Use a backup SPG with air integration?

[tbone1004]

Most of this conversation has centered on loss of connectivity between transmitter and receiver. What about hosed AI Dive Computers such as the Oceanic ProPlus3? For those who use hosed computers, do you also carry a backup SPG? One of the big reasons that I went with a hosed AI computer was that "transmitter failure", "receiver failure" and/or connectivity issues would never be issues that I would require a backup plan for.

Several issues I have with hosed AI computers
They do nothing for you other than provide instant SAC rate or DTR for you. They don't remove a hose, they don't make checking things more convenient, etc. all of which are advantages of WAI.
They bind the computer to the reg set vs. a WAI computer that can still function if you want to/have to use a different reg set and can't swap the HP hose over
They're quite expensive, and none of them are computers that I would want or recommend even if they were wrist computers...

 

[stuartv]

For recreational dives, no. I would keep diving and monitor my gas more closely, and if that failed, I'd be going to the surface in a slow, controlled manner. I always dive with redundancy (doubles or sidemount), except when teaching OW classes.

So, if an SPG blows out but you still have AI, you would just shut down the post with the SPG and continue the dive with whatever gas/cylinder remains available? (presuming a rec dive) I did previously specify "if an SPG blows out".

For many things in scuba, redundancy clearly makes you safer. For tank pressure, it seems like it's the opposite. 2 transmitter/SPG devices just means double the chances for a failure that results in gas loss... with the SPG being far more likely to lose gas if it fails versus the chance of losing gas with an AI failure. And, any failure that doesn't involve losing gas just seems like, one, very unlikely, and two, no big deal. And, actually, the worst case scenario for a failure of an AI/SPG that doesn't involve losing gas is a failure where it reads higher than your actual tank pressure. And that seems MUCH more likely to happen with an SPG than with AI. An SPG that "sticks" at, for example, 700 psi doesn't seem that far-fetched at all. Not common, but not at all unheard of. OTOH, an AI transmitter that reads accurately throughout a dive and then just "sticks" at a number - or reads progressively more "off" during a dive seems much less likely and, in fact, is something I have not personally heard of (though my experience is fairly limited).

 

[dmaziuk]

An SPG that "sticks" at, for example, 700 psi doesn't seem that far-fetched at all. Not common, but not at all unheard of. OTOH, an AI transmitter that reads accurately throughout a dive and then just "sticks" at a number - or reads progressively more "off" during a dive seems much less likely and, in fact, is something I have not personally heard of (though my experience is fairly limited).

Also the technology hasn't been around for quite as long as bourdon tubes.

I can't think of a technical reason why a pressure sensor couldn't fail like that: the part that bends under pressure still offers non-linear resistance, just like copper tube, and material fatigue could still make it "not unbend all the way" at low pressure end of the bend. There is less material in there to go wrong, that might be the only difference.

Not that this makes it any worse than the copper tube -- in fact, they usually come spec'ed for 1M work cycles min. and +/- 1% accuracy over the operating range. I wonder what the specs on brass and glass SPGs are.

 

[wetb4igetinthewater]

So, if an SPG blows out but you still have AI, you would just shut down the post with the SPG and continue the dive with whatever gas/cylinder remains available? (presuming a rec dive) I did previously specify "if an SPG blows out".

For many things in scuba, redundancy clearly makes you safer. For tank pressure, it seems like it's the opposite. 2 transmitter/SPG devices just means double the chances for a failure that results in gas loss... with the SPG being far more likely to lose gas if it fails versus the chance of losing as with an AI failure. And, any failure that doesn't involve losing gas just seems like, one, very unlikely, and two, no big deal. And, actually, the worst case scenario for a failure of an AI/SPG that doesn't involve losing gas is a failure where it reads higher than your actual tank pressure. And that seems MUCH more likely to happen with an SPG than with AI. An SPG that "sticks" at, for example, 700 psi doesn't seem that far-fetched at all. Not common, but not at all unheard of. OTOH, an AI transmitter that reads accurately throughout a dive and then just "sticks" at a number - or reads progressively more "off" during a dive seems much less likely and, in fact, is something I have not personally heard of (though my experience is fairly limited).

I was more reading the issue with the SPG that it fails to read the proper depth. If it failed and was leaking air, I would not continue that dive, as that is a whole 'nother matter.

Now again, I am dive more and more sidemount. If an SPG was to "blow" and leak gas, I would terminate the dive. If the needle was just stuck, I'd keep diving.

 

[kelemvor]

Thinking about the perdix AI. I get it that two is one and one is none when it comes to equipment failures, but what is the consensus for warm rec diving only?

