Amberjack:
When we logged our last two cert dives, we were slightly off the tables, but well within the limits of the instructors computer.
I am not sure what you mean with being "slightly off the tables". If you meant that you did not stick to your dive plan and went deeper or stayed longer than planned, then that is still fine. You simply recalc your pressure gruop after the dive, based on the actual dive, as usual.
However if you would have gone into a decompression dive based on the tables, it is less than ideal. Off course you are well advised by others to buy a dive computer but you may also want to look at this from a different perspective.
Before the dive you should know the planned depth and duration of the dive. Even if it is a training dive. The instructor or DM briefing should include that. Then look at your tables for that depth and see what your maximum safe time is for that depth.
Even if you dont plan to stay the maximum time, now you know your limit. If the dive site has deeper depths, then also look at the safe depths for the deeper depths. If you make a note of these you will know the limits of your dive profile.
It is a simple but effective way of avoiding mistakes. For example: You are diving in Fort Lauderdale (my home base
) on a wreck that is 100ft deep. Looking at the tables you will see your NDL is 20 minutes. Just remember 20 minutes and you know your safe limit. Even if you have enough air and your instructor is busy teaching, when you approach 20 minutes, you can indicate to your instructor that you are going to end the dive and ascend (the thumbs up sign).
You will see many divers with computers just jump into the water and only check their computers and air, without having an idea before the dive as to how long they can stay at depth. Almost all computers allow you to plan the dive and get the same result as the tables, check your NDL BEFORE the dive and remember that.
If your computer breaks down while you dive, you cal always end the dive safely because you remember your NDL. Even if you have to check the dive duration on your buddy's timer - hope you dont have to.
I suppose I am trying to stress that you should base your dive on a planned duration and not on your dive computer's remaining NDL.