How much air to leave in reserve with different sized tanks?

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I have an absolute minimum of 300 psi to get on board with. This covers gage error of up to 10% with me being onthe surface. If no error i still have gas to breath till my feet gets out of the water. You reg doea not want to work so well when secondary pressure gets below 150 psi. It cant supply air fast enough to breath easily. So now add the 300 psi to the 150 psi and you get 450 psi. This is tank specifics out of the picture. Now you have to add to that how much gas you need in cu ft to get from bottom to the surface with a buddy on your tank. (Rock bottom) . If that is another 400 psi for your tank then then you start your ascent based on your tank at 850psi. Your buddy may have say an lp120 on him and his start to surface psi may be 600. Between the 2 of you you start up at the highest PSI because the highest psi is the smallest tank.
 
Don't confuse "turn pressure" with the pressure you might have when beginning your ascent.

Turn pressure is when you "turn" your dive and start heading back to the exit point from the furthest point of your dive. A direct ascent from that furthest point may not be practical/desirable. Consider a case where you're on a 400ft wreck, with the dive boat tied in on the stern of the wreck. You're swimming along and reach the bow. You see that you have 1400psi remaining. Is that enough to get you back to the upline, do your ascent, make your stop, and get out of the water... or do you intend to do a free ascent 400ft away from the dive boat?

Similarly you might be in Bonaire. Do you want to do a nice leisurely swim back to where you parked your truck, or would you rather ascend at the furthest point of your dive and walk back?

I think this story is fitting. Two Divers, No Air
 
To answer your question properly you have to look at a few things
First what size tank are you diving with
Second how deep do you plan to go
Third what is the temp of the water
Fourth how long do you plan to stay at said depth (are you within ndl)
Fifth how long can you dive on said tank
All of these factors are part of a predive plan
So in other words plan your dive and dive your plan planning your dive means we turn around at said pressure Wethersfield it be 1/2 or 1/3 or 3/8 and whoever gets to predetermined turn back pressure first notifies the other and you turn back without hesitation

I can't stress this enough though plan your dive and dive your plan
Don't deviate that's when you get into trouble
 
Totally agree.

I've realized that my gas consumption is decent, but not stellar (partly, but not totally, due to the fact that I dive dry, and for several reasons diving dry often comes with a somewhat increased gas consumption compared to WS diving). Since my computer automatically gives me my average consumption I have a decent idea of both the average and normal range, so I've made myself a small table giving min pressures at some relevant depths. I've assumed a worst case scenario, having to share gas with my buddy, and both having an elevated consumption due to stress (when the bovine manure intercepts with the rotating ventilation device, your BP and breathing rate goes up).

For a 10L tank (about 10% less than the water volume of an Al80), my personal min pressures are:
30m/100': 110 bar/1600 psi
25m/80': 100 bar/1400 psi
20m/65': 85 bar/1200 psi
15m/50': 70 bar/1000 psi
10m/30': 60 bar/900 psi
5m/15' (safety stop): 50 bar/750 psi

Google "minimum gas" or "rock bottom" for more info. If you're living in imperial unit country, you might want to take a look at NWGratefulDiver.com

Yes, it depends on depth and whether you're solo or buddy diving and may need to provide air to share. A rule of thumb I use to always have at least in PSI (depth in ft)*10 + (300-500) as Rock Bottom to allow air sharing with a single tank. So at 90 feet: 900+300 to 500 = 1200 to 1400 psi. It's roughly in line with the table you listed. Use the 500 psi in the formula to be on the conservative side.
 
Glad that this is a topic on the Board. The traditional way of teaching a safe reserve within the major agencies is very flawed, in my opinion. I teach Rock Bottom to all my students as it is truly the safest strategy out there.
Rock Bottom- the calculation of enough gas for 2 divers, in an emergency, to ascend at the proper rate and conducting all required/safety stops to the surface.
To keep this simple we have to establish a few things, standard SCR rates: 0.5 cuft/min resting(deco),
0.75 cuft/min working (swimming), 1 cuft/min emergency (OOA).
Tank Factor for an AL 80 rated to 3000psi is 2.5
Formula is as follows:
SCR for Emergency Rate
# of divers
Average Depth of Ascent (ATA)
Total Time of Ascent
Looks like this for 100 ft dive:
1 cuft/min Emergency SCR
2 Divers
2.5 ATA
8 Minutes= 40 cuft/2.5= 16x100=1600 psi as rock bottom for you and another diver to share air to the surface making all the appropriate stops.
**Disclaimer- this method is utilized based upon the application of Min-Deco ascent strategy and not the generic safety stop at 15ft for 3-5 minutes. For 99% of dives, you will not have to "share air" therefore you will typically use only 25% of your rock bottom when there is no emergency.

Let me know if you have any questions. This is very similar to what some of the other posters have put but I feel this may be a bit easier to understand written out this way.
 
Of course pressure alone is not the only factor. I used to dive with a HP120 rated at 3500 psi. At 500 psi on my gauge (assuming it was accurate), I'd have approximately 17 cu ft of gas. With my current 80 rated at 3300 psi, I'd have about 12 cu ft left at 500 psi. As others have pointed out, the characteristics of your reg set must also be considered.
 
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