Closed-circuit divers fear loop floods for a reason. The unit’s quiet efficiency depends on a closed gas path between the diver’s lungs and the scrubber, and any water that enters that path changes the chemistry inside the canister, coats the surfaces of the oxygen cells, and forces a decision in the next thirty seconds. AP Diving’s Inspiration and Evolution chassis are built around redundant valves and tight tolerances, but no rebreather is sealed against a careless mouthpiece bite at the surface or a missed pre-dive pressure check. When the loop gets wet, the diver’s response decides whether it stays a minor incident or becomes a real in-water emergency.
This article walks through how a flood actually starts on a CCR dive, what happens inside the scrubber and the sensor stack once water arrives, how to read the difference between a wet breath and a flooded loop, when the decision tips toward bailing off the loop, and what the unit needs from the diver back on the boat and in the days that follow.
How Does a CCR Loop Actually Get Wet?
The most common cause of a wet loop is also the simplest. A diver removes the mouthpiece at the surface to talk to the team or take a sip of water and forgets to close the dive surface valve. The mushroom valves in the mouthpiece are designed to direct gas flow in one direction during a dive, not to seal against open water above the head, and when the unit goes back below the surface they let a half-cup of saltwater into the inhalation hose. The first deep breath on the descent draws that water past the mouthpiece and into the loop. This single sequence accounts for the majority of minor flood incidents that experienced CCR divers report on debrief.
A second mechanism is a torn or unseated exhalation diaphragm. The exhalation counterlung sits below the chest on the Inspiration and Evolution platforms, and the diaphragm has to flex on every breath. If the diaphragm was folded wrong during a service swap, compressed against a sharp object in a packed travel case, or stretched past its working envelope, a small tear can develop that lets water past on each exhale. The diver may not feel the leak immediately. The loop fills slowly across the first ten minutes of the dive, and the first signal is often a change in the breathing rhythm or a touch of weight in the counterlung that did not exist on the descent.
Less common mechanisms include a leaking automatic diluent valve seal, a hose pinhole that opened on the last descent rather than the current one, a mis-routed inflator that took the loop’s QC6 fitting rather than the drysuit or wing port, and a counterlung breach against fixed structure during a hard entry. Each one shares a single feature: the loop is normally a sealed gas system, and any path that opens a connection between the loop and the surrounding water creates a flood path. Knowing where those entry points live is the first step in catching them before they become an in-water problem.
What Happens to the Scrubber Once Water Reaches It?
The scrubber is the part of the loop that responds most violently to water, and it is the reason a loop flood is treated as a serious incident rather than a minor nuisance. The CO2 absorbent inside the canister is a calcium hydroxide and sodium hydroxide formulation that becomes strongly alkaline once wet. A small amount of moisture is fine, because absorbing CO2 produces water as a byproduct and the absorbent is designed to tolerate that condensate. A flood is different in scale and concentration, and the difference is what changes the loop from a working breathing system into a chemistry experiment.
When sustained water reaches the absorbent, two things happen in sequence. First, the dry granules at the contact surface can liquefy into a hot caustic slurry that the diver tastes as a sharp soapy bite on the inhale. This is the caustic cocktail that every CCR course warns about, and it is a genuine medical risk when even a small amount makes it past the mouthpiece and into the mouth or throat. Second, the chemistry that keeps your scrubber working through a dive collapses as the active surface sites are saturated with water rather than gas, and CO2 begins to break through far faster than the dive plan expected. This is why a wet loop and a CO2 hit can arrive almost together on a deep dive.
The behavior depends on how much water reached the absorbent and how active the chemistry was when it arrived. A scrubber that has already been working on a long dive is hot, and the reaction with incoming water is faster and more intense. A scrubber that was freshly packed at the start of the dive is cooler and slower to react. Either way, once a flood has touched the absorbent the canister is compromised, and the diver who finishes the dive on a flooded scrubber is exposing themselves to a CO2 risk that grows minute by minute even after the water entry has stopped.
How Do the Oxygen Cells and Electronics React?
