Computers for Deco

[Andrew H]

Does anyone use computers for deco? How about for nitrox accellerated deco? Is it worth buying a mixed gas capable computer or nitrox or trimix? How about for CCR diving? Would I be wasting my money on technology which nobody belives in? Does it make more sense to get a gauge type computer or gauge capable computer like the viper or stinger or do people belive in the VR3 style all singing all dancing types? What do you do if you and your buddy have different computers etc? I realise that you should follow your computer because it has done your dive but on a 40m 60 min nitrox dive with deco gases there could be big differences... It must be best to agree this beforehand. I guess DIR has a good point in this respect. Most of my diving has been done with a depth gauge so I'm not too clued up here. In the interests of planning for the future I would rather be looking ahead with my gear and not buying twice.

Andrew

[Mark Chase]

Tables are available for constant PP02 diving so there is no reason why you cant do OC and CCR with tables and a depth / bottom timer.

The VR3 is capable of doing set point decompression and also constant PP02 monitoring deco with an option to switch to OC bail out at any time and continue giving useful deco data. I don’t know enough about set point dive planning to understand the problems or lack of them in bailing out to OC on tables so this may or may not be an issue.

The VR3 is upgradeable so you can get it in OC Nitrox mode and upgrade to Trimix or CCR Nitrox then CCR trimix with or without the fourth cell link. Personally I prefer computers to tables and my research has found the VR3 to be the best available computer in the UK. There is an American computer in a similar vain (The Abyss Explorer) but its rarity rules it out. The VR3 is widely used and discussed and is standard kit on most of the top deep diving expeditions. That kind of exposure brings confidence. NO ONE gets a VR3 to do bimble dives so you can be assured that the users out there are doing deep and deco. Problems with the algorithm will pop up pretty quickly in that game.

Delta P appears to have a grip on early unreliability problems and upgrades are available for older models.

As for buddy diving the normal rules apply. You always go on the most conservitave model. If your buddy is a regular on your deep deco dives then I would match equipment and gasses for redundancy.


ATB

Mark Chase

[Narked@50]

Originally Posted by Andrew H

Does anyone use computers for deco?

Yes, probably 90% of recreational divers not including the deep techies.

How about for nitrox accellerated deco?

Yes, but you have to get a computer capable of gas switching like the Vytec or VR3

Is it worth buying a mixed gas capable computer or nitrox or trimix? How about for CCR diving?

If you see yourself in the future using nitrox, many will, then get one that can do it now. Tri-mix and CCR are a bit different and the choice of computer limited to a couple of specialities, the most common is the VR3 which is around £500/600 and will cost more as you upgrade it depending on use. For CCR and O2 testing it could be around £700/800 depending on spec.

Would I be wasting my money on technology which nobody belives in?

Most respected technical divers use a VR3, not everyone agrees, but they are probably the best currently available.

Does it make more sense to get a gauge type computer or gauge capable computer like the viper or stinger or do people belive in the VR3 style all singing all dancing types?

If you are going "all the way" and can afford £600 for a computer you may want to get a VR3, but you will get funny looks by divers "in the know" as you gain the experiance to ever use it. I would start on something like a Vytec if you are going to go technical, it's half the price and more friendly for recreational diving, more importantly, you can use it as a bottom timer on deep dives as you progress to a VR3 and 100m.

What do you do if you and your buddy have different computers etc? I realise that you should follow your computer because it has done your dive but on a 40m 60 min nitrox dive with deco gases there could be big differences... It must be best to agree this beforehand.

The standing rules of diving apply. You stay with your buddy and do the longer of the two decos as gas permits, regardless of the difference. If you are doing this kind of dive you should be diving similar configurations and gas, it should be planned and there should be little difference in deco times. If you find yourself in your scenario you screwed up the planning.

I guess DIR has a good point in this respect.

DIR is just one way to dive, it does not suit everyone, it actualy suits very few if you count the numbers, most divers are not DIR and dive well and safely. If you are realy interested in DIR you should speak to some of the guys on here who are into it, they will better be able to explain the concept, but do it before you buy kit as it may not be acceptable to DIR.

Most of my diving has been done with a depth gauge so I'm not too clued up here. In the interests of planning for the future I would rather be looking ahead with my gear and not buying twice.

