Kharon
Contributor
Something has been percolating in my head for a while now. I understand the reason for the rule of thumb its easier to teach, easier to understand, easier to put into practice, and in most instances its workable. The problem is that it is simply wrong in a lot of cases.
What Im talking about is the rule of thirds. It is a more or less reasonable rule of thumb if were talking 80 cf tanks when diving with a buddy on reasonably deep dives say 60 to 90 feet. We are taught to reserve one third of our air supply for emergencies. That should cover ourselves and a buddy to get us both back to the surface.
One third of an 80 cf tank is just shy of 27 cf. If we are diving a 100 cf or a 120 cf tank wouldnt make better sense to reserve the same 27 cf rather than an arbitrary third? After all the dive parameters haven't changed. What if we are carrying a pony? Does that reduce the amount of main tank air we need to reserve? What if we are diving a 64 cf tank or as I often do a 50 cf? One third there is only 21 cf of a 64 cf or less than 17 cf for my 50 cf. What about a diver using 80 cf doubles? Should they hold back just over 53 cf?
How about the issue of diving solo. There is no buddy to reserve air for. If we have a sufficiently sized pony why should we reserve a rather large amount of main tank air to cover a non-existant buddy?
What about a diver limiting their depth to 20 or 30 feet? Do they need to hold back the same reserve as a diver hitting 90 or 130 feet? Do we need the same reserve in 80°F water as in 50°F water? The same reserve in water with 100 foot visibility as in water with 10 foot visibility?
It seems to me that the rule of thirds is a necessary evil to make the OW classes less complicated. However, once weve got our skills down, I believe we need to start thinking, rather than blindly following an overly simplistic generalization.
What Im talking about is the rule of thirds. It is a more or less reasonable rule of thumb if were talking 80 cf tanks when diving with a buddy on reasonably deep dives say 60 to 90 feet. We are taught to reserve one third of our air supply for emergencies. That should cover ourselves and a buddy to get us both back to the surface.
One third of an 80 cf tank is just shy of 27 cf. If we are diving a 100 cf or a 120 cf tank wouldnt make better sense to reserve the same 27 cf rather than an arbitrary third? After all the dive parameters haven't changed. What if we are carrying a pony? Does that reduce the amount of main tank air we need to reserve? What if we are diving a 64 cf tank or as I often do a 50 cf? One third there is only 21 cf of a 64 cf or less than 17 cf for my 50 cf. What about a diver using 80 cf doubles? Should they hold back just over 53 cf?
How about the issue of diving solo. There is no buddy to reserve air for. If we have a sufficiently sized pony why should we reserve a rather large amount of main tank air to cover a non-existant buddy?
What about a diver limiting their depth to 20 or 30 feet? Do they need to hold back the same reserve as a diver hitting 90 or 130 feet? Do we need the same reserve in 80°F water as in 50°F water? The same reserve in water with 100 foot visibility as in water with 10 foot visibility?
It seems to me that the rule of thirds is a necessary evil to make the OW classes less complicated. However, once weve got our skills down, I believe we need to start thinking, rather than blindly following an overly simplistic generalization.