Galileo Sol, Remaining Bottom Time and Rock Bottom

[Hatul]

The Galileo Sol is smart enough to to tell us when a solo diver must start an ascent and still have tank reserve at surface. That time of course is RBT=0.

If were diving with a buddy there must be enough gas to allow a safe ascent sharing air. That amount is called Rock Bottom. Rock Bottom has four parts:

1. time to sort problem and swab reg (30-60 sec)
2. time to ascend
3. safety stop
4. tank reserve (300 psi)

At any depth the amount of air used is time(min)*(depth/33+1)* SAC(cf/min). Where depth/33+1 is the pressure in bar. Looking at my SAC in California diving it's an average of 0.75 cf/min and peaks at 1.3 cf/min. So I shall assume my buddy is stressed and is breathing at a SAC of 1.3 cf/min.

At safety stop the gas consumption is 3*(15/33+1)*1.3 for the buddy= 5.7 cf
I'm going to use an al80 for the calculation which is between HP80 and HP100 in pressure/vol.
5.7cf * 3000 psi/77.4 cf= 220 psi In other words my buddy needs 220 psi to complete a 3 min safety stop. Round up to 250 psi

If I set my tank reserve at 550psi that will leave me 550-250=300 psi which is my Rock Bottom tank reserve goal.

How much extra RBT do I need to allow for my buddy to have enough gas to make an ascent to surface:

the amount of gas in cf= d/30 * (d/2/33+1) * 1.3 where d/30 is the time in minutes to ascend at 30 ft/min and d/2 is the ave depth

now if the Sol is showing a RBT >0 how much extra gas do I have over RBT=0? it's RBT*(d/33+1)*0.75

It turns out if you leave at least d/30 minutes of RBT you will almost have enough air for the buddy. I made a table on Excel of the gas remaining vs buddy needs at each depth and the maximal shortfall is 32 psi at 90 ft. (It's zero at surface and at 180 ft). A 32 psi shortfall can be ignored because I'm leaving a reserve of 300 psi at end anyway.

About the half minute to sort things out: just add a minute to RBT.

So in summary for Sol user to manage Rock Bottom to rescue buddy (Imperial units):
1. set tank reserve at 550psi
2. at each depth divide depth in feet by 30 to get the time to surface
3. always require RBT>= depth/30 +1

If you do the above you'll meet the requirements of Rock Bottom and be able to surface with a tank reserve of just under 300 psi

One other point. In the above I assume we need d/30 minutes to surface, but actually we need less because the ideal ascent rate by the Sol is actually faster than 30 ft/min (p44 of manual). So that gives a further small reserve.

Adam

[Jax]

? You just figure what you and your buddy need to ascend, and set that as your reserve. When RBT = 0, then you will ascend with a full 1/3 for you and for your buddy.

[Hatul]

? You just figure what you and your buddy need to ascend, and set that as your reserve. When RBT = 0, then you will ascend with a full 1/3 for you and for your buddy.

You're suggesting you leave a reserve equivalent to a pony bottle for the buddy. So if you're doing a deep recreational dive take a 20 cf reserve for your buddy, which works out to 20* 3000/77.4=775 psi for al80.

So set tank reserve to 300+775 or 1100 and now you can dive to RBT=0. That would work as well but you'd have to change the tank reserve for each dive or be leaving a large reserve for shallower dives.

With my method the reserve for the buddy is tailored to the depth. So if I dive to 130 feet I leave a large reserve, but if I ascend to 50 feet on the same dive I no longer need such a large reserve. With your method you're always carrying a fixed reserve for the max depth.

Adam

[Jax]

Well, as I change my settings to suit each dive, it suits me.

Except for the #$%^&* alarms. I hate forgetting to turn the alarms off.