ABSTRACT The status of underwater heat sources being developed for U.S. Navy divers is reported. Specific heater types are-propane catalytic combustion heaters, seawater hydraulic heaters, and magnesium-oxygen combustion heaters. Portable underwater energy source concepts are reviewed. The review is limited to theoretical figures regarding size and heat output. Principles of operation, hardware design, engineering prototype test results, and heater interfacing with the diver are described for the specific heaters being developed for the Navy. From the overall theoretical predictions and actual performance data of the various underwater heater concepts, it is concluded that new diver heater concepts presently under investigation by the Navy will enhance the mission capabilities of the Fleet diver. Furthermore, these concepts may be readily adaptable for commercial application and could solve diver thermal protection problems associated with commercial diving in cold water. INTRODUCTION In the past, diving missions have been limited to a certain extent by breathing gas duration, carbon dioxide scrubbing capability, depth, distance to be traveled, and decompression constraints. Through research and development, the limitations in these areas have been reduced so that diver thermal protection has become one of the single most limiting factors in Navy diving missions. NCSC has evaluated potential heating techniques and devices relative to diver missions and applications to determine the course of action the Navy should follow in diver heater research and development. This report summarizes the characteristics of various potential heat sources for underwater applications, and reviews the results of past and present efforts in Navy diver heater development. DIVER THERMAL REQUIREMENTS Before considering a supplemental heat source, the diver thermal requirements must first be established for determining the amount of supplemental heating, if any, required for a given diving mission. Generally, if a mission can possibly be performed without supplemental heating, it is more economical and practical to depend on the garment insulation alone to keep the diver warm by retaining body heat. If the garment insulation is not sufficient, it may be more practical to use two or more shifts of divers to accomplish the mission. If it is not practical to use more than one team of divers or if the mission conditions are extremely severe, supplemental heating may be the only way to successfully accomplish the mission. To determine the diver's ability to perform an assigned task, the mission conditions must be known specifically, (1) water temperature, (2) water depth, (3) breathing gas, (4) diver work rate, and (5) diver dress. All play a key role in determining a diver's performance on an assigned job. Once these parameters are known, an estimate can be made of the expected duration of effective work by the diver. Tables 1 and 2 give minimum durations which might be expected from a typical diver for various conditions without supplemental heat. Table 1 gives results for a diver working in shallow water while breathing air, and Table 2 gives results for a diver working deeper than 61 meters and breathing heliox.