As an industry, we tend to do things in the margin of safety. Diving with partners, redundant air and other equipment, etc. When we, especially when we do repetitive diving, one of the most important functions of our dive plan revolves around nitrogen absorption. When we make our first dive we limit our down time based on our maximum depth. During our surface interval, we calculate next down time based on our total surface interval and the depth that we will dive. Why, because we want to keep a check on our nitrogen absorption. By making our repetitive dives shallower then the last, just keeps that partial pressure of nitrogen down, reducing the of nitrogen absorbed, and reducing the total residual nitrogen in our system. It for safety reasons so that we have less of a chance of DCS.
We also have to remember that the tables and computers are guides, not laws. Everyone absorbs and sheds nitrogen at different rates. Age, circulatory function, respiratory function, underlying medical conditions will all affect the rate that nitrogen is released. So its only natural that we take as much precaution, and add safety to our dives. By the way, most computers do a pretty good job at monitoring/adjusting for you during a reverse dive profile, but remember, were all different and its only a guide.
Can we do reverse dive profiles? Yes, the SCUBA Police will not come and arrest you. But, you have to be more concerned that you do not overstay your welcome. Remember, the deeper your, the less down time your going to have, the shorter the amount of remaining time your going to have when you start up (because your going to try to squeeze as much time in as you can, statistically speaking), the more nitrogen that going to be in your system, and lets hope you have enough air to do your safety stop.
So with that said, dont do the reverse dive profile for the safety factor, and if you do, just lean more to the conservative side.
Stay well and dive safe
Bill
