DIR mental math for gas consumption

[tbone1004]

@elgoog the time reference you use is going to be based on the specific variables and your comfort level with both the dive and mental math. 1-5-10 minute intervals are good to keep written down for referencing

 

[TerryC]

@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.

Think that I will stick to Metric (at least for my own calculations).... SO MUCH EASIER.
Yes, I can do it in cu ft and psi, but only when I'm teaching!, not in my head, on the fly.

 

[PfcAJ]

Figure out how many cubic feet you consume at various depths every 5mins.

Figure out how many psi that is in whatever tanks you’re using. Write it down.

Now you’ve got a handy chart in your wetnotes. Before you get in the water you can easily reference it and say “ok this dive is to x feet, I’m using y tanks, so I expect to use z psi every 5mins”.

Simple and easy.

Get used to looking at your bottom timer before looking at your spg. Looking at your spg will confirm what you already know because of the above exercise. If it doesn’t line up you should investigate (physically or mentally) why that is.

 

[dave22387]

Figure out how many cubic feet you consume at various depths every 5mins.

Figure out how many psi that is in whatever tanks you’re using. Write it down.

Now you’ve got a handy chart in your wetnotes. Before you get in the water you can easily reference it and say “ok this dive is to x feet, I’m using y tanks, so I expect to use z psi every 5mins”.

Simple and easy.

Get used to looking at your bottom timer before looking at your spg. Looking at your spg will confirm what you already know because of the above exercise. If it doesn’t line up you should investigate (physically or mentally) why that is.

Is this part of your standard planning? At least before you had it memorized? I like the idea

 

[tbone1004]

Is this part of your standard planning? At least before you had it memorized? I like the idea

yes, we also teach it at our basic diver levels. Not an agency requirement, but a requirement to create independent divers. They are required to calculate their SAC rates for all of their dives and submit it in their logs and also use it to plan their dives

 

[dave22387]

yes, we also teach it at our basic diver levels. Not an agency requirement, but a requirement to create independent divers. They are required to calculate their SAC rates for all of their dives and submit it in their logs and also use it to plan their dives

Ya I was trained to and do record my rmv. My question was do you write down in your wetnotes your planned consumption per unit of time for that specific dive? For example my rmv is . 57, I round up to . 6. On a 100 cf tank that boils down to a sac of 21psi/min. @ 150 (roughly) 5.5 ATA, that would be roughly 120 psi/min or about 600/ 5 minutes. Cut in half for doubles... Are those numbers pre planned and written?

 

[tbone1004]

@dave22387
what I would have in my wetnotes is the following

Depth (ata) 1min 5min
1 21 100
2 42 200
etc etc

If I use the same tanks on a regular basis, then I'd create that table permanently. I.e. for me I dive basically 3 tanks. Single AL80 when travelling, double HP120's in OW, and LP121's for OC sidemount.
In that case my chart columns would be AL80, HP120's, LP121 and it would all be in 5min intervals to have. I use a nav slate for most of my OW dives where I have the dive plans, site info, etc. instead of using wetnotes so I just write that information in a corner of the slate. In a cave, I use a wrist slate and I have it written in sharpie on the last page of the wrist slate

 

[helodriver87]

I just keep charts in 10' increments for all my standard tank sizes. It lists the depth, the RMV (I vary between .5 and .6 typically) and psi/5 min segment. Sometimes I remember the number and don't need to reference it (like in Ginnie). Sometimes I have to look it up before the dive. Then, I'll just check my time, do a rough calculation, and compare to the SPG.

 

[rjack321]

Is this part of your standard planning? At least before you had it memorized? I like the idea

Yes the only way to "get it" is to use it.

For tech dives when you might be nervous or edgy, or the site is just overall more demanding (lots of stuff going on, higher than normal currents, new, unknown, etc, not exactly "relaxed") we usually rounded up to be more "worst case".
My normal cold water consumption is 0.6 (cfm @ 1ata). For planning purposes we used 0.75. If I was a hair under this every 5 mins yay for me. If I met that, well that is probably just normal for this day. If I was exceeding even 0.75 that indicated perhaps a problem worth STOPPING and thinking about. The stop & think is probably the most important thing you can do in deco and cave diving.

Recreationally, on an otherwise "normal" causal dive with no obvious leaks or free flows, the biggest deviation is probably being deeper than you realized. Isolator closed is a whooper of an oops that you should catch though. Beware, do not open a closed isolator underwater unless you analyzed both sides of your doubles.

 

[CptTightPants21]

Is this part of your standard planning? At least before you had it memorized? I like the idea

It can be used for standard dive planning. It can also be used to gauge your progress during a dive and assist on the fly dive planning.

Your mistake is that you are apply ATA before the time. You should get the 5 min consumption rate in PSI and then apply the ATA.

There are a few reasons 5 min blocks are preferred:
1. Time enough to generate a noticeable change in the SPG--I think you can see 20 PSI per min is nothing, 100 PSI per 5 min block however can be useful.
2. Segment is small enough that it can be used for easy calculation and/or added up for long dives. I was initially taught 10 min check gas, but what I found is that on deeper dives and/or with small tanks (like AL80s). The 10 min blocks would be a significant amount of usable gas/the planned bottom time.

You can put it in your wet notes as a permanent chart for the different tank configurations. Put once you know what your 5 min consumption rate is on the surface in PSI/Bar is it is fairly easy to apply the average ATA for a dive.

You take your RMV at the surface--lets say .6

.6/3 (tank factor of hp100/LP85) x 100PSI = 20 PSI per min on surface

20 PSI per min on surface x 5 min segment = 100 PSI per 5 min segment on the surface.

Now just multiple 100 PSI by ATA..

Double HP100s would turn the tank factor into 6 and make it 50 PSI per 5 min segment on the surface

Single HP100, 5 Min Segment

ATA .5 .6 .7 (SAC RATES) x 100PSI
1 85 100 115
2 170 200 230
3 255 300 345
4 340 400 460
4.5 383 450 518
5 425 500 575
5.5 468 550 532
6 510 600 690
6.5 553 650 748

These are the actual numbers, but really it is just a planning tool. This is a range to let you see how the dive is progressing. After 2x 5 min segments at 100 feet where are you? Breathed down 700 PSI--ok you are at the lower end of you range breathing close to a .5, more gas, potentially more time at depth. Are you down to 900PSI--ok, you are breathing harder his dive closer to .7, dive time should be shorter. If I was to memorize this chart/this configuration, I would make the numbers 90, 100, and 120 PSI per 5 Min and then 45, 50 and 60PSI for double HP100s.