Stage bottles transmitter plus SPG setup...

[rddvet]

I'm actually not a fan of AI ini the overhead. If you're talkiing ow stage then maybe. I've had a couple of oring failures on the ai unit on my rebreather. A stage bottle is going to be manhandled way more than your bm, sm primaries, or your onboard rebreather oxygen. I think AI on a stage bottle is asking for an issue. You can do your sac calculations on your own very easily. As said before, an spg is the bwst way to get a veriification you're using the right reg. I don't want to sit and wait 5 seconds (yes, it's not long but longer than my ADHD would like).
If you're going to do it, then at least direct connect to your first stage as opposed to being on a hose where you're addinig more orings. All of the failures I've seen have been the oring on the transmitter side not the hose, so I think a failure with it direct mounted is ineviitable as well. I'm not old school or against technology. I just don't see the need for ai on open circuit gear in the overhead when it can increase risks to add something that's not really needed. On my ccr I added one to the oxygen because it was either put a long hose with an spg which iis an annoyance or add the ai.

 

[stuartv]

It seems very wrong to use a transmitter on a stage/deco bottle UNLESS you have a normal SPG attached.

Reasoning: stage/deco bottles may need to be handed off to other divers. The gauges are clearly visible in front of you (as in a short 15cm/6' HP hose doubled back). Secondly your whole dive plan is based around having enough gas for deco, with reserves for team member/buddy should that "one" failure occur.

You've planned out all your gas consumption and contingencies ahead of time. What scenario might occur where being able to see an SPG on your deco/stage/BO cylinder might change what you are doing? I.e. something goes wrong. You're now into a contingency plan. You can see an SPG on the cylinder you're now using. What do you do? Same scenario except you can't see an SPG on that cylinder. What are you doing differently?

I'm not trying to be contentious. I'm trying to understand what scenarios might occur where having an SPG (versus AI or a button gauge or AI and a button gauge) would make a difference regarding how the scenario plays out.

Personally, I don't use AI on my BO or deco bottles. I just don't feel any need. If I wanted to know my exact consumption of my deco or BO gas, I would note the cylinder pressures before and after the dive and then use my dive profile data - including the data on when I did my gas switches - to calculate it (or, really, plug the numbers into Subsurface and let it tell me). Otherwise, I planned it out ahead of time, I start with full cylinders, and I either have enough gas or I don't.

My thinking is that being able to look at an SPG during the dive is not going to change anything if I screwed up and didn't bring enough gas. If I run out in one cylinder, I'll switch to some other cylinder. If I have to switch to a way-too-hot mix and make a very rapid ascent to get to the MOD of that gas, I'll do that before I try to breathe water. If I run out of all my gas in all my cylinders, well, by the time I'm breathing on my last cylinder that has any gas in it, I'm going to have gotten shallow enough that, if it runs out too, I can get to the surface and deal with possibly being bent there.

In short, I think having an SPG on a cylinder on a tech dive seems far less important that having an SPG on a cylinder being used on a recreational dive. If I planned my tech dive correctly, and dive my plan, why do I even need ANY SPGs (beyond one on my bottom gas, to know if I'm at my turn pressure)? I'm not saying to dive without SPGs. I'm just saying that the importance some people place on a full-sized SPG and also the importance some people place on having a redundant SPG (if using AI) seems out of proportion. A good quality button gauge on deco/BO cylinders seems adequate for most all scenarios.

However, I am VERY open to explanation of why an SPG is needed. An actual example, with details, would be very helpful, in that case.

 

[Wibble]

However, I am VERY open to explanation of why an SPG is needed. An actual example, with details, would be very helpful, in that case.

Example; rebreather deep diving with bailouts. The general challenge is being able to carry sufficient bailout to "safely" return to the surface. Rebreathers can last a lot longer underwater, so bottom times are often pushed -- to the extent that some people dive what they call "Alpinist".

Sucking on bailout deep when under stress can easily deplete your gas (at 70m/8ATA an ali80's only got minutes of gas) and you need to make decisions should you run short before your switch depth. Similarly for deco bailout; do you start mitigation procedures or just chill out? (e.g. changing GF-hi, searching for additional gas, ascending to 3m/10' to minimise depth, applying zen to reduce your breathing, inventing a new God to prey to...)

The other use of an SPG is the simple checks that it's on and working. OK, a bit of a safety blanket, but I like to know that my stageS are pressurised even if turned off (as in rich gasses).

 

[rjack321]

I'm with @Wibble here. I wouldnt even try to use AI on a 3rd cylinder. Partly because you're diving 1/2s+200 on the stage but something 1/4ths on the backgas (depending on their size - you need to take away the 35cf used in the stage so you dont break 1/3rds). And you'll be dropping and picking up the stage and/or potentially handing it off. Just use a full sized gauge on the stage and leave the AI on your personal cylinders. Simpler is better.

 

[rddvet]

I'm with @Wibble here. I wouldnt even try to use AI on a 3rd cylinder. Partly because you're diving 1/2s+200 on the stage but something 1/4ths on the backgas (depending on their size - you need to take away the 35cf used in the stage so you dont break 1/3rds). And you'll be dropping and picking up the stage and/or potentially handing it off. Just use a full sized gauge on the stage and leave the AI on your personal cylinders. Simpler is better.

Completely agree. The scariest thing is that most people aren't taught to calculate stage gas used into how much backgas is usable. I've asked a ton of people and I commonly get: 1) I was taught 1/2 + 200 plus 1/3 of back gas (or conservative 1/3s) or 2) 1/3s on stage and conservative 1/3s on backgas. II would say the number of people that actually have told me they were taught to calculate gas consumed from their stage when calculatiing back gas is riidiculously low. That's a whole different conversation, but it's something I find scary.

