Failure points vs redundancy

[Kharon]

I would need to grossly deviate from my dive plan.

And that never happens? Either by wandering too far or being pushed somewhere you didnt intend by unexpected current? Or so many other things. What is most likely to kill you is the unthinkable.

 

[datho87]

Why would you thumb the dive?

If I was on a technical dive and an o-ring went on my spg, I would not continue the dive, even if minor champagne bubbles. You would?

 

[datho87]

OK, try reading my post again.

Yes, SPGs can fail, but if you are using one as a redundancy for your air integration, then in that case you are left with the same situation you would be in if you didn't take one with you because of your fear of a failure point.

You’re not left in the same situation. You’re dealing with a failure you wouldn’t have otherwise. Hence the whole concept around failure points. If the backup spg was a completely redundant system, then yes you would be where you are had you not taken it. But it’s attached to your rig…

 

[inquis]

And that never happens? Either by wandering too far or being pushed somewhere you didnt intend by unexpected current? Or so many other things. What is most likely to kill you is the unthinkable.

The hypothetical scenario to which I responded was that I would somehow wind up with zero air without my knowledge. (This was in contrast to SM/pony when cluelessly draining one cylinder at least gives you that notice.) And yes, "wandering" far enough to exhaust my return AND reserve gas before I realize it is without a doubt a serious deviation from the plan.

 

[tursiops]

If I was on a technical dive and an o-ring went on my spg, I would not continue the dive, even if minor champagne bubbles. You would?

LOL, you just moved the goal posts. Boulderjohn talked about an SPG failure; now you have moved to an o-ring failure. An HP hose failure to the SPG will take at least 20 minutes to drain a full cylinder, so it is not an emergency if you see massive bubbles, but yes you do need to end the dive, just not in a hurry. With the "champagne" bubbles from a bad o-ring to the SPG, you may not even notice any gas loss. Whereas, if the SPG fails -- typically getting stuck at some pressure reading -- there is no need to end the dive....you still have the AI info. All "failures" are not created equal, nor are all responses to failures.

 

[datho87]

LOL, you just moved the goal posts. BoulderJohn talked a bout an SPG failure; now you have moved to an o-ring failure. An HP hose failure to the SPG will take at least 20 minutes to drain a full cylinder, so it is not an emergency if you see massive bubbles, but yes you do need to end the dive, just not in a hurry. With the "champagne" bubbles from a bad o-ring to the SPG, you may not even notice any gas loss. Whereas, if the SPG fails -- typically getting stuck at some pressure reading -- there is no need to end the dive....you still have the AI info. All "failues" are not created equal, nor are all responses to failures.

Haha good point. I did move the goalposts somewhat. Apologies boulder John. However, an analog spg requires a hose, so my point remains

 

[tursiops]

Haha good point. I did move the goalposts somewhat. Apologies boulder John. However, an analog spg requires a hose, so my point remains

Yep, it remains...irrelevant, pretty much. The HP hose (along with AI) is an additional failure point, but it is of no great consequence if it should fail, and of vanishing consequence if the SPG fails or you get champagne bubbles from the o-ring. BJ's point is that it may be an additional failure point, but the tradeoff is that it will allow you to continue a dive should he AI fail, whereas if the SPG fails there is no downside.

 

[Whitrzac]

Yep, it remains...irrelevant, pretty much. The HP hose (along with AI) is an additional failure point, but it is of no great consequence if it should fail, and of vanishing consequence if the SPG fails or you get champagne bubbles from the o-ring. BJ's point is that it may be an additional failure point, but the tradeoff is that it will allow you to continue a dive should he AI fail, whereas if the SPG fails there is no downside.

You can not continue that dive because you have no way of verifying that the spg actually works. You haven't looked at it in 150 dives, who knows if it's accurate.

Spg stays in the save a trip kit.

 

[tursiops]

You can not continue that dive because you have no way of verifying that the spg actually works. You haven't looked at it in 150 dives, who knows if it's accurate.

Spg stays in the save a trip kit.

Not so hard. Glance at it; if it agrees with the AI, it is working. If it doesn't agree, ignore it.
If the AI fails, use the SPG.
Do you trust the SPG if that is the only pressure reading you are carrying? Do you use two (or 3!) SPGs just to make sure?

 

[johndiver999]

You use redundancies for situations that are potentially deadly. For contingencies that are inconvenient, then you should think hard if it is worth the trouble and/or added complexity.

For example, I generally always carry two cutting devices. The added clutter, complexity and expense of a second cheap knife on my harness (that is not THAT easy to access) is inconsequential.

But I remember a solo dive where I did not have a back up knife and a large fish wrapped me up (in 300 lb fishing line) and tied me to a shipwreck (after it knocked my mask off and regulator out of my mouth) and then I dropped the knife to the ocean floor below. I recovered the reg, reset the mask, vowed to always carry a second knife (if I lived) and then worked on the line and weaseled my way out of it and was on my way.

I have had my mask knocked off against my will so few times, that I have never taken a spare; but for a long time I dove with a marine radio in a canister, which was far more expensive and bulky than a mask, because for the dives I was doing at that time, the radio seemed like a good redundant rescue device (on top of a couple smb's).

I carry a small light on all dives, even if I have no use for it, because I want it in case I am lost at sea and the search continues into the night. The cost and complexity of tucking a small light into the harness is minimal and for me, it has the right risk/reward ratio.