Which equipment is essential for monitoring a diver's air supply during a dive?

The essential equipment for monitoring a diver's air supply during a dive is the pressure gauge. This device measures the remaining air in the scuba tank, allowing divers to keep track of their air consumption and ensure they have enough gas to safely complete the dive and return to the surface. Understanding one's air supply is crucial for safety, enabling divers to manage their ascent and avoid potential emergencies related to running low on air.

While other equipment, such as the dive computer, can provide valuable information related to dive time, depth, and no-decompression limits, it does not directly measure the air supply. Similarly, a depth gauge serves its purpose by indicating how deep the diver is, while a compass aids in navigation underwater. However, none of these instruments provide the critical information regarding the amount of air available, which is why the pressure gauge is indispensable for safe diving practices.