 

[Manatee Diver]

Completely agree. The scariest thing is that most people aren't taught to calculate stage gas used into how much backgas is usable. I've asked a ton of people and I commonly get: 1) I was taught 1/2 + 200 plus 1/3 of back gas (or conservative 1/3s) or 2) 1/3s on stage and conservative 1/3s on backgas. II would say the number of people that actually have told me they were taught to calculate gas consumed from their stage when calculatiing back gas is riidiculously low. That's a whole different conversation, but it's something I find scary.

I normally like to leave things to flow except to answer direct questions, but I am slightly confused.

The way I was taught stage planning for the 1/2+2. Is to take how much gas that would be, so assuming full,
1300 psi
Which is 13* 2.5 = 33cuft
33cuft / 6.4 = 5.1
Round that up to 600 psi.

So if you are starting with 3,600psi, you have 3,000psi to work, or 1,000psi if you are diving hard thirds.

 

[Wstern5]

You've planned out all your gas consumption and contingencies ahead of time. What scenario might occur where being able to see an SPG on your deco/stage/BO cylinder might change what you are doing? I.e. something goes wrong. You're now into a contingency plan. You can see an SPG on the cylinder you're now using. What do you do? Same scenario except you can't see an SPG on that cylinder. What are you doing differently?

I'm not trying to be contentious. I'm trying to understand what scenarios might occur where having an SPG (versus AI or a button gauge or AI and a button gauge) would make a difference regarding how the scenario plays out.

Personally, I don't use AI on my BO or deco bottles. I just don't feel any need. If I wanted to know my exact consumption of my deco or BO gas, I would note the cylinder pressures before and after the dive and then use my dive profile data - including the data on when I did my gas switches - to calculate it (or, really, plug the numbers into Subsurface and let it tell me). Otherwise, I planned it out ahead of time, I start with full cylinders, and I either have enough gas or I don't.

My thinking is that being able to look at an SPG during the dive is not going to change anything if I screwed up and didn't bring enough gas. If I run out in one cylinder, I'll switch to some other cylinder. If I have to switch to a way-too-hot mix and make a very rapid ascent to get to the MOD of that gas, I'll do that before I try to breathe water. If I run out of all my gas in all my cylinders, well, by the time I'm breathing on my last cylinder that has any gas in it, I'm going to have gotten shallow enough that, if it runs out too, I can get to the surface and deal with possibly being bent there.

In short, I think having an SPG on a cylinder on a tech dive seems far less important that having an SPG on a cylinder being used on a recreational dive. If I planned my tech dive correctly, and dive my plan, why do I even need ANY SPGs (beyond one on my bottom gas, to know if I'm at my turn pressure)? I'm not saying to dive without SPGs. I'm just saying that the importance some people place on a full-sized SPG and also the importance some people place on having a redundant SPG (if using AI) seems out of proportion. A good quality button gauge on deco/BO cylinders seems adequate for most all scenarios.

However, I am VERY open to explanation of why an SPG is needed. An actual example, with details, would be very helpful, in that case.

All team members need to know what resources are available so the best decisions can be made.

If you are by yourself just trying to get to the surface by any means necessary then you’re right, it doesn’t really matter.

 

[stuartv]

So far, nobody has given a specific, detailed example, of how having a full-sized SPG on their deco or BO would matter. Let me give one to show what I mean:

I'm doing a dive to 200', carrying a cylinder of 50% and a cylinder of 100% for deco.

Just when it's time for me to start my ascent, the tank valve O-ring on my O2 blow out and I lose all my O2.

I ascend to 70' and switch to my 50%, per my plan.

I do my deco stops up to where I am at 20'. I don't have O2 to switch to, so I keep breathing my 50%. My buddy switches to his O2.

Depending on how I planned my dive, I might have planned Lost Gas so that I can finish all my deco on 50%, or I might have planned it knowing that I would have to get gas from my buddy in this scenario.

In the former case, I deco out on 50%. In the latter case, I keep breathing my 50%. If it runs out, I switch to back gas briefly and then swap 50% bottles with my buddy and then switch to breathing that. When my buddy is clear on deco, he gives me his O2 and I finish my deco and we get out.

Nowhere in there does having an SPG or not on my deco bottles make any difference.

Realistically, I wouldn't plan the dive this way (as it's less conservative with respect to deco), but if it actually happened, I would probably immediately go to riding my GF99 at somewhere between 85 and 95 to get as shallow as possible as quickly as possible - to maximize how long my gas will last - and then stay at my last stop as long as possible (or until my SurfGF came down to the low side of what I normally look for before getting out).

What's an example, with details, where having an SPG would (or could) make a difference to how you finish your dive? What scenario could happen where you would look at a deco/BO SPG and say "if it's above X, we'll do this but if it's X or less, we'll do this other thing instead"?

 

[PfcAJ]

I normally like to leave things to flow except to answer direct questions, but I am slightly confused.

The way I was taught stage planning for the 1/2+2. Is to take how much gas that would be, so assuming full,
1300 psi
Which is 13* 2.5 = 33cuft
33cuft / 6.4 = 5.1
Round that up to 600 psi.

So if you are starting with 3,600psi, you have 3,000psi to work, or 1,000psi if you are diving hard thirds.

What he’s talking about is if you dive half + 200, some divers don’t adjust their backgas usage.

They should.

If you dive 1/2+200 (which I do believe to be the best stage technique), you need to reduce penetration gas on your back so you don’t exceed thirds of your total starting volume.

 

[PfcAJ]

Having an SPG gives you info if/when your computer fails.

too many eggs in one basket not having an SPG. And if I’m going to take an SPG, I don’t need another way to view the contents of the cylinder.