What indicates that gas in the tank has significantly decreased?

The indication that gas in the tank has significantly decreased is represented by a pressure gauge reading around 1000 psi. In dive physics, understanding the pressures and their corresponding gas volumes in tanks is essential. A reading around 1000 psi suggests that the tank is nearing a low pressure situation, as most scuba tanks are typically filled to around 3000 psi when full.

A pressure gauge reading around 1000 psi indicates that there's a limited amount of breathing gas left, which necessitates that the diver should consider ascending or getting ready to replenish their tank to ensure safety during the dive. Monitoring pressure readings is a critical skill for divers because it helps in managing air supply, preventing potential emergencies due to depleted gas levels.

Other options, such as a high-pressure warning light or a reading over 1000 psi, indicate either full pressure or an abnormal condition that requires attention, rather than a significantly decreased gas level. Rapid drop in buoyancy, while relevant to dive safety, doesn’t directly correspond to gas pressure in the tank, but more to the underwater dynamics and equipment used, such as buoyancy compensators.