Air Starvation at Depth: Equipment or Human Failure?


  • Quarry dive in 70°F weather.
  • Diver experienced a sudden out of air at 85fsw, temperature at depth was 54°F.
  • Unable to locate his buddy, diver chose to rapidly ascend to the surface.

Reported Incident:

After an uneventful descent on the first dive in a flooded quarry, a diver experienced a sudden air-out at 85 fsw (26 msw). The environmental air temperature was 70°F (21°C) and the lowest underwater temperature was 54°F (12°C).

Poor visibility prevented immediate contact with his buddy and the diver proceeded with a rapid ascent to the surface. No air was available through the regulator throughout this ascent, which was recorded by the dive computer as 230 feet/minute (70 m/min). He was clearly not able to carry out a safety stop and he kept his weight belt on.

Air was then available through the regulator on the surface and the diver waited five minutes for his buddy to resurface. He was also able to re-inflate his BCD at that time.

The only post incident symptom was a headache, which resolved after five minutes of surface oxygen.

For precautionary purposes, the diver was evacuated to a hospital where a CT scan and continuous, 24-hour observation was done. Oxygen was administered throughout this period.

There were no further symptoms and the diver was discharged without needing any recompression treatment.


The list of typical causes of an air-out at depth would include the cylinder valve being inadequately opened; a perforated diaphragm or damaged piston O-ring, failure of any of the regulator springs, or an inadequate inter-stage pressure setting in the first stage regulator, or simply degraded hose material lodging before the regulator.

An ambient water temperature of 54°F (12°C) would not cause freezing or even sticking of the piston-sensed first stage regulator in use at the time of this accident.

In the past, the practice taught to new divers was to open the cylinder valve fully and then back it off a quarter-turn; the concern being damaged to the regulator components due to excessive mechanical force. However, due to reported incidents where the valve was actually only a quarter-turn open causing starvation of gas when at depth, current practice is to open the cylinder valve fully.

In this case, the diver stated that the cylinder valve was fully open.

The inter-stage pressure is achieved through either a flexible diaphragm, or a metallic piston. Although very reliable, poorly maintained regulators can fail due to fatigue or o-ring damage.

Although unlikely, a poorly serviced regulator without the inter-stage pressure being set accurately on the workbench, could result in insufficient gas as depth increases.


There have been numerous reports of late where the inner lining of breathing air hoses has degraded, allowing sizeable debris to move in the direction of gas flow. With increasing depth, the force holding the debris in a place increases with depth. Once at the surface, the inter-stage pressure is the lowest. It is possible that this would then allow the debris to shift and gas to flow again.

The sudden nature of the failure at a specified depth could tend to indicate either mechanical damage inside the first stage regulator, or debris blocking the flow path to the breathing regulator.

The case notes in this incident do not provide sufficient information, such as a post incident investigation of either the first stage regulator or the demand valve. Hence no specific cause can be attributed to this accident.