Backup gauges for air-integrated computers: a poll

[scubafanatic]

......well, I run a 'hosed' Oceanic Datamax Pro Plus AI computerover the right shoulder, and a wireless Cochran AI computer over the left shoulder...as well as a back-up Halcyon SPG on the left side......yeah, I've a gear freak!

Karl

 

[shurite7]

I don't carry the spg as backup - I keep it on my reg because it is easier to switch over to doubles.

I have several hundred dives on my AI and have never had an issue other than the battery dying on my computer just before a dive.

C

 

[yyidus]

I carry no backup. My reasoning:

3 possible failures:

1) transmitter fails, computer still working.
If this is the case, the computer will alert me rather loudly that it can't
get a pressure reading (0 psi). At this point, I know that before it lost connection,
I still had enough air to safely make an ascent, safety stop, and reach the top with
more than 500 psi. So I simply call the dive.

2) receiver fails (we're talking catastrophic failure). This is extremely unlikely, but
in theory could happen. At this point, I have to make a safe ascent without a depth
gauge or timer. Most likely I am with a buddy and can easily inform him of the
situation, and simply call the dive, and use him as a reference for ascent speed, and
safety stop.

3) buddy fails AND computer fails. This one sucks... But I dive recreational limits
anyways.. so I do my best to do a slow ascent. I probably will miss the safety stop,
but odds of DCI are still pretty low.


In any case, I don't do cave diving. I don't need to carry a backup for any possible failure point. Nobody I know that has a hosed AI computer carries two of them. So really it's just
people whining about the transmitter (envy?). That failure mode is #1, and is trivial to deal with. (yes you might lose one dive... but we're talking a VERY rare failure anyways).

 

[MRPerkins]

I use a Suunto D9 and a Suunto combo gauge as a back-up. But I'm obsessive/compulsive anyway...

 

[DA Aquamaster]

I have had three failures in about 25 years of diving with either electronic SPG's or air integrated computers.

None were a major event because I monitor my guages on a regular basis and anticipate what I shoudl see next based on depth, time and workload. Consequently, after numerous dives of anticipating the reading and then validating that and/or correcting the assumptions you use to estimate, if and when the display ever is blank when you go to look at it, you will have a very ood idea what you have left. More importantly, if you look at the display and see a number that does not make sense, you know the gauge is fubar and you abort the dive but still have a good idea what you have left to abort with. (The same approach applies to deco status on your computer, if it fails you should still have a good idea where you are in terms of NDL's.)

An example of this occurred during a dive when a QD fitting partially unscrewed and essentially trapped the gas in the SPG hose. I had 1820 psi at 104' which seemed within reason but was a little on the "too good of SAC for the dive" side of things. A couple minutes later when I noted I still alledgedly had 1820 psi on the guage I knew something was wrong and aborted.

The moral of the story is a backup is a waste of time on single tank recreatonal dives. If one spg develops an error you have two different readings to chose from and will have to base that decision on prior experience in observing SAC and SPG readings at various depths and workloads and on a common sense test. Those are the same set of skills you'd be using when you abort after a failure with a single SPG as well. If you dive with two because redundancy is improtant by definition you will abort when one of them fails anyway. So if you are paying attention to your SPG during the dive, you gain nothing by having two and by having two you double the odds that one will fail and double your odds of needing to abort at some point.

In addition, if you use two identical models of similar ages and battery life, there is a good possibilty that both may fail under similar conditions. For example on the boat in 80 degree temps the battery may produce enough voltage to not even activate any low voltage warning. But 15 minutes into a dive in 45-50 degree water, both may fail once the battery cold soaks and no longer produces enough voltage to run the computer. So if you feel the need to have a backup, make it a mechanical gauge, not another air integrated or wireless computer.

 

[Saltwater Junky]

Because of a water leak into the transmitter I went ahead and purchased a back up SPG; however, I plan on becoming wreck cert'd. So, I'd been purchasing a backup soon anyway.

The leak was my own fault. Changed out the battery and did not tighten the cap enough. I was afraid of over tightening. A hard lessen learned w/o incident.

 

[ATL_Diver]

I just put a little pony gauge on my 2nd HP port as a backup. I'm confident in my computer and don't think I'd have any problems getting to the surface if it died but $40 for the gauge is a small price to pay for knowing that I can always find out how much air is left. I went with a pony gauge instead of a full-size SPG because I could avoid another hose (the whole point of going with a wireless computer is fewer hoses, right?) and because, as a backup, I just need an "oh *****" reading on my pressure. I don't really care if I've got 1400 psi or 1480 psi - I'm still heading topside. I'm a casual diver so deco stops aren't usually a concern but I figured that if my computer ever really borked itself, I can call the dive and have my buddy check my air on the button gauge (I don't dive solo but I could also take off my BCD and check it myself, I guess) before heading to the surface.

As an added bonus, I can also check the tank pressure when I set my rig up without turning on my computer. And just so the Safety Police don't try to bust me, yes, I check the reading on the computer before I hop in the water.

 

[mruseless]

If you dive with two because redundancy is improtant by definition you will abort when one of them fails anyway. So if you are paying attention to your SPG during the dive, you gain nothing by having two and by having two you double the odds that one will fail and double your odds of needing to abort at some point.

I love your logic! I never thought of it that way.

 

[scubafanatic]

...so, if 2 computers are 'bad'...why are 2 second-stages 'good' ? ...also, the space shuttle uses 3 main flight computers...in the event of a computer failure/malfunction, the computers 'vote' and the best 2 computers of the 3 determine the course of action...so if it's good enough for NASA I guess it's good enough for me!

 

[yyidus]

...so, if 2 computers are 'bad'...why are 2 second-stages 'good' ? ...also, the space shuttle uses 3 main flight computers...in the event of a computer failure/malfunction, the computers 'vote' and the best 2 computers of the 3 determine the course of action...so if it's good enough for NASA I guess it's good enough for me!

Ok. First of all. Someone here just failed a basic probability class. Having two computers does not double the odds of a failure. This is only true if failures are independent. Given that failures are usually related to abuse of the device and batteries running out, they are probably more closely related (and therefore less than double the chance of failure).

Now. For your question: When a computer in space fails, you can't just 'land the shuttle'. If a regulator fails, you can't just 'safely make an ascent, deco stop, and return to the boat'. If a computer fails, you know it's failed, and simply safely call the dive. You still have everything you need to safely ascend, make a safety stop (possibly not depending on if you can tell where 15 ft is), and return. For the same reason I don't carry two safety sausages, or two whistles, or two wetsuits.