Traditionally recreational scuba divers have carried a depth gauge, dive watch and dive tables to dive to a plan, but the advent of low-cost computers designed to monitor depth and time on a second to second basis means that divers can now have longer dives, and if correctly used, stand much less chance of suffering a diving related injury.

The way in which divers using the standard tables dive, compared with how divers using a dive computer is slightly, but significantly different, due to the fact that in the former the whole dive is planned on the maximum depth reached.

For example, using the PADI tables to dive to 30 metres (100 feet) the no-decompression time is 25 minutes. That means that the total time from starting the decent to the point at which ascent begins can be no longer than 25 minutes, and at 42 metres (140 feet) the no-decompression time is just 10 minutes – considering that you are always advised to dive conservatively and so not to the limits of the tables in reality your dives would be even shorter.

However a dive computer works differently. Rather than base the dive time on the maximum depth reached, it continually computes the theoretical nitrogen absorbed by different parts of the body. This means that although a maximum depth of 30 metres may be reached during the dive, if much of the dive is at a shallower depth then the body's absorption of nitrogen will be much less than that assumed by the dive table.

To return to the example of a 42 metre (140 feet) dive, which gives a maximum bottom time of 10 minutes if diving to the limit of the table, I'd like show how a real dive to the same depth using a computer can give a much longer dive time. My deepest dive last summer was in Cap de Creus (Costa Brava, Spain), to a depth of 42.1 metres according to my computer.

The maximum depth was reached nearly nine minutes into the dive and I remained at that depth for about two minutes and then ascended slowly to about 30 metres for 5 minutes. Slowly ascending again, much of the remainder of the dive was completed at a depth of 15 metres, before ascending to 5 metres for a safety stop and then breaking the surface, a total dive time of 45 minutes.

That illustrates how diving with a computer can dramatically increase your dive time without requiring decompression stops and provided you don't dive to the limits of the computer, complete safety stops when required and ascend slowly you can enjoy more time under the surface enjoying the sport you love.