Chris,
Rock Bottom is a variable, not a constant. No such matrix would be useful across the board for all divers.
The concept is that for any specific dive [given discrete parameters: planned depth, anticipated current, planned overhead (or not), individual SAC/RMV (gas consumption rates in psig or volume), etc.] you first calculate an estimated volume of gas that would be required to get two divers back to the surface from the furthest planned point in the dive - should one of the two suffer a complete gas failure at that furthest penetration.
After calculating that estimated volume of gas, it is subtracted from the total gas available to each diver (so that either diver could rescue both). The dive is then planned using only the remaining volume of gas. The divers might plan to use all the remaining gas, or halves or thirds.
Since the calculation must take into consideration whether the divers must exit an overhead (or not), deal with current (or not), while consuming gas at potentially heightened breathing rates (or not), the amount of gas set aside is going to vary from dive to dive based on the different data about the divers, the depth, and the parameters of the dive site. Even for the same two divers, Rock Bottom could calculate out to be quite different for two completely different dives.
One reason why Fundies courses use an aluminum 80 as an example is to demonstrate to students that by the time you subtract a Rock Bottom gas volume from the available gas in an 80, then divide the remaining gas into thirds, you don't have a whole lot of gas available with which to execute the dive
The rationale for humping doubles, and planning dives using the much greater total gas volume, becomes self evident.
Hope that helps. YMMV.
Doc
Rock Bottom is a variable, not a constant. No such matrix would be useful across the board for all divers.
The concept is that for any specific dive [given discrete parameters: planned depth, anticipated current, planned overhead (or not), individual SAC/RMV (gas consumption rates in psig or volume), etc.] you first calculate an estimated volume of gas that would be required to get two divers back to the surface from the furthest planned point in the dive - should one of the two suffer a complete gas failure at that furthest penetration.
After calculating that estimated volume of gas, it is subtracted from the total gas available to each diver (so that either diver could rescue both). The dive is then planned using only the remaining volume of gas. The divers might plan to use all the remaining gas, or halves or thirds.
Since the calculation must take into consideration whether the divers must exit an overhead (or not), deal with current (or not), while consuming gas at potentially heightened breathing rates (or not), the amount of gas set aside is going to vary from dive to dive based on the different data about the divers, the depth, and the parameters of the dive site. Even for the same two divers, Rock Bottom could calculate out to be quite different for two completely different dives.
One reason why Fundies courses use an aluminum 80 as an example is to demonstrate to students that by the time you subtract a Rock Bottom gas volume from the available gas in an 80, then divide the remaining gas into thirds, you don't have a whole lot of gas available with which to execute the dive
The rationale for humping doubles, and planning dives using the much greater total gas volume, becomes self evident.Hope that helps. YMMV.
Doc
