Why I am getting a backup pressure gauge for my hoseless air integrated wrist dive computer.

[Joneill]

OK, just so we are all on the same page: 'what' don't you have before you splash? And, 'who' won't give you more? I simply disagree that 'the fact is I can't'. Even if a dive plan changes under water, there should be a plan to be changed, developed in advance.

For example, do you approximately know your SAC / RMV? That is a straightforward matter to calculate, before you arrive at whatever unknown site you may be diving, before you even sign up for the boat trip. And, I am not disagreeing with monitoring your gas, nor that gas = fact. But, I am saying that this monitoring should be a form of redundancy, used to confirm whatever plan (time) you had before the dive.

I have an AL80 (or sometimes an AL100) filled to a noted psi. That is what I have before I splash and cannot get any more of from the boat crew - so my dive "plan" ultimately is to manage my time at depth to avoid deco and not run out of the air in that tank.

I have a decent sense of RMV and SAC from calculating it over my 50 dives. But that changes dive-to-dive based on a lot of factors - so unless I'm diving a very similar profile (again, not really known definitively in advance - it's somewhat of an educated guess) a plan based on assumptions could be way off.

My point is that with today's tools that provide you with realtime feedback, a recreational NDL diver does not need to have a incredibly detailed plan in advance. As long as you understand the impacts of depth and workload on gas consumption , know how much air you are starting with and align on turn and ascent pressures in advance, you are pretty set. After that, you really need to monitor your air and depth (more often when new and less often as you gain experience) to decide when to turn and when to ascend.

One could calculate expected turn times based on depth and SAC but that is ultimately just an indirect way to predict your remaining air - so why not just monitor the thing that drives your behavior directly - your air supply.

For tech diving, my understanding is that you determine what you want to achieve, make a plan based on the planned profile and then use that plan to determine how much gas you need to execute that plan (including deco) and ensure you have that much gas (and the right mixtures) plus a reserve on hand - either on you or staged at various points in the dive.

For single tank, tropical diving, you just don't need to "plan" gas like that. Have a basic plan and then monitor how you are going through your gas.

 

[joe10540]

I have an AL80 (or sometime and AL100) filled to a noted psi. The is what I have before I splash and cannot get any more of from the boat crew - so my dive "plan" ultimately is to manage my time at depth to avoid deco and not run out of the air in that tank.

I have a decent sense of RMV and SAC from calculating it over my 50 dives. But that changes dive-to-dive based on a lot of factors - so unless I'm diving a very similar profile (again, not really known definitively in advance - it's somewhat of an educated guess) a plan based on assumptions could be way off.

My point is that with today's tools that provide you with realtime feedback, a recreational NDL diver does not need to have a incredibly detailed plan in advance. As long as you understand the impacts of depth and workload on gas consumption , know how much air you are starting with and align on turn and ascent pressures in advance, you are pretty set. After that, you really need to monitor your air and depth (more often when new and less often as you gain experience) to decide when to turn and when to ascend.

One could calculate expected turn times based on depth and SAC but but that is ultimately just an indirect way to predict your remaining air - so why not just monitor the thing that drives your behavior directly - your air supply.

For tech diving, my understanding is that you determine what you want to achieve, make a plan based on the planned profile and then use that plan to determine how much gas you need to execute that plan (including deck) and ensure you have that much gas (and the right mixtures) plus a reserve on hand - either on you or staged at various points in the dive.

For single tank, tropical diving, you just done't need to "plan" gas like that. Have a basic plan and then monitor how you are going through your gas.

A baseline gas plan would be to say ok the bottom is at 80 feet and I need 200psi to make a normal safe assent from that depth and make a 3 minute safety stop let's double that for stress caused by an emergency and then add whatever my buddy would need. In this case we can say that 1000psi would be our absolute turn pressure 1500 would get us back on the boat in an emergency with a reserve. Based on a 3000psi fill that gives me 1500psi based on 2.5xSAC I have a rough idea of how long I should have available for sight seeing before making my ascent. 5 minute gas plan and that's always open for adjustments based on higher than expected air consumption or lower than expected air consumption but gives a more concrete plan than first one to hit 1000 calls the dive

 

[tbone1004]

@barth we'll have to agree to disagree on that. Using WAI for dive time remaining? not interested. Using it to track things like consumption trends in varying dive conditions and getting resolution that's better than the entire dive? that's nifty. Shearwater now has the first computer with WAI that I'd actually dive on a regular basis, but unfortunately for the dives I do, getting 4-5 transmitters is a bit impractical. I won't invest yet, but I have no issues with it.
If I was doing dives that were doubles+one deco bottle, or single tank diving, I have to look at the following
Shearwater Perdix-$850
Shearwater Perdix AI w/ transmitter-$1175

That's a $325 premium for something nifty. You still need an analog SPG and hose so you can't deduct that from the delta, but I'd seriously consider paying it for the convenience.


to your argument, if you plan your dive properly, you don't need anything more than a J-valve...

to add to what @Colliam7 wrote, if you have avg depth indicated on your computer in the dive logs, then you can use that to get an average SAC for the whole dive which is how I track it.

