Rock bottom - exact definition

[mania]

OK, lately we had discussion what exactly the rock bottom is.
One option (mine) is that - rock botto is the amount of gas needed for reaching this deco stop where we have to change into deco gas. So when planning the dive I have to take into account the amount of gas needed for me and my partner (plus the stress factor) to reach this stop. Then we switch into deco gas.
The second is that this is the amount of gas needed for myself and my partner to reach the surface after doing the whole deco

need your help in solving or getting the clear definition of rock bottom

Mania

 

[ianr33]

Well I have always figured it is the amount of gas needed to get you and your buddy,in a stressed situation to the next gas source (either the surface or deco gas ) Coincidentally this is also what V-Planner uses !

Basically always planning to allow for one complete failure.If backgas fails for whatever reason the rockbottom calculation allows you to safely get to the deco stop. If the deco gas fails you should have enough backgas to deco from that if the dive has been planned properly. (gets more complicated with 2 deco gasses)

If backgas AND deco gas both fail then you have a problem! However 2 failures should be about as likely as being hit by lightning.

 

[miketsp]

OK, lately we had discussion what exactly the rock bottom is.
One option (mine) is that - rock botto is the amount of gas needed for reaching this deco stop where we have to change into deco gas. So when planning the dive I have to take into account the amount of gas needed for me and my partner (plus the stress factor) to reach this stop. Then we switch into deco gas.
The second is that this is the amount of gas needed for myself and my partner to reach the surface after doing the whole deco

need your help in solving or getting the clear definition of rock bottom

Mania

Assuming you (and your buddy) are carrying your deco gas with you then your definition makes more sense.
And of course your bottom gas (hypoxic) may not even be suitable for use at shallower depths.

 

[DiverBuoy]

This was a post from MHK a DIR Instructor.

The three different methods all are derived from the Rock Bottom concept. Once you've figured out your Rock Bottom, which is the gas supply that you need to reserve for you and your buddy to surface in an OOA emergency, you need to figure out how to best utilize the remaining "useable" gas supply.

1) All useable, for Cayman, Cozumel type diving where the boat follows you and you don't need to get back..

2) 1/2 useable meaning getting back to the boat is optional but not critical, such as the type of diving we do in SoCal.

3) 1/3rds, when getting back to the line or exit point is necessary..

Hope that helps..

Let me know if you need anything else..

Later

 

[DA Aquamaster]

Basically always planning to allow for one complete failure.If backgas fails for whatever reason the rockbottom calculation allows you to safely get to the deco stop. If the deco gas fails you should have enough backgas to deco from that if the dive has been planned properly. (gets more complicated with 2 deco gasses)

That's essentially the approach I use. As a general rule I plan to end with a 1/3rd back gas reserve, which for moderate deco dives usually allows enough gas to do the entire deco on backgas in the event the deco gas is lost.

If the longer backgas deco required is in excess of what can be done on the 1/3rd reserve, then I add a second deco gas which again should create a situation where you can complete the deco on the back gas and either one of the two deco gasses. This works well as when the deco is too long to allow deco on the 1/3rd back gas reserve, it is almost by default long enough to benefit from the efficiency of having two deco gasses anyway.

Obviously you need contingenecy deco schedules for each possible situation so you have your primary plan and a back gas contingency with one deco gas and a primary plan and up to three contingency plans with two deco gasses.

But to address the original question, to do it this way requires a minimum amount of reserve back gas at the point where you anticipate switching to the (first) deco gas. The amount of gas remaining at that point then needs to be adequate to complete the various deco contingency plans. So in my opinion, your "rock bottom" number needs to be derived with that in mind and consequently must either:

1. reflect enough remaining back gas to get you all the way from your first deco stop to the surface in the event you lose your only deco gas, or

2. reflect enough remaining back gas to do the required deco if either one of your deco gasses are lost or unavailable.