[-]Deco [/-]All gas is planned to be adequate (or, more precisely, MORE than adequate) for the contingency bottom time. Honestly, knowing your pressure in your [-]deco[/-] bottles, absent malfunction, is almost irrelevant. It's enough, if you planned properly; it isn't, if you didn't. You should ALWAYS have something else you can switch to, if you empty [-]the[/-] a bottle.
Just a quick edit that more accurately reflects gas management for technical diving.
Whether bottom or deco gas, the SPG exists to tell you something you should already know. In the rare event that gas consumption is impacted by unusual factors, such as a failure/leak or escalated consumption rate, then the SPG is consulted and a pre-planned contingency is enacted. Nonetheless, in all reasonable circumstances, and assuming the correct protocols are performed effectively and in a timely manner, the reserve should be sufficient to ensure a safe conclusion to the dive.
Beaverdivers claims an approximately equal failure rate between SPGs and transmitters. I'd like to see those statistics. In all honesty, I doubt that claim.
By the term "failure", we must include any event that causes the gauge to fail, whether permanently or temporarily, to provide the diver with immediate and accurate information of the gas volume in a given cylinder - the loss of which would warrant an immediate abort of the dive and/or would compromise the diver's safety and reduce their confidence and situational awareness in the water.
---------- Post added September 30th, 2013 at 01:51 PM ----------
I do not use a computer with a transmitter, and when I did (on rec dives), I had several problems with the system's reliability. However, in terms of complexity of use, I do not see your point. If the diver is using the computer out of gauge mode, he will have to make the gas switches on his computer, no matter if it monitors the cylinders' pressure or not. Therefore, I do see an increase in complexity from using a computer in bottom timer mode to "standard mode", but as I see it having a transmitter would simply tell the computer to start showing the deco cylinder's pressure in addition to using the new mix to compute deco obligation.
When setting up the transmitters onto cylinders, the diver will have to confirm which transmitter is on which gas. This has to be programmed into his computer. An error made at this stage can be catastrophic later on the dive. That potential for error simply does not exist in gauge mode, or without the transmitters used.
For this reason, when we set-up cylinders, we confirm the gas and mark them. It doesn't end there though. We don't dismiss the process as 'finished'. We also perform very proscribed gas switching procedures, which serve to provide a final in-water/concurrent confirmation and security against human error. The process of checks does not conclude at set-up, but rather, it spans the entire dive; pre-dive and in-water.
When conducting the actual gas switch procedure, the diver will conduct that actual, formal gas switch (same as normal), conduct the switch on his computer (same as normal, if computer is used). If a transmitter is used, he will then have to confirm that the transmitter is correctly aligned to the tank he is using. How?
We can mark tanks with %/MOD to prevent such errors. We have formal gas-switching protocols to confirm that an error doesn't arise, or can be swiftly resolved, to prevent a fatal outcome. That is a core focus in all technical diving training. What measures do we have, as a standard, to confirm no error has been made in the configuration the transmitter relationship between cylinder breathed-to-gas content-to-display/computation?
Or are we suggesting that either; (1) an error couldn't occur, or (2) an error could be tolerated?
If not, the diver has to enact a further protocol/s as part of their gas-switch procedure to guard against expected, reasonable human error. This is the added complexity to which I refer...