@Joneill read what I wrote in the post that I linked to. The argument isn't necessarily to have a turn time based on the conservative numbers, it's to have an understanding of how long you actually have, and a contingent time.
In that post it talks about estimating your bottom time based on reality, then giving a conservative time value in the event of an AI failure so you don't have to abort the dive. Your example of a 60 minute dive doesn't do much for you if the gas isn't going to last long.

the issue with analog spg's in particular when you don't do this kind of gas planning is when people get surprised by how little gas they have left. having the estimated dive times based on your conservative plan means that if you know that conservatively you have 20 minutes, realistically you probably have 25mins with rock bottom, and sh!t hits the fan at 33mins. When you glance down and see the time at 18 minutes, you verify your SPG and make sure that your gas is moving as planned. You check about 5 minutes later and verify again then turn the dive if all is well. What you shouldn't be doing is checking your gas every 3 minutes while you're diving

 

[Joneill]

@barth we'll have to agree to disagree on that. Using WAI for dive time remaining? not interested. Using it to track things like consumption trends in varying dive conditions and getting resolution that's better than the entire dive? that's nifty. Shearwater now has the first computer with WAI that I'd actually dive on a regular basis, but unfortunately for the dives I do, getting 4-5 transmitters is a bit impractical. I won't invest yet, but I have no issues with it.
If I was doing dives that were doubles+one deco bottle, or single tank diving, I have to look at the following
Shearwater Perdix-$850
Shearwater Perdix AI w/ transmitter-$1175

That's a $325 premium for something nifty. You still need an analog SPG and hose so you can't deduct that from the delta, but I'd seriously consider paying it for the convenience.


to your argument, if you plan your dive properly, you don't need anything more than a J-valve...

to add to what @Colliam7 wrote, if you have avg depth indicated on your computer in the dive logs, then you can use that to get an average SAC for the whole dive which is how I track it.

@Joneill read what I wrote in the post that I linked to. The argument isn't necessarily to have a turn time based on the conservative numbers, it's to have an understanding of how long you actually have, and a contingent time.
In that post it talks about estimating your bottom time based on reality, then giving a conservative time value in the event of an AI failure so you don't have to abort the dive. Your example of a 60 minute dive doesn't do much for you if the gas isn't going to last long.

the issue with analog spg's in particular when you don't do this kind of gas planning is when people get surprised by how little gas they have left. having the estimated dive times based on your conservative plan means that if you know that conservatively you have 20 minutes, realistically you probably have 25mins with rock bottom, and sh!t hits the fan at 33mins. When you glance down and see the time at 18 minutes, you verify your SPG and make sure that your gas is moving as planned. You check about 5 minutes later and verify again then turn the dive if all is well. What you shouldn't be doing is checking your gas every 3 minutes while you're diving

I did not give an example of a 60 minute dive - I said the tropical boat dives usually instruct to keep to 60 minute max dive time before they want everyone back on the boat. However, depending on max depth, profile/avg depth, current, etc., I've actually done anywhere from 42 to 64 minute dives based on monitoring how my gas consumption tracks during the dive.

With the type of dives I am doing and a decent understanding of my average consumption, I really see no need to do the kind of time based planning you are referring to. I have a general plan for the dive, which will always be a little different (or a lot) in reality - almost guaranteed. As long as I then periodically check my air, I'm good. Based on what my gauge is showing me in real time during on the actual dive, I adjust the time at which I turn or/or ascend.

That is why I like the backup analog SPG to my Perdix AI. In the VERY unlikely scenario that I lost both my AI and SPG, I'd just abort the dive. I would not fee comfortable continuing based on what I estimated/ guessed my dive time might be - not worth it to me. Of course, that is my approach - your mileage may vary.

BTW - I don't check my air every 3 minutes, but honesty not sure why that would be a problem? It might even be a way to find out you have a leak (assuming you have unattentive dive buddies of course) or are huffing through your gas faster than you expected for some reason that might not be obvious to you.