When I first went AI I had a backup SPG on my regulator. Now I do as tbone suggests and just keep the backup in my SAD bag. As long as you've been watching things, you should have a good idea how much air you've got in the tank in case it does crap out. Obviously you end the dive if you realize your AI has crapped out. Unless you're cutting things far too close, it shouldn't be anything more than an annoyance.

 

[stuartv]

Also the technology hasn't been around for quite as long as bourdon tubes.

I can't think of a technical reason why a pressure sensor couldn't fail like that: the part that bends under pressure still offers non-linear resistance, just like copper tube, and material fatigue could still make it "not unbend all the way" at low pressure end of the bend. There is less material in there to go wrong, that might be the only difference.

Not that this makes it any worse than the copper tube -- in fact, they usually come spec'ed for 1M work cycles min. and +/- 1% accuracy over the operating range. I wonder what the specs on brass and glass SPGs are.

I could totally be wrong, but from anecdotes I've read I have the impression the more likely reason for a mechanical SPG to read falsely high would be contamination inside the gauge mechanism. Not a failure of the Bourdon tube itself. More like a salt crystal or piece of grit happens to get inside and causes it to mechanically bind. That kind of failure or any type that is analogous to that seems pretty unlikely to happen to a pressure transducer.

 

[stuartv]

Side note: When I only had 1 transmitter, I would use it on the right post of my doubles and I still had a mechanical SPG on my left post. Now that I have a Perdix AI and 2 transmitters, and have been dabbling in sidemount, I have removed the SPG from the left post reg and have a second transmitter there. So, when I'm diving back mount doubles, I have redundancy, by having both transmitters, with both showing on my Perdix. In back mount doubles, they should always read the same. If they don't, that would tell me that I have failed to make sure my isolator is open. Or that one is bad. If one just dies, I still have the other one and I'm no worse off than a "classic" rig which would only have 1 SPG on the left post and none on the right. Plus, if my left post were to ever blow a HP seat or other 1st stage or 2nd stage failure, I can shut down the post and still know what my cylinder pressure is, from the AI on my right post. And all with less failure points than if I just had 1 regular SPG.

 

[Dan]

I could totally be wrong, but from anecdotes I've read I have the impression the more likely reason for a mechanical SPG to read falsely high would be contamination inside the gauge mechanism. Not a failure of the Bourdon tube itself. More like a salt crystal or piece of grit happens to get inside and causes it to mechanically bind. That kind of failure or any type that is analogous to that seems pretty unlikely to happen to a pressure transducer.

How likely a grit will get into the SPG? Have you read about it?

 

[Dan]

Side note: When I only had 1 transmitter, I would use it on the right post of my doubles and I still had a mechanical SPG on my left post. Now that I have a Perdix AI and 2 transmitters, and have been dabbling in sidemount, I have removed the SPG from the left post reg and have a second transmitter there. So, when I'm diving back mount doubles, I have redundancy, by having both transmitters, with both showing on my Perdix. In back mount doubles, they should always read the same. If they don't, that would tell me that I have failed to make sure my isolator is open. Or that one is bad. If one just dies, I still have the other one and I'm no worse off than a "classic" rig which would only have 1 SPG on the left post and none on the right. Plus, if my left post were to ever blow a HP seat or other 1st stage or 2nd stage failure, I can shut down the post and still know what my cylinder pressure is, from the AI on my right post. And all with less failure points than if I just had 1 regular SPG.

What less failure points are you referring to?

 

[kelemvor]

Side note: When I only had 1 transmitter, I would use it on the right post of my doubles and I still had a mechanical SPG on my left post. Now that I have a Perdix AI and 2 transmitters, and have been dabbling in sidemount, I have removed the SPG from the left post reg and have a second transmitter there. So, when I'm diving back mount doubles, I have redundancy, by having both transmitters, with both showing on my Perdix. In back mount doubles, they should always read the same. If they don't, that would tell me that I have failed to make sure my isolator is open. Or that one is bad. If one just dies, I still have the other one and I'm no worse off than a "classic" rig which would only have 1 SPG on the left post and none on the right. Plus, if my left post were to ever blow a HP seat or other 1st stage or 2nd stage failure, I can shut down the post and still know what my cylinder pressure is, from the AI on my right post. And all with less failure points than if I just had 1 regular SPG.

That's a cool use case. I wondered about multiple transmitters when I started in on technical diving.

Another use case I have for transmitters that I love is single tank recreational diving. When you combine it with an "air2" type device you only have two hoses coming off your first stage. It just makes everything a little easier to manage. Maybe manage is the wrong word because it's not particularly difficult to dive with a drysuit inflator, spg, and regular spg. Maybe I'll go with clean and enjoyable or streamlined or something like that.