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Course Outline

However, it is important that you know a little about this method of dive planning. The dive table example following shows a first dive (having NOT dived within 12 hours) to 15 meters/49 feet/2.5 bar. The time is more restrictive or conservative than the times allowed on a PDC. This is because the PDC makes continuous adjustments based on your actual depth at that time. Your manual dive tables can plan only for a continuous dive at one depth. So, you assume the deepest depth for calculation. The table shows a part of the air-diving tables.

Note: Because there is no 15 meters/49 feet shown, you will calculate your NDL status at the next greatest depth, which is 16 meters/53 feet. The maximum allowable no-decompression dive time is 60 minutes.

When you become an Open Water 20 diver, you will be certified to 20 meters/66 feet depth based on sea-level diving. This may also be restricted by any local laws, standards, or codes of practice.

Open Circuit Gas Supply Limit

How long will the air supply in the cylinder last?

You will apply what you learned earlier. The breathing rate is changed slightly for practice: 25 liters per minute/0.88 cubic feet per minute.

A cylinder that is 11.1 liters/80 cubic feet holds about 2,220 liters/80 cubic feet of gas with a pressure of 200 bar/3,000 psi. You know that you have an SAC rate of 25 liters per minute/0.88 cubic feet per minute. This will increase proportionately with depth, and the RMV depends on the depth, in this instance, at 15 meters/49 feet/2.5 bar.

25 L p/m × 2.5 bar = 62.5 L p/m

0.88 cu. ft. p/m × 2.5 bar = 2.21 cu. ft. p/m

Therefore, how long will your cylinder will last? You must consider a 50 bar/750 psi reserve for emergency.


200 bar - 50 bar = 150 bar

150 bar × 11.1 L = 1,665 L

1,665 L ÷ 62.5 L p/m = 26.64 min. (~27 min.)


80 cu. ft. × 750 psi ÷ 3000 psi = 20 cu. ft.

80 cu. ft. - 20 cu. ft. = 60 cu. ft.

60 cu. ft. ÷ 2.21 cu. ft. p/m = 27.37 min. (~27 min.)

Note: There is a slight difference between the metric and imperial results due to the rounding, but they are only estimations and do not have to be accurate.

In summary, the following are the limitations when diving at 20 meters/66 feet.

  • NDL: 60 min. (maximum dive time)
  • Gas supply: Approximately 27 min.
  • Planned depth: 15 m/49 ft. (planned depth), 20 m/66 ft. (max allowed)

Therefore, the gas supply is the limiting factor with 27 minutes.

All other factors aside, a 27-minute dive to 15 meters/49 feet is within the limits of a no-decompression profile.

However, note that it is recommended that divers conduct at least a 3-minute safety stop after any dive to 15 meters/49 feet. This does two valuable things: It may lessen the occurrence of DCS. It will give you an opportunity to fine-tune your buoyancy control.

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