Real-Life Math
Mathematical reasoning is an important aspect of diving.
Among the calculations a diver must make is how long they'll be underwater, how deep they'll dive, and how much air they'll need.
If a diver is going to go deeper than 131 feet, they also have to figure out how much time to allow for decompression.
Decompression is necessary in order to avoid the potentially fatal illness known as decompression sickness, also called the bends.
Decompression sickness is caused by a build-up of nitrogen in the body. In order to release the nitrogen from the body, a diver has to ascend slowly enough to give the nitrogen time to "gas off." This requires divers to stop ascending at a certain depth for a specified amount of time in order to allow nitrogen to escape: this is called decompression.
Divers use charts known as "dive tables" to help them determine how long they'll have to decompress as they rise to the surface.
In order to use the tables, the diver must know their "actual bottom time." This is the amount of time underwater before a diver decompresses. The diver must then add the time for decompressing to this number. Doing this calculation is necessary to ensure your tanks have enough air for the duration of the dive. The "total bottom time" is the total time spent underwater -- decompression time plus actual bottom time.
Air consumption is one of the most important issues for any diver, says diving instructor Mary Brill.
"On every dive we have one person who really sucks up the air and we have to go up early," Brill observes.
Below is a partial dive table taken from a diving manual.
Note that the table is in metric. Most of the world uses dive tables that are written in metric. Since scuba diving is an international sport, it's important to be able to use the metric tables.
Depth
(in meters) | Actual Bottom Time
(in minutes) | 10 meter stop
(decompression time in minutes) |
| ---------- | ---------- | ---------- |
| 40 m | 200 | 0 minutes |
| - | 210 | 2 |
| - | 230 | 7 |
| - | 250 | 11 |
| 50 m | 100 | 0 |
| - | 110 | 3 |
| - | 120 | 5 |
| - | 140 | 10 |
| 110 m | 20 | 0 |
| - | 25 | 3 |
| - | 30 | 7 |
| 150 m | 5 | 0 |
| - | 10 | 1 |
| - | 15 | 3 |
Using the tables, figure out how long you need to decompress for the following dives. How long will your "total bottom time" be for each dive?
- A dive 50 meters below the surface with a bottom time of 140 minutes.
- A dive of 110 meters below surface with a bottom time of 30 minutes.
- A dive of 150 meters below the surface with a bottom time of 15 minutes.