It's not just how likely we consider the failures to be. It's how we are breaking down the possible failures and how important we consider them.
Is it possible that my computer and transmitter will lose connection during a dive? Yes. But, it has never happened for more than 1 - 2 seconds. If I look at my computer and it takes 1 or even 2 seconds to reconnect and show me my tank pressure, that doesn't bother me at all. You seem to be taking that as a dive-ending failure.
No, I'm taking losing connectivity for an extender period of time, losing the computer, or losing the transmitter as a dive-ending failure in general, of course this is the first time you mentioned using redundant computers using the same transmitter...
I use 2 computers that both read the same transmitter. So, if one computer dies, I still haven't lost my SPG.
I have heard of computers that totally lose their connection to their transmitter and have to be reprogrammed before it works again. But, I have never heard of that for a computer that reads the type of transmitter that Shearwater uses. Have you? I don't think it's really possible, because of the system design for the PPS transmitter (that Shearwater and others use).
Yes, I watched it happen to my brother just last December between the 2nd and 3rd dives we did at Ginnie Springs on a Sunday. Took us about 20 minutes on the surface to figure out what the problem was so we could fix it for the next dive.
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You also did not address the issue that I raised regarding gas loss from failures. If my transmitter battery dies, I'm not going to lose any gas. I might end my dive, but it does not turn into an emergency. If your hose gets cut or ruptures or a spool O-ring blows out, you have a potential emergency on your hands.
I did address it, with the discussion on anticipated likelihood of the issue happening, I find it to be an extremely low-likelihood event.
With your experience as a nuke, surely you have seen things where the Navy was slow to adopt something that was obvious to everyone was actually an improvement...? Is that always a good thing?
When it comes to safety items, and when the existing equipment has a proven track record of reliability, I agree with the Navy's way of erring on the side of caution, even if you could probably consider something reliable before there was hard data to compare the relative reliability of the methods employed.
Bottom line, these are the ways I would possibly not know what my tank pressure is:
- transmitter battery dies - I replace the battery per the maintenance schedule and that is a non-concern. My CCR only has one battery in it. Am I really going to not dive something just because one battery COULD die? No. That's what checklists and maintenance schedules are for.
- BOTH my computers die - a non-concern.
- I had a momentary loss of connection - it happens, but it always comes back within 1 second or so. A non-concern.
That is has always returned in a second or two for you doesn't reflect the information provided by others over the years imo, and makes it much more of a potential issue in my opinion and certainly not a "non-concern". Obviously this is something based on our individual opinions/research/experience and thus purely subjective.
- transmitter floods or otherwise just craps out - given proper maintenance, this seems to be even less likely than an SPG spool blowout.
I've seen 4 ft across heat exchanges with o-rings fail at depth on a submarine after being serviced by trained and qualified people with an independent inspector there to verify everything was done properly when it was put together. As such, I'd give them relatively similar likelihoods.
So, to me, given how unlikely it is to really have a dive-ending problem with a PPS/Shearwater transmitter, adding an SPG is just adding something else to my rig that could cause a problem. And if it does, it's most likely to be a problem that involves losing gas, which could turn into an emergency.
That's your opinion, and though it differs from mine I won't disagree with it, I just also don't agree with it.
Steve Lewis (aka @Doppler) says in his book, The Six Skills, that part of being streamlined in the water is not taking things with you that you don't need. To ME, when you have a PPS transmitter and 2 computers to read it, a physical SPG is one of those things that you don't need and the concept of being streamlined dictates that you leave it on the boat.
While I agree it "can" be left on the boat, I don't think it's necessary or prudent to do so for my diving. There's a reason that my training material, despite acknowledging AI exists, never suggests that a SPG is optional tec diving equipment.
My gut says that you are more likely to end a dive because of some issue with your redundant SPG than you are because a PPS transmitter simply crapped out. But, it is certainly the case either one could happen. Or, in your case, both could happen - on the same dive, even.
And if only one happened, and it wasn't causing me to lose air, then I can continue the dive... which is an idea I like and and comfortable with.
ps. I am definitely not saying that I would never end a dive early if my transmitter died. If it's a technical dive, then the dive is over, period. Thems the rules. If it's a recreational dive, then there are a lot of dives I do where I would not end it early. Some where I would end it early, but not immediatley, just to err on the side of caution. And some where I would end it immediately. It just depends on the specific circumstances.
Every diver chooses their gear, their level of risk, and what they consider appropriate. That's why any of us can thumb any dive at any time and we're all okay with it. That we have different opinions on this particular issue isn't a surprise. I'm sure if we polled SB on the use of redundant equipment, and what they consider a good idea or bad idea, we'd get plenty of people with varying opinions. And that's fine. As the Chairman says, "dive and let dive". I'm not going to try and convince you to take a SPG along with you, and I don't think you're going to convince me that taking one is a bad idea. It's nice to hear other points of view though, as they give everyone reading (including those involved) a different side of things to consider.
