What is the main difference between a "J" valve and a "K" valve?

The main difference between a "J" valve and a "K" valve lies in the reserve feature. The "J" valve includes a built-in reserve feature that provides an additional, limited supply of air after the main supply has been depleted. This allows a diver to access a small amount of air even when the tank is nearing empty, which can be crucial for making a safe ascent. The reserve feature is designed to give divers a safety margin, allowing them time to reach the surface or to a safer depth when they might otherwise be out of air.

In contrast, the "K" valve does not contain this reserve feature, which means that once the air supply is depleted, there is no additional air available. This makes the "J" valve a more safety-oriented option for divers who may find themselves relying on that extra air in emergency situations.

While pressure gauge type, material construction, and weight may differ between various valves, they do not represent the defining characteristic that sets the "J" valve apart from the "K" valve. Understanding the importance of the reserve feature is crucial for divers as it directly affects diving safety and emergency preparedness.