DIR mental math for gas consumption

[dave22387]

Quick question really aimed at you DIR guys... I really like the concept of your brain being your primary SPG. I already kind of do this, meaning when I look at my guage I expect to see around a certain number. This basically comes from experience.

My question is since it is specifically taught to GUE divers how do you guys do it? Is it just experience or is there math involved? The only way I can think of is multiplying sac/rmv by ATA. I really don't want to do that underwater if I don't have to. How is it taught to you guys? I consider myself pretty good at mental math considering I have to do it for work but I'd rather not underwater if there is some rule of thumb etc.

Someone school me please and thanks!

 

[BDSC]

I really like the concept of your brain being your primary SPG.

Why?

I suspect your SPG or computer is far more reliable than your brain during a dive.

 

[seeker242]

The only way I can think of is multiplying sac/rmv by ATA. I really don't want to do that underwater if I don't have to. How is it taught to you guys?

Yes, it's RMV x ATA. But you generally don't do it underwater. You plan what depth you will be at ahead of time and follow that plans depth.

 

[dave22387]

Why?

I suspect your SPG or computer is far more reliable than your brain during a dive.

So that you stay within your dive plan and can determine early and quickly if things are going as planned. At this point in my dive I should have 2600psi... Crap I have 2200 what's wrong here.

Best way to deal with an emergency is to avoid it...

 

[dave22387]

Yes, it's RMV x ATA. But you generally don't do it underwater. You plan what depth you will be at ahead of time and follow that plans depth.

Thanks for the info. So you basically figure out your "equation" as part of your plan and just constantly do quick math at depth. Makes sense thank you!

 

[tbone1004]

@dave22387 many of us teach it at our basic classes outside of GUE.

So here's roughly how it works.
My SAC is roughly .6cfm and I'm diving an AL80. .6cf in an AL80 is roughly 25psi, from .6*3000/77.4
I'm diving on a reef to say 70ft which I know I'll be a few feet off the bottom so call it 3ata. 3*25=75psi/min expected consumption.
I have a full fill on my AL80 of 3200psi and I look down and see that my computer says 13 minutes
Quick mental math a la common core style says that 10*75=750, and 3 is roughly a third of 10, so a third of 750 is 250, total of 1000.
3200-1000=2200 so when I look at my gauge I should see roughly 2200psi. If it's higher, I should think back on the last 15 minutes about what may have caused me to be more excited which yielded a higher SAC rate, a bit deeper because I was hunting for a shell or lobster, etc. If it's a bit lower, I need to be thinking about whether I was abnormally calm, if it was a drift dive, I was hovering higher over the reef for whatever reason, etc etc.

Important part of that is converting your SAC rate from cfm to psim for the specific tank that you are diving, and then start thinking about your depth in ata's for easy math.

 

[_Ralph]

..
My question is since it is specifically taught to GUE divers how do you guys do it?
...

Take fundies and all the secrets will be revealed!

In short, you’re already thinking how to do it.

_R

 

[elgoog]

The way we did it was to calculate our RMV over the course of a few dives and, based on our tank factors, convert it to psi used every 5 min at 30ft (2 ata). Then, using the average depth from your dive computer at any point during the dive, you can get an estimate of what your and your buddies' SPGs reading should be.

These are all approximates due to rounding off to make underwater mental math easy but I found it pretty useful to use on almost every dive I do. I think the 5min thing makes the math easier than 1min but, obviously, more of an approximation.

For me, mostly it's an indicator that I'm working hard for some reason without realizing it and try to figure out why. Also, it's pretty nice to have a general idea of where your buddies' are going to be and just confirming that when you ask them.

Honestly, I've gotten used to it enough that it takes me just a few seconds. Also honestly, it's just as easy to unclip your SPG to look at it, but it gives me the warm and fuzzies when they match because it's an indicator that everything is going as planned.

 

[dave22387]

Take fundies and all the secrets will be revealed!

In short, you’re already thinking how to do it.

_R

Tbh I took fundies a long time ago when I was dabbling into dir. Granted I tightened my skills a good bit and its a great course but decided to not dive dir for personal reasons. That being said, I do not remember the instructor going over the how to do mental gas monitoring, just that your head is your primary SPG. Like I said though this was a long time ago so I may have forgotten...

 

[dave22387]

I know my rmv and use it for initial gas planning and cylinder selection but never really used it consciously "during" the dive. There's alot of things I like about DIR and have adopted, and some that I don't like. This is one of those things I would like to implement more to increase safety and the situational awareness of my dives. @elgoog & @tbone1004 thanks for the great explanations!