If a small compressor cannot maintain the pressures recommended above, always maintain a pressure of at least one atmosphere in excess of absolute bottom pressure. This is necessary to provide the diver with immediate available pressure in the event of a fall, thereby possibly preventing barotrauma, or increase flow requirements.

Adequate ventilation is required to keep CO2 concentration at a tolerable level. For free-flow mask and lightweight helmet, the volume of air available should be at least 4.5 cfm at depth. The free-flow/demand mask used in the demand mode will require from 1 to 3 cfm to depth depending upon the diver's activity level.

The volume of free air (as measured at the surface) required by a diver may be calculated using the following formulas.

A diver doing heavy work at a depth of 100' using a demand mask will require

Ra=3×4

= 12 cfm

10.3.3 Surface-Supplied Requirements Using High-Pressure Air Flasks

When conducting surface-supplied diving operations in depths less than 130 feet, a four-to-twelve 240 or 350 cu ft cylinder high-pressure air cascade system is satisfactory for diving with a free-flow/ demand mask. Six 240 or 350 cu ft cylinders are mounted on a cradle fitted with a manifold system. Several 6-cylinder cradles may be used for extended operations. A two- or four-cylinder unit is used for small boat operations (usually limited dive durations and/or shallow water). The high-pressure cylinders are charged using a high-pressure air compressor driven by an electric motor or diesel engine. This system provides a sufficient air supply and enables divers to operate without the excessive noise of a compressor motor during diving operations. This enhances communications considerably and provides more pleasant conditions for surface personnel. On large operations, one cradle unit can be recharged while others are in operation. In addition, the same system may be used to support scuba diving operations.

A pressure reduction regulator system is required to reduce the high cylinder pressure to working pressure. For shallow-water work, a single one- or two-stage gas reduction regulator (4000 cu ft per hour or higher and 250 psi outlet pressure) with a high flow capacity is satisfactory. Two or more cylinders are connected in series and the regulator is attached to the manifold. When using a single regulator system, the diver should be equipped with a self-contained emergency air supply and a lowpressure volume tank on the surface.

For deep water, a more elaborate air regulator system is recommended. Air control systems are not standardized; they are usually designed to the specifications of individual divers or diving firms. The system should include two regulators, two diver outlets, and connections for primary and emergency air supplies. Either regulator may be supplied by air from the primary or emergency air source at a given time. In the event of a regulator malfunction, the Dive Master or tender may immediately activate

the diver to the stand-by regulator. Furthermore, in the event of primary air supply failure (e.g., line rupture), the emergency air supply may be activated and the primary system isolated for repair. Even with the redundancy in the air control system, it is recommended that divers be equipped with selfcontained emergency air supplies (Somers 1972).

The amount of time a diver may spend on the bottom when using air supplied from a high-pressure air flask can be calculated using the following equation:

In using the decompression tables in Appendix D the following instructions must be adhered to:

All dives which are not separately listed are covered in the tables by the next deeper and next longer schedule. Do Not Interpolate.

Enter the tables at the listed depth that is exactly equal to, or is the next greater than, the maximum depth attained during the dive.

Select the bottom time from those listed for the selected depth that is exactly equal to, or is next greater than the bottom time of the dive.

Use the decompression stops listed on the line for the selected bottom time.

Ensure that the diver's chest level is maintained as close as possible to each decompression depth for the number of minutes listed.

Commence timing each stop on arrival and resume ascent when specified time has elapsed. Observe all special table instructions. Always fill out a Repetitive Dive Worksheet or similar systematic guideline.

For cold or arduous dives and under conditions that prohibit accurate decompression, the next deeper and longer schedule should be used. For example, for a dive to 110 feet for 30 minutes in cold water, decompress using the 120 feet for 40 minutes schedule. Diving at high elevations requires the use of special tables to determine equivalent sea pressure depths for decompression (See Section 9).

10.5 SURFACE DECOMPRESSION FOR AIR DIVING

For routine dives it is standard practice to carry out decompression by stopping at the prescribed depths in the water. Under normal conditions this procedure is safe if proper surface support facilities and personnel are available. There are occasions however, particularly after deep dives or relatively shallow dives of long exposure time, where sea conditions such as strong currents, cold water, heavy

weather, or other emergencies prevent a diver from taking adequate decompression time in the water. Other conditions may also dictate the use of surface decompression. These may be summarized as follows:

■Treatment of decompression sickness.

Cases requiring medical care in which decompression has not been completed.

■ Programs in which numerous divers are in the water making repeated dives requiring decompression- thus taxing surface support. Extended deep diving in cold water.

■ Diving conducted from shipboard where the ship cannot remain on station.

