SPG or not

[Pearlman]

I actually once had a dive shop insist that they won’t let me dive without SPG - I had to lift my SPG to their face and show them I always have both setup as standard configuration.

 

[aedwards92]

I have my dive computer with AI and I usually have an SPG since I rent my gear and my LDS provides one. I'll occasionally check the SPG to see if it matches my computer but as I've become more comfortable with AI I've found I check it less often. But overall, it can't hurt to have a backup right?

 

[Badger7]

I'll occasionally check the SPG to see if it matches my computer but as I've become more comfortable with AI I've found I check it less often. But overall, it can't hurt to have a backup right?

I share that opinion. I've got an SPG, I've got a hose, together that's negligible weight and clipped out of the way on the hip. I can't think of any good reason I would leave it home or in the truck rather than take it with on the dive. If what you're seeing on your computer from AI doesn't feel right, being able to immediately compare sounds better than not.

 

[CuzzA]

I can't think of any good reason I would leave it

One less failure point.
One less hose to replace every few years.
One less thing clipped off.

When they do fail, they fail the wrong because they stick, indicating you have more gas than you do. AI fails in a safe manner.

If you don't trust the air integration, then why use it? Stick to an SPG.

My fins could break, I don't carry an extra. My mask could break, my wetsuit, et cetera. I don't carry extras.

If I were exclusively a solo diver I wouldn't carry an extra second stage either. Redundant first stage makes the most sense.

If your SPG gave a you a drastically different reading than the AI, what would you do?

 

[scubadada]

I have used my SPG backup a few times with either one or two AI computers. I will continue to use it, a small thing on my left chest D-ring. I have never missed a dive or a series of dives due to a computer or transmitter related issue.

 

[inquis]

I can't think of any good reason I would leave it home or in the truck rather than take it with on the dive

Running both allows the dive to continue when then transmitter battery is totally dead or the antenna fails. This is pretty low on the probability scale. (Usually there is some advance notice before total failure, allowing for corrective action between dives.) However, I'm not disputing there is a benefit.

OTOH, the odds of a dive-ending gas leak from either SPG or transmitter are obviously higher than from the transmitter alone.

In my view, the likelihood that the dive ends goes up more than the likelihood I can continue with an AI failure. In other words, the juice ain't worth the squeeze. YMMV, of course, but that's my reasoning.

 

[Ontwreckdiver]

I haven't used a SPG in about 20 years.

 

[Umuntu]

Running both allows the dive to continue when then transmitter battery is totally dead or the antenna fails. This is pretty low on the probability scale. (Usually there is some advance notice before total failure, allowing for corrective action between dives.) However, I'm not disputing there is a benefit.

OTOH, the odds of a dive-ending gas leak from either SPG or transmitter are obviously higher than from the transmitter alone.

In my view, the likelihood that the dive ends goes up more than the likelihood I can continue with an AI failure. In other words, the juice ain't worth the squeeze. YMMV, of course, but that's my reasoning.

When there’s a leak from the SPG it’s usually at the HP Spool O-rings … and it’s typically a tiny stream of bubbles that have no impact on gas usage for a NDL open water dive. Also, your AI transmitter has the same vulnerability and greater vulnerabilities as an SPG, judging from the recalls.

The HP system of your first stage and components are designed for pressure transmission, unlike the LP system which is designed for high volume air transmission.

If you look inside the HP port of a modern first stage you will see only a pin prick of a hole, designed to transmit pressure, but not a high volume of air.

 

[LI-er]

If you don't trust the air integration, then why use it?

Because it's convenient. Even if the signal drops periodically durning the dive necessitating a quick glance at the backup gauge.

 

[aedwards92]

One less failure point.
One less hose to replace every few years.
One less thing clipped off.

When they do fail, they fail the wrong because they stick, indicating you have more gas than you do. AI fails in a safe manner.

If you don't trust the air integration, then why use it? Stick to an SPG.

My fins could break, I don't carry an extra. My mask could break, my wetsuit, et cetera. I don't carry extras.

If I were exclusively a solo diver I wouldn't carry an extra second stage either. Redundant first stage makes the most sense.

If your SPG gave a you a drastically different reading than the AI, what would you do?

In my case I've only recently started to use AI, so still getting used to it to the point I trust it. As for replacing the hose, I rent so I don't have to worry about that.