Disabled deco gas on backup computer.

[Zi55ou]

Recently I've started beginning a dive with my deco gas disabled on my backup computer. Then when hitting my switch depth enabling the gas. I'm wondering if there is any downside to this practice?

Logic: were I to lose my deco gas I am aware of how much air deco I've incurred and can monitor this air deco throughout the dive.

Practice: dive on air with 80% as deco. Primary computer has both gases enabled. Secondary computer has both gases set up but 80% is disabled. On reaching 9m stop, switch to deco, switch gas on primary comp, then enable and switch to deco mix on secondary comp.

Other than it taking a touch longer to switch the second computer to the deco mix the advantages of knowing how much air deco I would be forced to do were something to happen to my deco gas allow me to shorten the dive accordingly eg it's cold and I'm not prepared to spend more than x time doing deco if something went wrong.

I'm just asking if there's something I'm not seeing and if there's some disadvantage to diving like this?

 

[rhwestfall]

I'm interested to see responses. I run my second computer on back-gas only as a "lost deco scenario"..

 

[broncobowsher]

What computer are you running?

With the Shearwater I just leave the deco gas on. For one when I reach the point I can use it, it reminds me. There is no "auto switching" feature. If you have an auto switching feature I could see where it would be good to disable it.

I like to have all gasses I am carrying switched on. Having to switch it on during the dive I could see the potential to switch on the wrong gas. If you got a bottle of 70% instead of the regular 80% one day, set the computer to the 70% before the dive. You are not having to remember is it 70 or 80% that you need to turn on during the dive. Reducing the potential in water errors.

Also on the Shearwater you can program in someone else's gas. Say you dive with someone who carries 50%. You can program that into the computer, leave it off. If something got really ugly and they could give you the extra deco gas, it is just a matter of going into the computer and turning that one on. Now if you are doing that (and it is a pretty good idea to) and you needed to turn on your gas. 50 and 80 look pretty close to each other when you are looking through murk, have a spot of water in the mask, have a little salt sticking your eyelashs together, or any other little issue.

But back to the original question, what computer. Because all my Shearwater references may be pointless if that isn't what you are running.

 

[Zi55ou]

What computer are you running?

Perdix as Primary, OSTC plus as Secondary

With the Shearwater I just leave the deco gas on. For one when I reach the point I can use it, it reminds me. There is no "auto switching" feature. If you have an auto switching feature I could see where it would be good to disable it.
I like to have all gasses I am carrying switched on. Having to switch it on during the dive I could see the potential to switch on the wrong gas. If you got a bottle of 70% instead of the regular 80% one day, set the computer to the 70% before the dive. You are not having to remember is it 70 or 80% that you need to turn on during the dive. Reducing the potential in water errors.

As in the OP all gases are set up on both computers and all gases are enabled on the primary

 

[ChuckP]

I can see where you’d want to know where you stood without your deco gas....

My understanding of the shearwater computers is that they calculate deco based on all gas entered in the computer, doesn’t matter if they are on or off. Based on that understanding, I just enter what I have and leave them on - I use two peridixs

 

[ahereng]

I can see where you’d want to know where you stood without your deco gas....

My understanding of the shearwater computers is that they calculate deco based on all gas entered in the computer, doesn’t matter if they are on or off. Based on that understanding, I just enter what I have and leave them on - I use two peridixs

This is wrong, TTS(Time to surface) is calculated just on the gasses that are turned on.

Also on Shearwaters you don't have to turn a gas on to select it, you can directly choose a gas that is turned off. The TTS will change accordingly as you now have turned on another gas.

 

[rhwestfall]

Shearwaters will use all active gasses to compute your DECO schedule....

edit: (actually, TTS. DECO is based in your gas in use, and it will indicate if you have a better one available,and if you switch, it then adjusts the schedule)

 

[king_of_battle]

If I am doing a 'simple' deco dive - which to me means 1 deco gas and I could reasonably/safely deco on back gas without issue - my backup shearwater will only have the back gas turned on. It helps me see the difference if i cut my initial run time short (usually because I'm cold up here in New England) and want to be lazy and just deco without a gas switch. If I do end up going to deco gas I'll turn it on on the backup - the nice thing about the shearwaters is that I can do the gas switch to an "off" gas and the shearwater turns it on for me. If its a more challenging dive or with multiple gas changes I have them all on on both computers.

 

[RyanT]

I run my second computer on back-gas only as a "lost deco scenario"..

My backup computer is a bottom timer. I carry a set of wet notes with my deco schedule plus lost gas scenarios on it. I can revert that those schedules if necessary.

 

[lermontov]

I'm interested to see responses. I run my second computer on back-gas only as a "lost deco scenario"..

wouldn't you do that calculation as part of the predive planning anyway? so you should know you can deco on your back gas ( or at least some sort of team share) as long as you have a TP for you back gas the deco gas is an extra margin of redundancy

The OP's idea has some merit - as shearwater in my opinion could do better in this regard.

The interface shows (if switched on) a better gas available when it reaches the MOD of that Fg in a block of colour however Ive been caught out a coupe of times of having a gas still switched on from my last dive and had a lot more deco than I was expecting - in fact recently encountered a diver at 45 m who was not carrying deco gas ( i was carrying 50% with already having 20 min deco ) and I swam up to him and alerted him to his deco obligation - he had a 50% gas switched on ( but not carrying ) which ended up being 38 min of real time deco
it would be so easy fro shearwater to have the active gas font display a different colour if you had another gas switched on that way it would alert you that you had one or more other gas' programmed in