Andrew

Hope the above helps, as for buying twice, I would generaly agree, but you have to be carefull to gain experiance before you get some types of kit. You would not pass your driving test and step into a formula 1 car? If you did, you would probably crash, the same is true of any sport, diving included. If you think you are just going to be passing through the lower ranks, then buy a second hand single 15l and a pony with DIN regs you can convert when you get twins as this will allow you to do shallow dives and work up a bit of depth and deco to include gas switching before you move to the twins/wing etc. The same is true for other items, you don't want to spend £500 on a torch only to find you don't like wrecks?

Best thing to do is talk to lots of people, get loads of opinions, but don't overstreach yourself at first. Of course, if you are loaded, let's go shopping...

James

[Abyss Divers]

When we do our Deco Stops after dives to 60 meters on air we use EANx 50% from the first stop on 9 meters still staying the computer Deco time on air this to be on the safe side! If you start with accellerated deco with Nitrox after a deep air dive you will for sure be to close to a Bend! Better to be conservative and knowing that you are safe! Even running up some stairs could cause a Bend after accellerated deco!

[Mark Chase]

Originally Posted by Abyss Divers

When we do our Deco Stops after dives to 60 meters on air we use EANx 50% from the first stop on 9 meters still staying the computer Deco time on air this to be on the safe side! If you start with accellerated deco with Nitrox after a deep air dive you will for sure be to close to a Bend! Better to be conservative and knowing that you are safe! Even running up some stairs could cause a Bend after accellerated deco!

This makes no sense but perhaps I have misunderstood

Air is a rubbish decompression gas because it has far too much nitrogen in it and that’s the stuff you’re trying to get rid of. 79% is bad. The best gas for off gassing Nitrogen is 100% 02. Off gassing is where the nitrogen is passed safely into the exhaled gas. In order to force the transfer of nitrogen it is better to have a higher pressure of oxygen to push it out. In order to achieve this we use high partial pressures of 02. 1.6 is used in 'normal' decompression diving and 2.0 in advanced decompression practice. The decompression chambers will use up to 2.8PP02.

Using 50% it is best to get on the gas at the highest safe PP02. Most divers would consider this as 1.6PP02. To achieve a PP02 of 1.6 with 50% you switch at a depth of 21m. Oxygen takes approximately 2mins to circulate through the system and provide benefit. As a result it is recommended that the 21m stop is extended to 5mins to open up the oxygen window and kick start the off gassing.

Using a 21m stop for 50% you could reduce the decompression for a 60m dive for 30 mins to 69mins. If you deco'd on air it would take 125mins. Starting your decompression on 50% at the 9m stop would take 86mins. (all figures created using decoplanner on a 20/85GF

What this demonstrates is that to utilise the 50% deco gas you should get on it as early as possible. The Hyperbaric docters recommend a minimum PP02 of 1.0 for oxygen for medicinal use. The PP02 of 50% at 9m is 0.96.

50% is a good single gas deco choice. However getting on it at 9.0m is not taking advantage of its full potential. Deep air using air for deco is bad. Using Nitrox is good. Personally on a 60m dive I would use two decompression gasses to keep the PP02 even higher. I use 32% switching at 40m and 80% switching at 10m. Using this I would require 52 mins of deco on a 60m dive for 30mins.

Modern thinking is that even better more effective deco can be achieved using Helium in the mix to further reduce the nasty Nitrogen. Helium is a fast gas, fast into the body, fast out. The up side is it is not toxic like O2. As a result the hyper baric community are looking carefully at using 50% 02 and 50% Helium to treat bends. This is not yet accepted practice but the DIR/ GUE boys and Mark Elliot are already recommending using Helium in the decompression mix to get rid of the nitrogen and allow effective off gassing to start deeper.

ATB

Mark Chase

[Narked@50]

Originally Posted by Abyss Divers

When we do our Deco Stops after dives to 60 meters on air we use EANx 50% from the first stop on 9 meters still staying the computer Deco time on air this to be on the safe side! If you start with accellerated deco with Nitrox after a deep air dive you will for sure be to close to a Bend! Better to be conservative and knowing that you are safe! Even running up some stairs could cause a Bend after accellerated deco!

Must be a bounce dive?

To do deco from a 60m dive on air calculations would run into hours for a descent bottom time of say 20/30mins? Are you talking about a multi profile dive where your max depth was 60m?

I'm also with Mark on the 50% at 9m but I'm a wimp when it comes to pp02 and stick to the 1.4 BSAC recomends, I know it's conservative, but I do so little of this kind of diving it does not realy effect me, it's good to know what your doing though!

