Here in the very simplest terms is A BASIC method to manage gas with SM
Begin with similar volumes in each cylinder.
Breathe one sixth of starting volume in cylinder A (A=2500, B=3000)
Switch regs
Breathe one third of starting volume in cylinder B (A=2500, B=2000)
Switch regs
Breathe one sixth of starting volume in cylinder A, TURN DIVE (A=2000, B=2000)
Breathe an additional one sixth of starting volume from cylinder A (it now contains three sixths (1/2) of starting volume (A=1500, B=2000)
Switch regs
continue to breath from cylinder B until exit (end of dive) (A=1500, B=1500)
if at some point during exit your buddy needs gas, she will have access to at minimum one half of starting volume
Only THREE reg switches
Come out and take part in a SM clinic at Dutch Springs in the Fall...
That's 50% more complicated than it needs to be.
Begin with similar volumes/pressures in each cylinder (we will say you have 2 AL 80s at 3000 psi starting pressure).
Breathe one third of starting volume in cylinder A (1000 psi, so you now have A=2000, and B=3000)
Switch regs
Breathe one third of starting volume in cylinder B (1000 psi, so you now have A=2000 and B=2000 psi)
Turn the dive
Continue breathing another third off Cylinder B (another 1000 psi, so now you have A=2000 and B=1000)
Switch regs
Breathe a second third out of tank A to the end of the dive, (1000 psi, so you now have A=1000 and B=1000 as a reserve at the end of the dive).
That's just 2 reg switches and it ensures that:
1) At the point of maximum penetration or distance where you turn the dive, the volume in each tank is equal to the total volume you used from both tanks to get there.
2) you never have a differential between tanks of more than 1000 psi, so lateral trim is not a significant issue,
3) you have ample reserve at the end of the dive, and
4) allows you to adjust the reserve up or down with only 2 reg switches and only 2 pressure numbers to remember.
If a 1000 psi per side is too much ( 50 cu ft in this case) in a purely recreational setting, you could reduce it by using larger "thirds" and switching accordingly. 1200 psi would give you "thirds" would leave you with pressure of 1) 1800, 3000, 2) 1800, 600, and 3) 600, 600, for a 600 per side reserve with a 30 cu ft total reserve with 15 cu ft in each tank.
On the other hand in a technical diving situation if a 50 cu ft (1/3rd) reserve is too small, you can reduce the "thirds" you use. In this case if you used 800 psi "thirds" you'd get switch pressures of 1) 2200, 3000, 2) 2200, 1400, and 3) 1400, 1400, leaving a 72 cu ft reserve. So you have the flexibility of tailoring the reserve requirements to the specific dive.
Where I may choose to use 750 psi thirds (A=2250, B=3000), (A=2250, B=1500) and (A=1500, B=1500) like doppler's example, in a
recreational setting, would be when diving side mount with AL 80s on a two tank/two dive recreational boat trip. If I do this on the first dive, I use 77 cu ft - a full AL 80's worth of gas, but with an enormous 50% reserve for the first dive. On the second dive, I'll switch to 500 psi thirds - (A=1000, B=1500), (A=1000, B=500), and (A=500, B=500), still giving me 52 cu ft to use on dive 2 with a 25 cu ft reserve. It works well as the greater gas used on dive one maximizes the available NDL on what is usually a deeper first dive. Ad the reduced gas used on dive 2 normally works better with the shallower depths or shorter NDLs you have on dive two, and doubles the reserve you'd have available compared to a single tank rec diver getting out of the water with 500 psi.
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Too many SM divers make the whole gas switch thing too complicated and use approaches that do not ensure an equal distribution of gas at the pint of max penetration, or require frequent reg switches to make that happen. I still prefer to minimize the workload, maximize the gas planing benefits, minimize the number of SPG checks and the risk of making an error by not making any more gas switches than I have to. But some divers still prefer to use an every 500 psi approach. To each his own, but be sure to at least understand why you do what you do as well as the pros and cons.
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Single tank side mount is also a great lighter and lower drag option on a recreational dive, but it has some limitations. With anything other than an AL 80 or an AL72 the lateral trim starts to become an issue and it won't work well with most steel tanks due to either the initial excessive negative buoyancy of the tank and/or the larger swing weight of the tank during the dive.
If you use a pair of 100 cu ft steel tanks in the recreational example above, you'll have bags of gas compared to an AL 80 on each dive, plus a fantastic reserve on each dive, and the configuration is still very clean in the water.
