Gas switching computer

[DevonDiver]

Originally Posted by Mark Chase

The problem with cheep gas switching computers is they dont show TTS (time to surface on all available gas) and / or dive planning on PC.....As a result i found my Vytec a tad useless. Id be at 45m with it telling me i ha two hours of deco on the 21% I was diving. Fortunatly i had a rough idea that i only needed 45mins deco if i did 45mins on the bottom so i could ignore it and it then becomes a very expensive depth and bottom timer.

Mark

I've found that ProPlanner gives a very accurate reflection/prediction of the deco imposed by Vytec/D9 computers. I normally cut tables from Proplanner and will use them as back up if reqd.

[Janos]

Originally Posted by GLOC

Yes, you can create ratio deco tables for whatever mix and depth you like but I very much doubt that someone who is doing AN & DP will be at that skillset. Indeed the published RD profiles are based on Trimix so it would be out of the scope of this discussion.

Don't forget DECO ON THE PIE

----

I have a Vytec, but I wouldn't follow its profile for deco dives with more than 10 mins of stops or so. It brings you up too quickly and the shallow stops are too long. The new DS model isn't much better. So what's the point of having a computer where I don't like the algorthmn it uses?

So if someone asks me for a recommendation for a computer, then I'd suggest either the Gekko (lovely computer which doesn't do accellerated deco) or Shearwater (full function computer). I'd consider the VR£ too. I don't see the point of any computers currently available inbetween these two.

Janos

[Mark Chase]

Originally Posted by GLOC

What I was trying to say was the ratio deco which most people subscribe to is based around a standard set of gases, standard rates of ascent and standard deco deco gases.

Yes, you can create ratio deco tables for whatever mix and depth you like but I very much doubt that someone who is doing AN & DP will be at that skillset. Indeed the published RD profiles are based on Trimix so it would be out of the scope of this discussion.

Regards

I have a bit more time to properly answer this now so here you go:

If (DP)decompression procedures doesn't qualify someone to do gas planning and adhere to pre set ascent rates then Id like to know what the hell it is supposed to do. AN and DP alowes multiple gas decompresion which is about as technical as decompresion gets

Which non GUE course do you think is supposed to prepare you for this?

With reguard to the suitabuility of ratio deco to non trimix dives in the max 50m range.... (bout as far as anyone pushing a nitrox dive should go)


Using deco planner set to 10/100 (simila to ratio deco)

Gas plan for 21/35 and 50% 45 mins at 50m

1 @ 33
1 @ 30
1 @ 27
2 @ 24
2 @ 21
2 @ 18
3 @ 15
5 @ 12
7 @ 9
35@ 6

Total deco 60mins

Gas Plan for 23% and 50% 45mins at 50m

1 @ 33
1 @ 30
1 @ 27
1 @ 24
2 @ 21
3 @ 18
3 @ 15
5 @ 12
8 @ 9
37 @ 6

Total deco 63mins



Ratio deco for 45mins at 50m using 21/35 & 50%

Set point 45m + 5mins add 6mins so deco from 21 and up = 51mins


1 @ 36
1 @ 33
1 @ 30
1 @ 27
1 @ 24
5 @ 21
5 @ 18
3 @ 15
5 @ 12
7 @ 9
26 @ 6



Total 57min


Basicly bugger all in it

ATB

Mark

[GLOC]

Sorry for the delay Mark in replying, been busy with the kids.

A couple of points on what you have said above.

1. Ratio Deco works because, within a certain window of operation, a certain increase in bottom time, increases the deco obligation by the same amount. I ran through some examples last night using 45m (limit of DP qualification) and 25% & 50% alongside 21/35 & 50%, GF 30/85, to see what figures came out. You are correct that the times are very similar but if you the depths are changed the deco time changes are not quite linear (and neither are the standard gas deco times) but the way the times are spread/change would be harder to do from memory. As you increase times from 30mins to 50mins at 39m using 25% & 50%, the change becomes closer to 2:1 rather than 1:1 at 30mins whereas 21/35 and 50% still remains at 1:1.

2. I have not done enough investigation into the profiles to see what happens with regard to 'shaping a curve' on Nitrox mix but I would have thought that it would have been biased for a shallower deco profile making adjustments a little harder to do from memory. This would be especially true if diving a nitrox mix rather than trimix. Diving Nitrox to 40m+ in the UK is something which you (and quite a few people on here) agree is not a good idea.

3. When I talked about 'skillset' I meant the knowledge to go into Deco programmes and play with multiple profiles to spot the patterns which make up the basis for RD. You are correct, and many people have agreed, you can create a RD profile for any gas set but it does take time and you need to go through lots of profiles to see what the patterns are. The RD patterns from AG and GUE have had alot of scrutiny and people who use them know what the limits are and know how they can 'frig' the figures to take into account the agressive side of the scale. Creating your own RD profiles isn't an easy task. However, if you want to cut tables for longer, deeper, longer & deeper you will see what can be acheived. As long as you use the same gas each time, those tables are re-usable as Paul said.

As I said before, the best bet would be to go on the course, use tables and a timer (as that is most likely what will be taught), talk through with the instructor what gas computer options are available with their respective pros and cons and go from there.

[Finless]

Ref a post earlier in the thread

My Cochran Commander Nitrox shows time remaining at each stop and (if set correctly) both time remaining at the current stop and total time to surface.

Did I say I like my Cochran dive computer? Oh well, I forget and, just in case, "I like my Cochran dive computer".

The Commander Nitrox has now been superseded by a newer model. I can get a 'cheap upgrade' but why bother when there is nothing wrong with ...............

