Alternate Gas Backup Computer

[arew+4]

While I appreciate all the input from everyone, I believe many of you are over thinking things a bit. Our goal here is to reduce the risk of an "undeserved" DCS hit. All of the tables and models are based on statistical analysis, but I am sure you all know offer no guarantee. By reducing the the nitrogen concentration, you reduce the loading, by not adjusting the calculations you provide an additional "buffer". This is not some attempt to "game the system" somehow.
I have heard people in diving medicine have recommended this practice to people who have had DSC while diving within their computers limits and what would be considered conservative profiles. We are simply looking to proactively reduce our risk, and wish to track actual exposure while we do this.

The model/computer compatibility issues that drove me to post this are related to things such as repetitive dives, surface intervals, and ascent rates.

Any comments in this regard would be most welcome.
Thanks

 

[arew+4]

View attachment 692308

What an appropriate screen name.

 

[Belzelbub]

While I appreciate all the input from everyone, I believe many of you are over thinking things a bit. Our goal here is to reduce the risk of an "undeserved" DCS hit.

Actually, I think that it is you that is overthinking it. The goal you mentioned is to provide a safety buffer. There are easier ways to do that. Set one at the most conservative setting, the other at a more liberal setting. The conservative one is the one you follow, the other is just in case. For additional buffer, don’t dive to NDL on either one.

I have heard people in diving medicine have recommended this practice to people who have had DSC while diving within their computers limits and what would be considered conservative profiles. We are simply looking to proactively reduce our risk, and wish to track actual exposure while we do this.

But, you are only tracking actual exposure on one of the computers. The other has incorrect information.

The model/computer compatibility issues that drove me to post this are related to things such as repetitive dives, surface intervals, and ascent rates.

Any comments in this regard would be most welcome.
Thanks

I believe you were using an Oceanic ProPlus 3. In that case, you could use any Oceanic or Aqualung computer and use the same algorithm. Oceanics could use either DSAT or PZ+, Aqualung would use PZ+. DSAT is the more liberal algorithm and is based on the same studies used to develop the PADI RDP. PZ+ is based on the Buhlmann algorithm.

Another option would be the Shearwater Pereegrine. That won’t lock you out, and can be set to many levels of conservatism if you desire.

You should still set both to the actual gas you are breathing.

 

[Wibble]

Seems an odd thing to do: accept there's a benefit from Nitrox and then not exploit it.

As you appear to be doing shallow recreational dives within the NDLs; why bother.

The risk of DCS is very low in the shallows and especially if you're diving within the NDLs. Unless of course you're mitigating something like a PFO.

As most have said; don't lie to the computer(s); just set them to a conservative setting, for example a low gradient factor, etc.

Does seem to be a waste to buy nitrox and not use it.

Also, within those shallow and NDL constraints, you're not going to rack up any high levels of CNS. If you do and you lied to the computer, you wouldn't be able to use the computer's measurements.

 

[tbone1004]

While I appreciate all the input from everyone, I believe many of you are over thinking things a bit. Our goal here is to reduce the risk of an "undeserved" DCS hit. All of the tables and models are based on statistical analysis, but I am sure you all know offer no guarantee. By reducing the the nitrogen concentration, you reduce the loading, by not adjusting the calculations you provide an additional "buffer". This is not some attempt to "game the system" somehow.
I have heard people in diving medicine have recommended this practice to people who have had DSC while diving within their computers limits and what would be considered conservative profiles. We are simply looking to proactively reduce our risk, and wish to track actual exposure while we do this.

The model/computer compatibility issues that drove me to post this are related to things such as repetitive dives, surface intervals, and ascent rates.

Any comments in this regard would be most welcome.
Thanks

You are lookin at some pretty old recommendations. If your goal is to reduce your tissue loading, then you should do that with conservatism settings in your computer.
You should be using PZ+ as your algorithm on that computer which is roughly comparable to a Gf-Hi of 85 IIRC the comparison that @scubadada did and with the conservatism setting on you'll probably be around 75 or so. That is a far better approach that gives you predictable results to reduce your decompression stress vs running 2 computers where one can't really be used to track your decompression profile.
Diving nitrox causes a decrease in your nitrogen exposure by having you dive at a shallower equivalent air depth. Diving with the PZ+ computer set to conservative mode will cause you to be simulating diving at altitude which causes a deeper equivalent air depth. Using EAN32 these do not quite offset each other as 3000ft is used for altitude which is roughly 0.9ata.
Diving at 99fsw, the conservative setting will have you diving at 108fsw equivalent, and EAN32 will have you at an equivalent of 80fsw. 80fsw at 3000ft of altitude is roughly 87fsw equivalent.
This is not taking full advantage of the nitrox mix, but it is splitting the difference and the reality is that at 100ft we are talking about a minute or two, not 10, and at anything shallower you are probably going to run out of gas before you hit NDL anyway.

I would strongly recommend that you use fully understand your computers conservatism settings and not try to run computers with different information on two different units.

 

[tursiops]

I have heard people in diving medicine have recommended this practice

Old hearsay.

 

[scubadada]

Hi @arew+4

I'm with @tbone1004 on this topic. There is no reason you can't successfully use your current Oceanic Pro Plus 3s to dive nitrox conservatively, with or without a backup.

Your computer runs 2 deco algorithms, DSAT and PZ+. Each algorithm has a conservative setting, giving you 4 choices. From most liberal to most conservative: DSAT, PZ+' DSAT with C, PZ+ with C. DSAT is about a Buhlmann GF high of 95. PZ+ is around 85. I've not looked at the 2 with conservative factors but would guess they are probably around 75 and 65. Once you program your algorithm, you can run your NDLs on the planner.

So, I would recommend running your computer with the actual nitrox mix and choosing one of the 4 algorithm options that you feel is giving you the safety you want..

 

[Belzelbub]

@arew+4
You might take note that it appears to be unanimous that you should set your computer with the correct gas, and adjust conservatism to do what you are trying to do. A unanimous position rarely happens on Scubaboard. That should tell you something.

 

[ATJ]

Further, if you do adjust the conservatism on the computer as your safety factor, it will work no matter what gas mix you use (and set the computer to). i.e. you will get the safety factor without even needing nitorix, but also with whatever level of oxygen there is in the mix.

 

[broncobowsher]

Shearwater, programmed to the actual gas. Dial back the GF to make it conservativism as you like. Safe, sane, accurate (data).