PADI Divemaster Certification Practice Exam

During a predive safety check, observing the SPG needle fluctuate to almost zero and then rise back suggests that there is likely a problem related to the valve being only partially open. When a tank's valve is not fully open, it can create a situation where air flow to the regulator is inconsistent.

Initially, the diver may experience a drop in pressure as the air flow decreases when the valve is not opened all the way, resulting in the SPG reading dropping to nearly zero. When the diver adjusts the regulator or the valve inadvertently shifts further open, the pressure can then return to a more normal reading, causing the SPG to rise again.

This specific behavior of the SPG needle indicates that there is still a supply of air remaining in the cylinder, which rules out the cylinder being empty. A malfunctioning SPG would typically not show a return to a normal reading once the pressure fluctuates significantly. Similarly, if the regulator were broken, the air supply would likely be more severely compromised, leading to more pronounced issues in airflow rather than the specific fluctuation seen in this scenario. Thus, a partially open valve is the most logical conclusion here.