Oceanic. Then on another day choose the mid-level.
With your trip schedule be sure to keep an eye on your O2 toxicity levels and CNS loading.

I would not buy your son a new perdix. Let him get a job and buy his own.

Also, this is a great opportunity to do some serious dive planning and work on developing the discipline to follow the plan and not arbitrarily decide to go a little deeper or a stay down a little longer. I tell all my new divers to plan the dives as if they were tech dives. Develop discipline first and then start to extend your range.

 

[dmaziuk]

I am probably way overthinking this....
...
Am I overthinking this??

Yes.

DSAT was designed and tested specifically for multi-day diving; ZH-L was not. 8 days is a bit much for DSAT if you're pushing it, but should be fine if you're not (edit: 2 tanks/day is far from "pushing it" according to DSAT report). I'd dive Oceanics on DSAT, set your Perdix to 95/95 and use it as SPG. (45/95 and 95/95 should be identical unless you overstay the NDL: that's when "45" kicks in.)

 

[Jay]

I don't think you're overthinking this and knowledge is good

@scubadada has extensive experience diving DSAT and ZHL side by side, and I believe he mostly dives Nitrox so his information is probably the most accurate on SB and better than what I have below:

I've used both too, and had a similar question about transitioning (I've not done side by side). This info is for diving on air and is gleaned from a single set of profiles in this post (How to properly assess the relative conservatism of various algos/computers for Rec divers?). The Perdix (on x/95) will tend to have longer NDLs versus your two DSAT Oceanic's on repetitive dives with shorter SI's; with a SI's of 1 hour, you'll probably produce around ~2mins of NDL difference on a second dive, and ~4mins of NDL difference on a 4th dive. I don't think you'll be seeing 5mins+ deviation assuming your 2-3 dives/day will be well spread out. I think you'll be seeing 1-2mins (/rounding). In the link I posted ignore the Teric's data, I think it was on a different setting. Depending on what ScubaDada says re his results, you could set your Perdix on custom with a GF of x/94 to narrow any small difference.

Would love to hear how it went and Happy Diving to you both!

EDIT. p.s. the times above refer to and 'inflection point' on the dive when you think 'I've seen what I've come for now it's time to ascend and/or your NDL getting to some level that you would start to head up'

 

[scubadada]

For me, it is impossible to accurately mimic DSAT with ZHL with GF. DSAT tends to be more liberal on first dive. ZHL becomes relatively more liberal on repetitive dives, maybe with shorter SIs, which I often have in SE FL. DSAT seems to be more liberal at deeper depth, ZHL more liberal when shallower. When I dive my Nitek Q as a backup for my VT3, I often vary the GF high somewhere between 100 and 90. I have largely gone back to using my Geo 2 for a backup so that it matches my VT3 within 30 seconds.

 

[broncobowsher]

Set them both to a comfortable conservatism. Let them make different numbers. Follow the one that's most conservative.

My Shearwater (in default settings) is more conservative than my old Oceanic. I follow the Shearwater.
Remember the NDL times are not an exact science. Far from it. The simple fact that you can adjust the press your luck factor on the computers is telling you that it is a guessing game. Over here less likely, over there more likely. Like standing outside in a lightening storm. On top of a hill holding a metal golf club above your head, you have the conservatism down to nothing. You probably are not going to get hit by lightening, but if you do everyone will look at what happened and saw it was a disserved hit. Where do you want to be between the top of the hill and hiding inside a metal box afraid to go out?

 

[dmaziuk]

Where do you want to be between the top of the hill and hiding inside a metal box afraid to go out?

That's a really bad analogy: none of the existing algorithms puts you at the top of the hill holding on to a lighting rod. If you get an "undeserved" hit, chances are it's because of something no model is accounting for: PFO, drug interaction, whatever.

While ZHL is the "overall" reference model, the models designed specifically for repetitive multi-day diving tend to carry the loading over the longer periods. Hence you get longer NDLs from ZHL on the 2nd and subsequent dives: it "forgets" more. Upping conservatism with gradient factors does not change that dynamic. The difference is most likely academic over 8 days of 2-tank dives.

But if it weren't, then a more fitting description would be "on top of the hill while believing you're in a metal box".

 

[scubadada]

Set them both to a comfortable conservatism. Let them make different numbers. Follow the one that's most conservative.

My Shearwater (in default settings) is more conservative than my old Oceanic. I follow the Shearwater.
Remember the NDL times are not an exact science. Far from it. The simple fact that you can adjust the press your luck factor on the computers is telling you that it is a guessing game. Over here less likely, over there more likely. Like standing outside in a lightening storm. On top of a hill holding a metal golf club above your head, you have the conservatism down to nothing. You probably are not going to get hit by lightening, but if you do everyone will look at what happened and saw it was a disserved hit. Where do you want to be between the top of the hill and hiding inside a metal box afraid to go out?

I don't necessarily disagree. I have nearly 1700 dives using DSAT since 2002, about 5% light deco. It has served me very well. In general, I'm very comfortable diving ZHL with GF. However, on occasion, with repetitive dives, I have been given extremely long NDLs at 60 ft or so that I would be frightened to complete when compared to my DSAT NDL. Not that they would be unsafe, they are just outside my experience.

I would imagine that if my VT3 ever dies, my next primary computer will be running Buhlmann ZH-L16C with GF. I'm still not exactly sure how I will run it given my experience to date. I often run a first, deep dive at a GF high of 100 and end up diving later, shallower reef dives, at a GF high of 90.

 

[GameChanger]

OK, thanks for all the feedback and thoughts.
I think I may be overthinking it a bit...

 

[Diver0001]

I am probably way overthinking this....
Setup:
I just bought the Perdix AI. It will replace my Oceanic OC1. I plan on carrying the OC1 as a backup.
My dive buddy (18 yr old son) is a new diver and he has the Oceanic Veo 3.0.
100% of our rec diving is in warm Caribbean waters (Cozumel, Grand Cayman, etc). We will be in Cozumel next week for 8 days and have 6 2-tank dives planned. Maybe a night dive as well.
We do not push NDL's.
We use EAN32/PPO 1.4.

Question: The Perdix AI uses the Buhlmann ZH-L16C with 3 adjustable GFs and the Oceanic's are set to Pelagic DSAT. From what I have read, the Oceanic's Pelagic DSAT and the Perdix AI- set to GF of "LOW 45/95" - should be just about the same, give or take 3-7 min's NDL (Perdix AI with less NDL time) depending upon depth. While I am not too concerned with a 3 minute variance in NDL, 5-7 does give me pause for concern. Again, we do not push NDL's. What options or suggestion are available besides buying my son a Perdix DC? What other concerns are there using different algorithms with your buddy diver? (aside from always being the one to have to come up first).

Am I overthinking this??

I recently did some experimenting by wearing a Perdix on one arm and a old Vytec on the other through a series of 20 vacation dives, all but 1 of which were within the NDL's. On all of the dives I used EAN28. Depths varied from 10m to about 40m on various dives.

What I noticed in a practical sense is that the the Perdix, which I had set to 45/85 gave shorter NDL's on the deep part of deeper dives than the Vytec. Sometimes the difference was significant... maybe 8+ min at certain depths depending on which dive of the day it was.

Once going shallow the differences switched. The Perdix gave relatively more "credit" for going shallow than the Vytec did.

Depending on the gf's you choose you may find it doing that when compared to your son's computer too.

Best advice is precisely what Jim said; follow the more conservative of the two and you won't have a problem.

R..