Leaks are bad, no matter if they are in Congress or in your rebreather loop. They are annoying and dangerous.
Prior to diving we should all use a check list and do our pre-dive checks, two of which are the “positive and negative” tests. Their purpose is to detect system leaks before entering the water. We pressurize the loop for a positive test and draw a vacuum for a negative test. We then look for changes in counter lung volume and or listen for hissing sounds. If they deflate, inflate or hiss we suspect a leak. If they hold pressure and are silent we say they passed and are safe to dive.
But, are they? It is quite possible you have a leak that eludes this process.
Probably the most common leak is from a split or torn mouthpiece. It can also be quite difficult to find. A tiny tear or hole in a mouthpiece can present quite the mystery. The loop will pass positive and negative testing and seem fine. Then, during the dive you hear that ever so annoying “gurgling” sound. You listen for bubbles and hear none. You signal your buddy to check above you looking for leaks and he tells you there are none. But you keep draining the water from your loop so it has to be coming from somewhere. To further complicate things, the loop might only take in water on inhalation or when you tilt your head in a certain position. You may look for the leak on every exposed part of your rig and not find it.
So what Do You Do?
If you are getting water in your loop and cannot find the source, even after checking all of the prime suspects, change the mouthpiece, even if it looks good. This will often solve the problem or at the very least eliminate the mouthpiece as its’ cause. Sometimes the leak is so minuscule that only an exaggerated stretch, bend or twist will reveal the tear or pinhole.
Sometimes the cause of the leak is not a tear or a pin hole. Believe it or not, if your DSV or BOV is angled incorrectly it will cause the side of your mouth to twist open when you move in certain positions and allow water in. This could potentially cascade into a very unpleasant event if it causes the loop to pop out of your mouth. If you are fighting your loop to keep it in your mouth straight it needs to be adjusted to fit you properly. Loosen the hose clamps and move it so it sits at the same angle as your mouth and does not pull up, down or sideways. Of course be sure to retighten the hose clamps properly.
A small split in the crevices of a loop hose can be very difficult to find. It may only leak when the hose is stretched or turned in a specific way. All other times it may be fine. Because of this it is important to perform a visual inspection of your hoses as part of your pre-dive ritual. Do this when you check your mushroom or flapper valves by stretching the hose. Look for cracks or splits in the crevices between the ridges and where they are clamped to anything (DSV, TEE Pieces etc.). Anything suspect should be visually inspected and tested with a soapy water bubble test or by submersion. Remember, this type of leak can easily pass your standard positive and negative testing, so while I am not suggesting it is something to be paranoid about, it is not something to ignore. This is why I do not like hose covers. They may look cool but can conceal dangerous leaks. High quality loop hoses are quite robust and do not need covers.
Be wary of rubber sleeves covering loop hose clamps. They make the loop look pretty but they can conceal a large tear. In the image below the loop hose was torn by the clamp but it passed a positive and negative test because the clamp was covered with a rubber sleeve which in effect, sealed the leak.
Should you find yourself in the water with this type of leak, the safest solution is to bail out. If that is not practical or possible, an alternate solution is to grasp the hose with both hands and hold it in a position where it doesn’t leak, IE, press the offending crevice together to stop the leak while you abort the dive. Obviously, this is only practical in a limited number of circumstances where the split is small and in which staying on the loop is a better choice than bailing out. Remember, a flooded loop can lead to a caustic cocktail and cascade into a series of very unpleasant, potentially fatal events so if you make this choice, do so judiciously.
The next source of leaks and water intrusion are orings. They should be inspected every time you build your unit. Change any orings that have flat spots, nicks or cracks. Lubricate any orings that require it but don’t over do it. Remember that lube is also a dirt magnet so use care to not pick up any grit while the orings are exposed to the environment. Also visually inspect oring grooves and clean any accumulated dirt or excess grease. If necessary, remove the oring and clean the groove. Use a plastic pick or something similar for removal to help prevent accidental damage.
Every rebreather diver should be proficient at removing water from the loop. He should also be cognizant of how water will collect in his particular rebreather so he does not do anything to worsen his situation. For example, if you suspect water is in the unit and proximate to the scrubber you need to understand the unit well enough to avoid positions that will facilitate water reaching the sorb. I will not go into specifics here because every unit is different. What works well on one unit might be bad on another. If you do not understand this process well or if you understand it but are not good at it you should remedy this deficiency as soon as possible. Practice the procedure often so it becomes second nature. If you do not understand the path necessary to facilitate water removal, call your instructor and ask him to clarify it for you. Once you thoroughly understand the flow of the unit, dewatering becomes very simple. If your rebreather does not permit removing water from its breathing loop you need to account for that in your dive planning and factor in extra conservatism to account for it. Avoid dives or situations where the probability of water ingress into the loop is higher than usual and access to the surface is limited. You don’t want a flood 5000 feet back in a cave with no well rehearsed plan of escape. Remember that a flooded unit becomes extremely negative, making swimming on open circuit bailout more difficult. Even if you can no longer stay on the loop, the ability to remove water from it is important.
The Best Leak Test
So, what is the best leak test? At the beginning of every dive buddies should hover above one and other and look for bubbles. The predive positive and negative tests will find any larger leaks but nothing surpasses an in water bubble check at depth to find any smaller leaks. This can be accomplished at 20 feet in conjunction with a cell linearity check. If this is not practical do it at depth before you begin the actual dive. In any event, this should always be done. Other indications of a leak include gurgling sounds in the loop, hissing noises behind your head, an unexpected change in buoyancy or an increase in the work of breathing. The silence afforded us by diving a rebreather permits hearing the smallest of leaks so be acutely aware of any unusual sounds. Do not ignore them.
I hope some of you, especially newer rebreather divers, find this information useful.
Until the next time, thank you for reading.