There are two tables used in surface decompression. These are:

U.S. Navy Surface Decompression Table Using Oxygen

U.S. Navy Surface Decompression Table Using Air (See Appendix D).

10.5.1 Surface Decompression Using Oxygen Following an Air Dive

The Surface Decompression Table Using Oxygen is used for surface decompression from an air dive. It is essential that only pure oxygen be breathed during this procedure. If the oxygen supply is interrupted or oxygen toxicity symptoms appear, the decompression may be completed on air. If this occurs, use the Surface Decompression Table Using Air disregarding the time spent on oxygen. The notes following the Surface Decompression Table Using Oxygen and the Surface Decompression Table Using Air are self-explanatory and should be observed.

10.5.2 Surface Decompression Using Air Following an Air Dive

The Surface Decompression Table Using Air may be used following an air dive. When surface decompressing on air, do not use the Standard Air Table, but adhere to the Surface Decompression Table Using Air.

10.6 NO-CALCULATION REPETITIVE DIVE

TABLES

A system has been developed that simplifies the process of computing allowable repetitive dive bottom times and residual nitrogen times (Reuter 1971). This system is a rearrangement of standard U.S. Navy Tables (See Tables 10-1 and 10-2). These tables, and the procedures for using them, are included in this simplified method.

10.6.1 Instructions

For a "no decompression" dive:

1. Find the depth to which you have dived along the top of Table 10-2.

2. Drop down to the figure which denotes your Bottom Time.

3. Go across to the right and follow the arrow upward until you find the time spent out of the water since the last dive (Surface Interval).

4. Go across to the right to find the allowable Bottom Time (white numbers) for the next dive. These are listed under the appropriate depths at the top of each column. The Black Numbers are "Residual Nitrogen Times" and are only important for figuring "Decompression Dives."

5. If the "no decompression" limits are exceeded, go to Table 10-1 for Decompression stops and times. 6. If the diver's surface interval is less than 10 minutes, add the Bottom Times of the preceding and following dives, use the maximum depth attained and consider the two dives as one.

7. SHORTENED OR OMITTED DECOMPRESSION: If a diver surfaces after a dive and finds he has not adequately decompressed but has no symptoms of decompression sickness, he has a maximum surface interval of 5 minutes to determine what his decompression for the dive should have been, get back in the water and begin the following decompression procedure:

a. Make a stop at 40 ft. for 1/4 the 10 ft. stop time.

b. Make a stop at 30 ft. for 3 the 10 ft. stop time.

c. Make a stop at 20 ft. for 1/2 the 10 ft. stop time.

d. Make a stop at 10 ft. for 12 times the 10 ft. stop time, then surface.

Modified U.S. Navy Standard Air Decompression Table

a. All decompression stops are timed in minutes.

b. Ascent rate is 60 feet per minute.

c. The chest level of the diver should be maintained as close as possible to each decompression depth for the number of minutes listed.

d. The time at each stop is the exact time that is spent at that decompression depth.

10.6.2 Definitions

1. Bottom time (in minutes) starts when the diver leaves the surface and ends only when the diver starts a direct ascent back to the surface. Always select the exact or next greater bottom time

exposure.

2. Depth (in feet). The deepest depth of descent. Always enter the tables on the exact or next greater depth reached.

3. Residual Nitrogen Time-Time in minutes that a diver is to consider he has already spent on the bottom when he starts a repetitive dive.

4. Surface Interval-Time in hours and minutes actually spent on the surface between dives.

5. Repetitive Dive-A dive begun within 12 hours of surfacing from a previous dive.

PLAN YOUR DIVE-DIVE YOUR PLAN

Always carry the Dive Tables on a divethey may save your life.

DACOR CORP.
Northfield, III, 60093 U.S.A.

10.6.3 Examples

There are four basic problems for which the U.S. Navy dive tables can provide answers. The "No-Calculation" Linear System can solve these problems very simply and quickly (Reuter 1971).

Let us consider a hypothetical diver who descends to 50 feet remaining at that depth for 60 minutes (bottom time). He then returns to the surface for three hours (surface interval) before starting his next dive.

A. FIRST REPETITIVE DIVE (Second Dive)

What will be his allowable bottom time for a nodecompression dive if he wishes to dive to 70 feet on his first repetitive dive (second dive)?

1. Drop down the 50 foot column in Table 10-2 to the 60 minute line.

2. Go across to the right to "H" Repetitive Group. 3. Follow the arrow upward until reaching the limits within which three hours falls (2:24 to 3:20).

4. Go across to the right ("D" Repetitive Group) to the 70 foot column where the bottom time limit is found to be 30 minutes (white number- black background). This is the maximum time that can be spent without having to make decompression stops. If less time was spent, proceed to example B. If more time was spent, proceed to example C.