Before personal dive computers (PDCs) became popular and widely used, divers calculated their NDLs using printed dive tables. Dive tables base calculations on the deepest dive point that you will reach during the dive. They provide you with a conservative estimate of the allowable time you have for that dive (NDL) based on time, depth, breathing gas, and any buildup from previous dives in the last 12 hours.
Although the dive tables help keep you safe, there is a huge downside to using them. Firstly, in a survey of both new and experienced divers, the majority responded that using dive tables was a challenge. A significant number of new divers wrote that dive tables were so difficult to understand, they rarely dived using them. Secondly, it is unlikely you will stay at the deepest point on the dive for the whole time. It is much more likely (and safer) for you to descend to the deepest point first and to gradually ascend as the dive progresses. This means that your air supply will last longer, your final ascent to the surface will be shorter, and you are exposed to less pressure at the end of the dive. The dive tables simply can’t calculate those changes in depth. These reasons are among those that point to why most people lean heavily toward using PDCs.