Each scuba tank is rated for a specific pressure (e.g., 3000 psi) and maximum pressure capacity. Air compressors to refill scuba tanks can include a water bath to cool tanks.
Air tanks are placed in a holding container filled with water (water bath) during the filling process. The water bath provides a cooling environment to stabilize the warmed tank and a medium to detect any minor leaks in the tank walls while cleaning away surface salt and residue.
Water baths sometimes are used in air refill stations to provide fast, hot fills for customers (divers) who are anxious to get back in the water. A major concern with water baths is the possibility of getting water into the tank, compressor or equipment, which could cause corrosion or contamination.
According to Sandiegodiving.com in the article, “The Art of a Good Air Fill, ” tanks should be filled to (or near) maximum working pressure. The maximum recommended fill rate for tanks is 300 psi per minute.
According to Fred Calboun, PE, in the article, “Scuba Tanks,” a water bath during filling provides very little measurable cooling to stabilize the air tank (which is filled with warm air).
Allow adequate time to fill the tank in stages to provide optimum air fills. Fast fills can yield short fills; cooled water baths can promote overfilling tanks beyond their capacity.