V-planner or Shearwater

[scubadada]

I have never heard this before. Do you have any source for this other than an unnamed rebreather instructor?

In a past thread on ScubaBoard, rebreather diver and well known dive theorist @Dr Simon Mitchell said he uses Multi-deco for dive planning. That is significant because in many threads, both on ScubaBoard and in other forums like Rebreather World, Simon and Ross have clashed repeatedly, with Simon clearly pointing out the many times Ross has made statements that reveal a serious lack of knowledge of decompression theory. If anyone in the world would be expected to distrust anything Ross wrote, it would be Simon.

The thread in which Simon said he uses Multideco was one I started because I was becoming afraid to use a program created by Ross and asked for alternatives. In that thread, no one mentioned anything about Ross corrupting Bühlmann and GFs that way.

It would be relatively easy to test this by using the Shearwater planning function vs. MultiDeco. I hope someone will do this and post their results.

I have often wondered if Buhlmann is implemented exactly the same on all computers and planning software. One difference, previously discussed on SB, is the implementation of GF lo on Shearwater, and Dive Rite Nitek Q, perhaps others, and MultiDeco. For the computers, GF lo does not kick in until you enter deco and are not able to surface directly without exceeding GF hi. For MultiDeco, it appears that GF lo is continuously applicable. Here is a very simple example:

 

[KenGordon]

Be aware (which I don't think many divers are) that running the Buhlmann GF algorithm in Multideco Ross H has incorporated an inherent deep stop bias. By this I mean a dive plan using say a GF of 50/70 on the Shearwater will produce a dive plan more like 40/70 in Multideco, so to match the Shearwater GF50/70 plan in Multideco you will need to plan with a GF 60/70, it does not make a great deal of difference just that the first stop of a minute might be 3 or 6m deeper.

This is definitely wrong. I have my own implementation of Buhlman ZHL16 GF and it doen't fudge anything but does agree with Ross's MultiDeco plans.

Keep in mind that a planner and a dive computer have different information to go on. A planner has all the information at once and it is idealized. In particular it knows the first stop depth and assumes you hit that on time. A dive computer only knows the first stop depth when you catch up with it. Unless you actually bust the stop then what constitutes reaching the stop? Within 3m? 1m? 0.3m? Since GF is based on the first stop depth this is important. What some implementations do is use the deepest ceiling as the first stop depth for the purposes of the GF calculation.

I do agree that Shearwater ought to offer a desktop or ipad/android dive planner.

 

[KenGordon]

I have no idea what is being said in these quotes. People are writing as if people who use computers just jump in the water with no idea what is going to happen. You do just as much planning for a dive using computers as you would for a dive using no computers.

When you use desktop software (at least the ones I have seen) or when you plan on the Shearwater, you input your expected RMV for the bottom time and the deco time, and the program tells you how much gas you should use for the dive, assuming all goes as planned. Using that basic information, you plan a reserve. You can use any system you want to plan that reserve. Many people use thirds, meaning they add 50% of that expected total so that if the dive goes exactly as planned, they will finish with a third of the gas with which they started, with that last third available for any issues that arise. I have done a number of dives with someone who always carries twice what the computer says.

What do past plans have to do with it, other than giving you the RMV you input when planning the coming dive?

Of course people do these things. If you did a properly planned 50m 30 minute deco dive last week why would you replan for a 45m minute dive with the same gas?

And if you know you have enough gas for a TTS of x minutes from 50m why would you expect to need more for a TTS of x minutes from 45m?

Personally I will get out my iPad and spend 5 minutes running the plan and run it for my buddy too. Mostly though, at reasonable depths, we could do three times the deco and still have gas (of course this would fail in the face of a CO2 hit which is why not to do it). The crucial bit, OC, is the backgas to the first change.

 

[KenGordon]

It would be relatively easy to test this by using the Shearwater planning function vs. MultiDeco. I hope someone will do this and post their results.

I have often wondered if Buhlmann is implemented exactly the same on all computers and planning software. One difference, previously discussed on SB, is the implementation of GF lo on Shearwater, and Dive Rite Nitek Q, perhaps others, and MultiDeco. For the computers, GF lo does not kick in until you enter deco and are not able to surface directly without exceeding GF hi. For MultiDeco, it appears that GF lo is continuously applicable. Here is a very simple example:
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The Shearwater people will say 'set the MultiDeco stop resolution to 1 second and notice the first stop is only one second'. Then by the magic of interpolating the next stop is at GF high and you are free to surface.

This is one reason why GF is rather bogus for NDL dives.

 

[boulderjohn]

Of course people do these things. If you did a properly planned 50m 30 minute deco dive last week why would you replan for a 45m minute dive with the same gas?