One other thing I like is the SAC display on the Perdix AI. On a recent dive in Aruba, I glanced down and noticed it was 20-22. I then consciously slowed it down and relaxed my breathing and a few minutes later it was down to 13. I also could see the impact of an unexpected current shift. The realtime feedback is good for helping you be more aware of how different factors impact your consumption. I'm not checking it constantly, but the occasional glance gives good feedback and an opportunity to react to it.

 

[The Chairman]

I dive either an analog gauge or a wireless. When I side mount, I use transmitters on each tank. I don't like danglies. Have I called a dive for a failed transmitter? Sure. I now use rechargeable batteries and change them daily. I would love to have them on my rebreather.

Having both is like seeing that guy wearing a belt and suspenders. Kind of makes me snicker a bit.

 

[buddhasummer]

I've read people say when traveling take an SPG and leave in in your bag and if need it then attach it. If you're carrying it anyway why not just attach it, assuming you have two ports, what's the problem? I really don't see what the issue is. If one feels more comfortable having both then go for it, it's not like you're increasing/reducing drag by using/not using one. It's an SPG not an elephant. I would imagine having an AI issue is more likely than an SPG issue, battery, rough handing, lost signal etc etc.

Personally I'll continue to use both if live aboard diving. Each to their own. Like I said before in almost twenty years of diving I've never had an SPG failure, sure I've had to replace a spool or two but that's always been caught before the dive, lucky I guess.

I didn't buy AI to replace an SPG, I bought one because I like all the info in one place and I like new toys. Of course YMMV.

As with most things equipment related it's about personal preference and what makes you feel more comfortable/happy.

 

[Cascade]

I dive with transmitters 4 years, used 3 air integrated dive computers — Suunto, Aeris, Shearwater. Transmitter is for convenience, but it is not quite reliable piece of equipment. I had a lot of cases with lost link, so, I can't rely on transmitter as an only pressure gauge.
This is why I always use analog SPG, clipped to left hip D-ring and don't feel any drag or other inconvenience.

 

[Kharon]

... It's exactly the reason why I don't care much about computer calculated time remaining and so on. I'am more than capable of doing (a better/safer) job myself. Safer in calculating not only how much air I need in emergency, but also take in account air sharing. Training takes away the need to rely on computer based info. The only thing you really need is an SPG to know how much air is left.

I think you are missing something here. On my last trip to Bonaire I dove (once) with a buddy who wanted to go to the second reef. I have both an AI computer and SPG. Both told me I had a ton of air left. However the computer told me that I had only 6 minutes of NDL at that depth. There is no way to estimate that. If I was simply going by how much air I had left I would have gotten my butt into a real bind.

 

[scubadada]

Hi @idocsteve

I've been diving a hoseless AI, Oceanic VT3, since 2010, 875 dives. Mainly, to avoid losing a dive or interfering with a series of dives, I have always dived with a backup computer, an Oceanic Geo 2, or, more recently, a Dive Rite Nitek Q, and a backup SPG. I dive the Geo 2 when I want the NDLs to match exactly, such as a liveaboard week. I dive the Nitex Q at other times to gain experience with Buhlmann ZH-L16C with GF.

For 6 1/2 years I never needed the backup computer or the SPG. About 6 mionths ago I started a week of diving in SE Florida without checking my primary computer battery, and it was dead. I did the 4 dives that day using my backup computer and SPG. A simple battery replacement fixed that.

Just this week, on my first dive, I was not getting cylinder pressure on my AI. Turns out my 7 year old transmitter has failed. So this time, I had 2 working computers but needed my SPG for my gas supply for the 4 dives of the day. I replaced the transmitter with another one from the family equipment closet and dived for the rest of the week without issue. I'll be communicating with Oceanic regarding any options I may have, I have had good luck with them in the relatively distant past.

Neither of these problems was very serious but it could have been more of a problem if I had been on my 2 weeks on the Red Sea or on my recent trip to Cocos. For the longest time, I thought I was being overly cautious, it ended up paying dividends

Good diving, Craig

 

[RayfromTX]

Nobody is saying to leave the spg at home, just on the boat in your kit with the allen wrench and the wrench required to add it to your reg if needed. That would have worked in each of the instances you mentioned and that's 875 dives without the minor annoyance of another hose and a gauge clipped off. Again, as always, each individuals choice but it is a story like yours that makes me think leaving it on the boat is just fine in open water if there are no other issues that might require it.