James

[Mark Chase]

Originally Posted by narkedat50

Must be a bounce dive?

I'm also with Mark on the 50% at 9m but I'm a wimp when it comes to pp02 and stick to the 1.4 BSAC recomends, I know it's conservative, but I do so little of this kind of diving it does not realy effect me, it's good to know what your doing though!

James

I don’t know BSAC stuff. Do they recommend 1.4 for deco and bottom gas or do they, A: not differentiate B: Recommend even lower for bottom gas? (If so what)

Just curious

ATB

Mark Chase

[Narked@50]

Originally Posted by Mark Chase

I don’t know BSAC stuff. Do they recommend 1.4 for deco and bottom gas or do they, A: not differentiate B: Recommend even lower for bottom gas? (If so what)

Just curious

ATB

Mark Chase

Hi Mark

1.4 is the maximum recommended pp02 for either bottom gas or deco gas. I think PADI have the same limit on their courses. I know it's a bit lower than the more technical agencies, but as I said, this is not a limiting factor for my normal diving though I can see how it would be for others.

James

[Andrew H]

Thanks for all the input guys. I'm a former PADI OWSI and have 150+ dives including 40m no deco and nitrox dives so not exactly new, but currently without gear. I am hoping to dive wrecks while in England, have no urge to dive below 40m on air, particularly not in these waters. But appreciate the need for increased runtime for dives to 30m plus. Hence deco and most likely accelerated deco, Maybe I should have mentioned that before... I would like to progress to Trimix in couple of seasons time experience permitting. There seems to be a strong move away from deep air and towards trimix at shallower depths. Oddly some agencies require you to have a number of dives beyond 30m on air while others recommend you maintain an END of less than 30m... confusing stuff! I think the recreational agencies will start to recommend normoxic trimix for dives which were formerly extended range. Does anyone use trimix with high oxygen for dives of 20-40m or is it considered too expensive? The CCR crowd must have experienced this using trimix diluents on shallow dives after deep dives, there must be a different experience even at 30m?
From past experience I know that a dive on a wreck in 27m of water would be planned on PADI table as 30m for 20min, But much of the dive will be done at 20m on an intact wreck so diving a computer pays hugh dividends in that respect. I am wondering diving to around 40m on nitrox with deco, the difference would be similar? I am also assuming that diving deeper on mixed gas say 60m there would be less change and at 90m much less change because of the reduced relative change in ambient pressure as the dive is deeper? Seems to me then that you need a fairly compelling reason to spend hundreds of pounds on a VR3 when you could cut tables for the dive effectively for free. Any way that's what I'm driving at, don't seem to be any nearer to solving my problem so far.

[Mark Chase]

[quote]

Originally Posted by Andrew H

. Does anyone use trimix with high oxygen for dives of 20-40m or is it considered too expensive? The CCR crowd must have experienced this using trimix diluents on shallow dives after deep dives, there must be a different experience even at 30m?

If i am planning wreck penitration I would use Trimix on a 30m dive. On a 40m dive I would use it any way. I will always run a max planned depth PP02 of 1.4 on wreck dives. On higher work load dives I would drop that to 1.3 and increase the He.

From past experience I know that a dive on a wreck in 27m of water would be planned on PADI table as 30m for 20min, But much of the dive will be done at 20m on an intact wreck so diving a computer pays hugh dividends in that respect. I am wondering diving to around 40m on nitrox with deco, the difference would be similar? I am also assuming that diving deeper on mixed gas say 60m there would be less change and at 90m much less change because of the reduced relative change in ambient pressure as the dive is deeper? Seems to me then that you need a fairly compelling reason to spend hundreds of pounds on a VR3 when you could cut tables for the dive effectively for free. Any way that's what I'm driving at, don't seem to be any nearer to solving my problem so far.

30mins at 90m using 14/60 32 and 80% 153mins deco
30mins at 87m using 14/60 32 and 80% 143mins deco
30mins at 85m using 14/60 32 and 80% 135mins deco

A varaible of 5m on the depth reduces the total deco by 18mins. It dosent sound like much but when i wave you good buy and you have still got 18 mins to go trust me you will be pissed off. Finding a secton of the wreck at 75m and doing 15 at 90 and 15 at 75 would reduce it to 128mins so thats 25 mins saved but I am still doing the same 20/80 profile.

Its worth it to me.


ATB


Mark Chase