And if you know you have enough gas for a TTS of x minutes from 50m why would you expect to need more for a TTS of x minutes from 45m?

Personally I will get out my iPad and spend 5 minutes running the plan and run it for my buddy too. Mostly though, at reasonable depths, we could do three times the deco and still have gas (of course this would fail in the face of a CO2 hit which is why not to do it). The crucial bit, OC, is the backgas to the first change.

I don't think I was understood properly in my response, and possibly I did not understand the posts to which I was responding. Here they are:

If you just go by the DC deco plan and you suddenly need or have to donate gas then how do you know you will have enough

People diving 2 computers is where things can get lost.
You know you have enough gas for 50m 30min in your setup from past plans.
Someone asks you to do a 45m for 30 dive. Would you cut tables or would you just trust your 2 computers.

Of course you know your gas needs from pervious dives of the same profile. The two people I just quoted seem to be saying that people who use computers have no way of knowing how much gas they need for a dive they haven't done before, as if having done a prior dive of the same profile is the only way you can know how much gas you will need.

 

[KenGordon]

I don't think I was understood properly in my response, and possibly I did not understand the posts to which I was responding. Here they are:

Of course you know your gas needs from pervious dives of the same profile. The two people I just quoted seem to be saying that people who use computers have no way of knowing how much gas they need for a dive they haven't done before, as if having done a prior dive of the same profile is the only way you can know how much gas you will need.

C) just do your gas planning and hop in and ride the computer (idk about this one but I'm old school)

If you just go by the DC deco plan and you suddenly need or have to donate gas then how do you know you will have enough?

Does it make sense now?

Then someone pointed out that if a dive is withing the envelope of a previous dive (all other things being equal) it can be done with the same gas so jumping in and swimming about until the bottom time is up is an ok plan with two computers.

 

[DanielRJones]

I don't think I was understood properly in my response, and possibly I did not understand the posts to which I was responding. Here they are:




Of course you know your gas needs from pervious dives of the same profile. The two people I just quoted seem to be saying that people who use computers have no way of knowing how much gas they need for a dive they haven't done before, as if having done a prior dive of the same profile is the only way you can know how much gas you will need.

Close to what i was saying but again my meaning got lost in my post. I need to write out check and then write again i seem to write with my accent and just what is in my head.
My post was a question which shoukd have read as follows
When you are diving one techincal comouter you have to cut tables for the dive as a backup from decoplanner or from your computer.
When you have 2 technical comouters running the same gf you can theoretically not use cut tables as you have a backup that is suitable for the task to get you to the surface.
If you knew that your back gas could last a 50m dive for 30mins and your ali80 had more than enough gas for the deco of that dive, would you continue to put future dives through deco planner or through the planning software on your computer that were less than 50m 30mins or would you just rely on your 2 technical computers.

Myself onky having 1 technical comouter still have to plan every dive no matter the depth or time and cut tables. I do not think i would be comfortable not having these in my wetnotes when i get my second petrel.

Boulder John i hope this makes a bit more sence to you now

 

[boulderjohn]

It would be relatively easy to test this by using the Shearwater planning function vs. MultiDeco. I hope someone will do this and post their results.

OK, I did it. I ran a profile with GFs of 50/80 going to 270 for 17 minutes breathing 14/55 and using 50% and 100% for deco. I did it for our altitude in Boulder. The 2 could not be perfectly matched because, for example, I had to input a rough altitude into multi-deco whereas the Shearwater used what it sensed as my altitude.

The two plans were almost identical. The Shearwater gave me an extra minute at 100 feet and Multideco gave me an extra minute at 40 feet, resulting in identical total decompression time. I would not have expected them to be any more exact.

 

[boulderjohn]

Does it make sense now?

No. I still don't understand the statement that you won't know if you have enough deco gas. Why not?

 

[KenGordon]

No. I still don't understand the statement that you won't know if you have enough deco gas. Why not?

The OP suggests a possible method of diving

C) just do your gas planning and hop in and ride the computer (idk about this one but I'm old school)

When are you leaving the bottom? According to the on the fly DC deco plan?

If you just go by the DC deco plan and you suddenly need or have to donate gas then how do you know you will have enough?

No, don't do that or you will be in peril of an inadequate quantity of gas in practice to make the first stop, due to overstaying the slate (gas) plan.

If by flying the computer he means leaving the bottom on time but following the ascent the I said this:

If the computer is really showing much shallower stops than the plan I might move to the computer earlier, but I will be stopping for gas switches anyway so I might as well stop as planned.

That might happen if you miss the wreck and bin the dive after a few minutes. No point following a 25 minute longer BT deco schedule.