Mismatched SAC rates and Rock Bottom

[elan]

When calculating the RB you need to take into account both divers consumption in distress. The difficulty though is to know how the person reacts to emergencies. If you have a better SAC rate than your partner not necessarily does it mean you will have a better SAC in disstress. We have been taking 1cufm in distress but that has proven to be on the lower scale and my rate in real emergency seemed to be a bit higher - around 1.2 (being around .5-.6 in normal conditions)

The min gas(rock bottom) from 100 ft should be in 35-40 cuf range.

 

[Mr Carcharodon]

You can always share air for badly mismatched buddies. Of course you still have to observe rock bottom. And of course rb is a dynamic number. It might be 1200 psi at 100 fsw and 600 at 30 fsw. So during ascent your available gas increases.

 

[eelnoraa]

So, since 99% of my dives are drift, excluding Bonaire, I'll start with a direct ascent calculation for a RB based on a no added stress dive at 100 feet:

Combined SAC of 1.12

1.12 x 4 = 4.48 x 1 min for preparation = 4.8
1.12 x 2.3 (43 feet average) = 2.58 x 3 mins = 7.74
1.12 x 1.45 = 1.62 x 3 mins = 4.86
Plus 2 cf for 1 min ascent from SS = 19.4 cf

When Eric dives rented
AL 100's that's 633 + 200 (reserve) = 822
Al 80 730 + 200 = 930

My HP 80's 826 + 200 = 1086
Al 63 904 + 200 = 1104

That's very interesting. I had not looked at it this way. Think I'll try calculating a RB with a stress factor next...

i might try Lowviz's out and back too once my eye's refocus. Have I mentioned I HATE math?

I think the "combined" SAC of 1.12 is an very optimistic number. 1.12 for 2 means 0.56 each. Remember, if things happen, you both will be under stress. It is not uncommon that your stressed SAC is 2x of your nomal and relaxed SAC. Persoanlly, my RB for 100ft is 40cf.

 

[uncfnp]

I agree, it is optimistic and as I said it is a no stress rate. I am calculating a "stressed" rate at a dive factor of 2 next.

 

[freewillie]

I hate math and calculations.

First person to 1200 PSI give turnaround signal. Everyone starts for home. If diving deeper or need more cushion then higher PSI 1400-1600 or whatever the group decides depending on depth and condition.

While the diver with the higher SAC rate might be calling the dive early, since you should have more air any concerns that they are using your reserve should be moot, you already have more air than they do. They should be concerned that you are their extra air supply.

Any diver can call the dive for any reason at any time.

 

[uncfnp]

Using a dive factor of 2, I get a total SAC rate of 1.75. So:

1.75 x 4 = 7 x 1 = 7
1.75 x 2.3 = 4 x 3 = 12
1.75 x 1.5 = 2.6 x 3 = 7.9

Plus 2 for final ascent = 29 cf

Al 100's - 966 + 200 = 1166
Al 80's - 1115 + 200 = 1315
HP 80's - 1260 + 200 = 1460
Al 63 - 1380 + 200 = 1580

The fact I find most interesting (if i have done this correctly) is that though the 2 80's have the same cf, you need a higher rock bottom pressure in the high pressure tank since the initial pressure is higher. I had not taken that into account before.

 

[lowviz]

...//... The fact I find most interesting (if i have done this correctly) is that though the 2 80's have the same cf, you need a higher rock bottom pressure in the high pressure tank since the initial pressure is higher. ...//...

You did it correctly.

 

[Hatul]

When I calculate Rock Bottom, I assume that I'm ascending at mildly stressed rate and my buddy ascends at moderate stressed rate. I know my SAC and I assume my buddy's SAC rate is roughly close to my usual max.

In practice (for rec open water) I use simplified formulas that I can use under water and that already take this into account. For example one formula for RB with an 80 cu ft tank is to have at any depth in PSI the depth in feet*10 +500. So at 100 ft you should have at least 100*10 + 500= 1500 psi. This should allow you to make a safe ascent with a 3 min safety stop.

If you have an AI computer there are also simple formulas that can be derived from the remaining Air Time.

 

[Adobo]

i might try Lowviz's out and back too once my eye's refocus.

I don't bother doing anything complex to determine "turn" pressures for recreational dives.

What is important in a recreational dive is you have determined what you feel is the appropriate rock bottom/min gas. And you understand the assumptions that were made when you did your math.

Once you have the min gas calculated and converted from absolute volume (expressed in either cu ft or in liters) to PSI or bar, whatever you have left is what is referred to as usable gas.

So let's say your min gas equates to 1000psi and your tanks are filled to 3200psi. Your usable gas is 2200psi. That is the amount of gas you can consume before you need to begin your ascent. If you are on a boat dive and descend on an anchor line, you can do what some refer to as a halves dive. That is, you use half of your usable gas to go out and then the other half to meander back to find the anchor line. So the math looks like:

Min gas: 1000psi
Current fill pressure: 3200psi
Usable gas: 3200-1000=2200psi
Turn pressure: (fill pressure) - (usable gas / 2) = (3200) - (2200/2) = 2100psi​

So on this dive, you descend on the anchor line, you go out away from the anchor line and then turn around at about 2100psi. At no less than 1000psi, you have hopefully found the anchor line again and can begin your ascent. But even if you did not find the anchor line, you have to begin your ascent because if you don't, you might not have enough gas to get your buddy and yourself to the surface should an emergency arise.

If your buddy has different size tanks and different SAC rate, it doesn't matter. He does exactly the same math. He figures out his min gas in absolute volume and then convert it to PSI (or bar) and does exactly the same calculations as above. Whichever of you reaches his/her turn pressure first will call turn for the buddy group.

The reason you don't futz around with variable tank sizes is that on a recreational dive, if something happens, you thumb the dive and you do your ascent. The concept of having to figure out correct turn pressures for divers with varying tank sizes is only important in overhead dives where you do have a choice but to exit at the same point that entered. Again, not applicable to recreational diving.

At no point during the dive should either of you violate minimum gas. This way, should an emergency arise, at least one of you will have enough gas to get both of you back to the surface.

 

[lowviz]

I don't bother doing anything complex to determine "turn" pressures for recreational dives. ...//...

You probably did, once or twice. And that is about all it takes to get a basic understanding of what is a 'safe' reserve for a given cylinder for few key depths.

...//... What is important in a recreational dive is you have determined what you feel is the appropriate rock bottom/min gas. And you understand the assumptions that were made when you did your math. ...//...

Yes, exactly. And it is an estimate that contains a lot of uncertainties, so no need to obsess on three decimal places...

...//... At no point during the dive should either of you violate minimum gas. This way, should an emergency arise, at least one of you will have enough gas to get both of you back to the surface.

Absolutely. This is what this thread is all about. Having even a rough estimate of RB/min gas for a particular dive plan is empowering.