[GLOC]

Originally Posted by Finless

Ref a post earlier in the thread

My Cochran Commander Nitrox shows time remaining at each stop and (if set correctly) both time remaining at the current stop and total time to surface.

Did I say I like my Cochran dive computer? Oh well, I forget and, just in case, I like my Cochran dive computer.

The Commander Nitrox has now been superceeded by a newer model. I can get a 'cheap upgrade' but why bother when there is nothing wrong with ...............

Finless, does the Cochrane take into account switching to the correct gas at the correct depth when it calculates TTS? This was Mark's point about the Vytec.

[Mark Chase]

[quote=GLOC;882174]Sorry for the delay Mark in replying, been busy with the kids.

A couple of points on what you have said above.

1. Ratio Deco works because, within a certain window of operation, a certain increase in bottom time, increases the deco obligation by the same amount. I ran through some examples last night using 45m (limit of DP qualification) and 25% & 50% alongside 21/35 & 50%, GF 30/85, to see what figures came out. You are correct that the times are very similar but if you the depths are changed the deco time changes are not quite linear (and neither are the standard gas deco times) but the way the times are spread/change would be harder to do from memory. As you increase times from 30mins to 50mins at 39m using 25% & 50%, the change becomes closer to 2:1 rather than 1:1 at 30mins whereas 21/35 and 50% still remains at 1:1.

To save time you could point me at the profile you think dosent work?

You can't realy use 30/85 to mimic ratio deco you need to use 10/100. I know they tell you to use 30/85 for comparison but it will only confuse people

The 30/85 GF deco for 45mins at 50m is 73mins with a first stop at 30m and 45mins at 6m. How on earth does that give you a comparison with the ratio deco profile which starts at 36m, 26 at 6 and a total of 57mins deco?

For the 39m dive your on the wrong gas. We use 1.4 at planned max depth so we would be on 28% Nitrox

45min @ 39m 28% and 50% 10/100GF

1 @ 15
3 @ 12
3 @ 9
17 @ 6

45mins @ 39 21/35 and 50% 10/100GF

1 @ 24
1 @ 21
1 @ 18
1 @ 15
3 @ 12
4 @ 9
24 @ 6


Ratio deco set point 45m - 5min for every 3m

39 m therefore take away 10


45mins @ 39m on 21/35 & 50 % Ratio deco


1@27
1@24
4@21
3@18
2@12
9 @ 9
18@6

If you look at RD for this dive compaired to the 28% profile you can see its much closer than the GF comparison with 21/35


OK if you say you dont want to dive 1.4 at max depth we could use 25%


1 @21
1 @18
1 @15
2 @12
4 @ 9
20 @6

Still prety good


The long and short of it is its close enough for jazz.


MUCH more importantly I was taught on my DP course to look at at least two generated profiles and compair notes. I used to use Proplanner and Decoplanner to pre plan deco.

Back in the day before DOTF was made public I had already figured out there was a ratio between depth bottom time and deco. I did this by looking at the profiles put out by the software. However I thought the ratio was more like 1.5/1 because I used more conservative settings.

I use ratio deco today on dives to 70m on CCR. I estimate my max and min bottom times and put the numbers in the computer then i work out the ratio. I see where its aggressive or not and make a mental note to run straight ration or to pad.

Then on the actual dive if my computer fails I switch to ratio deco but i already know to add 20% or what ever to my shallow stop times. Its a shotgun approach to deco but these days it doesn't bother me at all. The days where i felt deco was an exact science are long gone.


Nothing wrong with what your saying generally in the thread its all good stuff. I am just highlighting the issue of using RD for gas sets outside of GUE standards. For me this meant using 50% as a deco gas and best mix for the bottom and clearing deco before doing a 6min ascent from 6m to the surface.

If you do this it either works close enough or its very conservitave. Thats good enough for me.



ATB

Mark

[Digger]

At risk of bringing the thread back to the OP, I've dived a Quantum for a while in the past, and found it to be a great computer that costs very little, and frankly you can't get more functionality for your money.

The Apeks will happily get you through until trimix diving, and at that point you can make a call as to whether to get a VR3, Shearwater, use tables and BT etc.

Other thing which makes the Quantum even better is the DSS mount, puts it in a rubber boot and means you can use bungee straps which I find brilliant for bottom timers. Quantum is goign to be just as good.

Digs.

[TonyD]

Originally Posted by Finless

I'm confused, I could understand you losing your dive history but why should changing the battery cause the other problems? Which one have you got; the Commander Nitrox?

When you say you can not change settings do you use the side contacts or the Analyst software?

I found the Cochran people really helpful (I phoned them up in America) when I had a problem. Per their web site their UK dealers are:-

Comparison Results

Having said that, I don't remember either being the business name I was given when I went up to the Cochran stand at LIDS.

Apparently there is a new single battery unit for the Cochras which lasts 6 times longer than 'ordinary batteries'.

What happened was that sometime between mid November and early Feb. the user changeable batteries went flat, this then discharged the internal lithium battery (its only use is to act as a power source while you change the N size batteries) and corrupted the stored dive data - this then stops the PC acquiring (connecting with) the cochran.

This was a refurbished Commander+ bought from Cochran, 4 years ago.
I had bought a Commander Nitrox second hand, only to find, after buying the download kit (£110), that it had the above problem. Spoke to Cochran and they said send it to use to be checked out. Told it was beyond repair and all they could do was offer me a deal on a new computer or a refurbished one.

I may look at trading up to a EMC-20H, but it will probably be too expensive for the limited amount of diving I now do.

I always found the commander+ to be a very easy computer to use, although it could be somewhat annoying when changing nitrox settings between dives.