Training Fundamentals: Checking Your Cylinder Valve

It’s surprisingly common for divers to enter the water with a tank valve in the incorrect position. Make sure to check your tank valve before every dive.

Spend time with any seasoned professional diver and they’ll share some of the rookie mistakes they’ve seen while guiding certified divers. It’s surprisingly common, for example, for a diver to enter the water without his cylinder valve in the correct position. This can lead to embarrassment at best and a potentially serious accident at worst. How and why does it happen? And how can you avoid it?

Entering the water without checking your cylinder valve is a surprisingly common diver error. Many divers, if pressed, would concede that at some point they have shamefully slid into their scuba unit, only to have to climb back out of it to turn their gas on. However, if they don’t identify the problem early, a minor error can become a catastrophe.

Potential scenarios play out from the inconvenient to the deadly. Those who remember their core training signal their buddy and share gas while they resolve the problem. Or, if they’ve not stayed suitably close to their buddy they can make a controlled emergency ascent. If they’ve been even more careless and are also too deep to make an emergency ascent, they may release their weight system and make an emergency buoyant ascent. Some, tragically, panic and rush to the surface, succumbing to decompression illness or, alternatively, drowning.

In the DAN (Diver Alert Network) 2016 report on diving incidents for 2014,“low on air/out of air” incidents triggered 9 percent of the 68 reported fatalities. A proportion of these will be due to incorrect cylinder valve positioning. The problem isn’t usually caused by a diver heavy-breathing the cylinder dry in the first few minutes of the dive. Rather, the cylinder valve was likely closed or only partially open from the beginning.

Some of the most prevalent reasons for low-on-gas/out-of-gas situations involving cylinder valves are as follows.

The diver didn’t open his cylinder valve all the way

A diver may become distracted when assembling her equipment. Or, he or she may not do a full buddy check before entering the water. This may be particularly common among inexperienced divers, those who dive infrequently, or with divers at expensive resorts, who assume that the staff will prepare all their equipment. In some instances, the cylinder valve may be open far enough to supply the diver with air at the surface. However, as the diver descends and the ambient pressure builds in tandem with the cylinder pressure dropping, the tank will not deliver any gas even though plenty of air remains inside.

The half/quarter-turn back

Depending on agency and experience, many recreational instructors still teach new divers to open their cylinders all the way and then turn the valve back a half or quarter turn. This practice originated when the cylinder’s brass valves could become stuck if opened all the way and not backed off by a quarter turn, particularly during severe temperature changes. Technical-diving circles have abandoned this practice for two reasons. First, if you must immediately open or close a cylinder valve, you can be certain of which way to turn the valve, i.e. when completing gas shut-downs using a manifold twin-set. There is no way to mistakenly turn the valve the wrong way. Second, modern cylinder valves have evolved such that sticking valves — provided that the valve is regularly serviced and inspected — are a thing of the past.

Incidents occur when recreational divers using the ‘semi-turn’ method accidentally close their valve all the way and then open it a semi-turn instead of vice versa. A valve that is 100-percent open and then a quarter-turn off, and a valve that is 100-percent closed and then a quarter-turn on deliver the same results when breathing on the boat’s dive deck and monitoring the pressure. It will seem as though the valve is sufficiently open and ready for the upcoming dive. However, as the diver descends and the ambient pressure builds in tandem with the cylinder pressure dropping, the gas flow will disappear, despite plenty of gas remaining within the cylinder.

Cylinder valve amnesia 

If you’re leaving your scuba system for some time, i.e. on a long boat ride, during a long briefing or instructional session, it’s best to turn off the gas and purge the unit so that the pressure gauge reads zero bar/psi. When you’re ready to suit up, personally verify that the cylinder valve is fully open.

Some divers have the bad habit of either leaving their scuba system with the valve open and the unit under pressure. Alternatively, some will close the cylinder valve but forget to purge the residual gas so that the pressure gauge needle reads zero. Then, failing to check that the valve is open and skipping a thorough buddy check, the residual gas in the system will present a positive reading on the pressure gauge and even several breaths from the regulator before it fails to deliver gas. This leaves the error unidentified until several meters or feet beneath the waves.

Helpful buddy/dive crew

Sometimes other divers or members of a boat crew — attempting to be helpful — may close the valve of a seemingly unattended scuba unit. A well-meaning buddy may close the valve and purge the system, believing you’ve forgotten to do so. Or, more likely, if a vessel is traveling through choppy seas or for extended periods, the crew will close all cylinder valves on the dive deck. This helps avoid any unnecessary hissing or unintended emptying of cylinders as the vessel is in transit. If the diver skips the buddy check or doesn’t notice the closed cylinder valve while putting on their equipment, they may enter the water with a closed valve.

Avoid the closed cylinder valve scenario

Don’t put yourself in a low-on-gas/out-of-gas situation during your dive due to a cylinder valve error. Follow these steps to avoid this mistake:

  1. Always do a complete equipment assembly, personally if possible. Be methodical, and use a checklist if necessary to cover all the steps. Be particularly mindful if using rental equipment.
  1. If someone interrupts you during equipment assembly, repeat all the steps from the beginning. This will help you avoid accidentally missing a critical step in the chain.
  1. Open cylinder valves all the way. Eschew the historic practice of opening the cylinder valve then closing it a quarter or half turn. This helps to avoid confusion in all situations.
  1. Always do a thorough buddy check before entering the water. This is your responsibility as a certified diver, not that of the dive guide. Breathe from all second stages while monitoring your pressure gauge. If the needle drops or waivers with each breath, your valve is likely either fully or partially closed.
  1. Stay close to your buddy on descent and within a few strong fin kicks throughout your dive. Review your low-on-air/out-of-air procedures, and know the location of your buddy’s alternate-air source and how it functions. This must be instinctive if something goes wrong.

Entering the water and beginning the dive with the cylinder valve incorrectly positioned is a surprisingly common error. Be meticulous in your equipment assembly. Proper preparation for the dive and a final buddy check can help avoid a potentially hazardous situation.