of oxygen toxicity. Some models allow the diver to select a high or low range of O2 partial pressure for short or long dives, to accelerate decompression. 11.4.1 Advantages and Limitations

The major advantages with closed-circuit mixed gas diving are the conservation of gases and accompanying extended dive duration, quiet operation, the absence of bubbles, and the depths at which the equipment can be operated. The Navy Mk 10 system is designed to be used at depths of over 1000 feet. Nonsaturated shallow dives are practical from surface support platforms, and result in an extended gas supply duration. Closed-circuit mixed gas equipment has been tested in cold water and appears satisfactory; however, the life of the chemicals used to absorb carbon dioxide are significantly decreased and limit the duration of the dive.

The disadvantages associated with closedcircuit mixed gas scuba are both operational and mechanical. The equipment itself is extremely

delicate and complicated. For safe and proper use a diver must be specially trained, and qualified personnel are required for maintenance and repair. The cost of the equipment coupled with the costs of support and maintenance prohibit its use for routine diving tasks. The equipment itself is reliable at shallow depths, but some critical failures, such as fluctuations of oxygen partial pressure, have been recorded on deep cold water dives. The configuration and size of the units vary with some models being significantly more comfortable, and easier to swim with than others.

11.4.2 Duration

Because the oxygen partial pressure can be automatically controlled, closed-circuit mixed gas scuba can be used to greater depths than closedcircuit oxygen scuba. As no oxygen is wasted the duration of a dive is usually limited by the life of the CO2 absorbent (See Paragraph 11.5.2).

11.5 CLOSED-CIRCUIT OXYGEN SCUBA

DIVING PROCEDURES

Oxygen diving is unique in that it is the only technique in which a diluent gas is not used. Pure oxygen is used as a breathing gas only with the closedcircuit oxygen scuba (except when breathed during decompression). The closed-circuit oxygen scuba is described in detail in Section 4.

When using a closed-circuit oxygen rebreather it is necessary to purge both the apparatus and the lungs with oxygen prior to entering the water. This is necessary to eliminate nitrogen and air from the breathing system. If the excess air is not eliminated from the breathing bags and lungs prior to the initiation of oxygen breathing, sufficient nitrogen may remain in the system to provide a breathable volume after all of the oxygen has been used. During a prolonged dive, the nitrogen eliminated from the body can cause a measurable increase of nitrogen in the breathing medium. The danger of excess nitrogen in a closed-circuit system is that hypoxia (See Paragraph 2.1.3.1) may occur if the volume of nitrogen is enough to markedly dilute or replace the oxygen in the system. Hypoxia gives no warning signals to a diver. Unconsciousness or death may result from hypoxia (See Figure 11-1).

11.5.1 Advantages and Limitations

The advantages of closed-circuit oxygen scuba include freedom from bubbles, almost completely silent operation, and maximum utilization of the breathing medium carried by the diver. A small oxygen supply lasts a long time, and the duration of the supply is not decreased by depth. Divers are not subject to decompression sickness or nitrogen narcosis while using closed-circuit oxygen scuba due to the absence of an inert gas.

The major limitations of O2 rebreathers are related to the effects of oxygen on the human body. Hypoxia can result if a large amount of inert gas is allowed to build up in the breathing circuit. The system must be thoroughly purged at the beginning of each dive, after one hour of submergence, and again immediately prior to ascent. An excess of carbon dioxide can build up in the system as a result of absorbent exhaustion, wetting of absorbent, improper canister refilling or over-breathing the system.

Because of the chances of oxygen toxicity, the

rebreather is normally used to a working depth of 25 feet for a period of 75 minutes, and any excursions below this depth will result in a greatly decreased allowable bottom time. The maximum permissible dive using this apparatus is 40 feet for a period of 10 minutes. Its use beyond these limits. can result in serious or fatal accidents resulting from oxygen toxicity. The degree of training required to use the equipment, coupled with increased maintenance requirements further restrict its use.

11.5.2 Duration

The major factor influencing the duration of a closed-circuit oxygen dive is the time limitation which must be imposed because of oxygen toxicity as shown in Table 11-1.

When using any closed-circuit scuba, utilization of available oxygen is nearly 100 percent because no gas is expelled into the surrounding water except that which is willingly purged from the system or which is automatically vented as the gas expands when surfacing. This permits not only a greater duration of the gas supply, but allows a smaller quantity of breathing gas to be carried. O2 consumption will vary, dependent primarily upon the diver's exertion level, from 0.5 standard liters per minute (slm) to 3.0 slm. A mean figure of 1.5 slm is a reasonable value for planning purposes (See Figure 2-3).

11.5.3 Medical Aspects

In summary, there are several physiological

problems which may arise when using oxygen rebreathers. These include oxygen toxicity, carbon dioxide accumulation resulting from a failure of the absorbent material, and hypoxia. The precautions described in the preceding paragraphs should preIclude the onset of these problems. Each of these conditions is discussed in detail in Section 2. Because of the absence of inert gases, there is no requirement for decompression following an oxygen dive.

11.6 SURFACE DECOMPRESSION USING OXYGEN FOLLOWING A NITROGENOXYGEN DIVE

Either the U.S. Navy Surface Decompression Table Using Oxygen or the Surface Decompression Table Using Air may be used for surface decompression following a dive in which nitrogen-oxygen was used as the breathing medium. In using these tables, it is essential that an equivalent air depth be determined in selecting the proper decompression depth from the tables. The procedure for determining an equivalent air depth is outlined in Paragraphs 11.3.3 and 11.4.3.

11.7 SURFACE DECOMPRESSION USING AIR FOLLOWING A NITROGENOXYGEN DIVE

The procedures outlined in Paragraph 11.6 are applicable to surface decompression using air following a nitrogen-oxygen dive.