Here's why the lockout thing is a bad idea.
For ease of illustration, let's use VPM since it's easy to see conservatism. So you normally plan VPM+4 because you're a bit overweight, the water is cold, at the end of a long dive you're a bit dehydrated.
Let's say you're doing a nice smooth deco dive. All of a sudden you have a suit flood and all of a sudden what was a nice warm drysuit is slowly cooling you down. And maybe you get a regulator free-flow on your deco gas. All of a sudden you're in potentially some serious trouble. On a computer like the Petrel, you can switch to say, VPM+1 and it will recalculate. I guarantee that depending on the profile, that change will cause you to blow a deco stop for 3 minutes somewhere.
With a computer that locks out, now you're running gauge mode, on a table that isn't inline with your decompression schedule. Now you either run the risk of either running the entire deco schedule on a lost-gas profile with a flooded drysuit increasing the chance of getting bent. OR, you have a computer that is continuing to calculate proper deco, although at a less conservative level, getting you out of the water faster.
It's definitely a choice, but I don't want a computer that takes that choice away from you. That's what happens when you dive a computer that will lock you out.
We could dream up hypothetical situations all day long, but at the end of the day, it is far safer to have a piece of gear that gives you reliable information so that you can make an informed choice, than a computer that leaves you to your own devices.
Can't you just do that in your head? Or if skipping shallow is your plan then plan for it and put it on a slate.
You have two failures at once here. Are you also planning for both your first stages to die? Didn't you have a buddy with deco gas to steal? You have a lost gas plan?
For the diving i do (eg The Duke off Brighton, OC 55m leaving the bottom at 27, 18/35, 40% and70%) the difference between a VPM+1 and +4 profile is small below 9m (3 minutes).In fact my HelO2 will get me to 9m quicker than either. The real difference is at 6m where you'd spend an extra 5 or so for VPM +4 vs +1 and then another 10 minutes for the RGBM (P-2).
Having planned a few such dives, with both multideco and DM5 I know that I should be able to chop back the shallow suunto minutes and not get too far away from a reasonable profile.
Suunto really have three sorts of stop:
Deep
Ceiling
Safety
The deep stops are essentially optional. Skip past those and you might get a beep. No error modes or lock out.
The ceiling is the serious thing. Go above it and it thinks you will be likely to bend yourself. Do that for 3 minutes and it says all bets are off and it errors and goes into gauge mode. My backup Zoop does this at about 9 to 7m on the profile above when it thinks I did the entire dive on air.
Safety stops are totally optional.
So supposing some disaster that forces you shallow, but not to the surface on a HelO2. Say you decide to go shallow as fast as you dare. What happens? Eventually you reach your ceiling at (using the Duke example) something like 12 to 9m. Now you can choose to follow the computer to 6m or ignore it. Say you ignore it, go directly to 6 and bend it. Now your options are wait out whatever your slate says, or guess. Personally I'd follow the computer as shallow as fast as possible and then do as long as I could take in the cold. The only difference between that and the same with a VPM Petrel is that the Helo2 would be bent at the end.
I think the Suunto computers deserve some criticism but it is almost never what people on SB claim. I find they hate helium, hence my use of P-2 and the DM5 software has been exceptionally poor from time to time. On the other hand with the Petrel you need to spend extra for planning software and hope that it has a similar enough implementation, also I have had as much trouble with the Bluetooth on mine as I have with the poxy usb to serial nastiness for the Suunto.
I will not buy the eon, it is too expensive really. If it would do GF then it could back up my Petrel then maybe but that will never happen. Perhaps the new research on helium may change suunto's algorithm so,I could set it to be more aggressive than the petrel and so a useful backup.