Oxygen sensors in a CCR are galvanic cells with a sealed lead anode and a porous PTFE membrane that lets oxygen molecules diffuse through to the electrolyte. The membrane is hydrophobic by design. It has to be, because absorbing CO2 generates water that condenses inside the loop on every dive, and the cell is rated to handle that routine condensate without drift. Salt water reaching the membrane during a flood event is a different chemistry, and the failure modes are not always what divers expect.
When salt water deposits on the sensor face, the output usually does not drop straight to zero. The cell keeps reading, but its output current changes as the membrane surface chemistry shifts, and the value drifts. A cell that should be reporting 0.7 bar at the current setpoint can read 0.8, 0.6, or jump erratically across a single breath. The handset sees those readings, applies the voting logic the firmware is built around, and may suppress an alarm that should have fired because the other cells in the head appear to agree on a different value. The fact that oxygen sensors are rated to handle the moisture from normal breathing, but not the chemistry of a sustained salt water immersion, is part of why a wet loop is treated as a cell event in addition to a scrubber event.
The electronics around the sensors react in two ways. The wet cell can produce a transient short across the connector that the handset interprets as a fault and logs to the dive record. The handset bus and the cell-head wiring, if they take any direct water during the flood, may begin to corrode in the hours and days after the dive even when the dive itself completes cleanly. Both effects are why post-dive inspection after any flood is non-negotiable, and why most experienced divers will dry the head, swap or test the cells, and run a calibration cycle before treating the unit as ready to dive again.
When Does a Wet Loop Force You Off the Loop?
Not every wet loop is a bailout event. A single mouthful of water at the start of a descent, with a competent diver who clears the mouthpiece, switches briefly to the open-circuit dive surface valve to purge the loop, and returns to the loop without any taste change or breathing resistance change, is a routine recovery. The unit was not flooded in the operational sense. A wet breath was taken, the loop drainage handled it, and the diver continued the planned dive without losing the bottom phase.
A partial flood is the next category up, and it is where the decision gets harder. The diver feels resistance on inhale that was not present a minute earlier, the breath has an off taste that may or may not be caustic, and the loop drainage is not clearing it. At this point the discipline is to hold depth, switch to the open-circuit bailout regulator, breathe normally for thirty to sixty seconds, and assess. If the taste is sharp and soapy, the scrubber has been compromised and the dive should finish on bailout. If the wetness came from one swallowed mouthful and the loop has visibly cleared, the diver can sometimes recover the loop and continue the dive, but always with the bailout regulator on a shorter leash than usual for the remainder of the profile.
A full flood, any caustic taste at any time during the dive, or a breathing pattern that the diver cannot regulate is a dive-ending event. The broader decision of when to bail off the loop rests on the same principle. The cost of an unnecessary bailout is one ruined scrubber and a longer deco hit on open circuit. The cost of staying on a loop that is producing caustic chemistry at depth is measured in the diver’s airway and judgment, and it is not a tradeoff worth making. Practice the bailout response in training and trust it in the water.
What Should You Do With the Unit After You Surface?
Get the unit out of the water cleanly. Carry it back to the boat with the mouthpiece closed and held high to keep additional saltwater from running into the loop on the surface swim. Once on the boat, the temptation is to start disassembly immediately. Resist that unless the team has the rinse water and the dry workspace to do it correctly. A botched in-the-field disassembly is one of the few ways to turn a recoverable flood into a serviceable-component loss, and most operational protocols call for stabilizing the unit on the boat and doing the real cleanup back at the dive base.
At a minimum, separate the scrubber canister from the chassis as soon as it is practical, and assess what is inside. A compromised absorbent canister should be drained outside the boat, because the slurry inside the canister is alkaline and corrosive and should not go into the bilge or the scuppers. Replace the absorbent with a fresh charge before the unit will dive again. If the diver does not have a spare scrubber on the boat or in the dive base, that unit’s dive day is over.
After the scrubber is handled, dry the head completely, inspect the oxygen cell connectors for any white-yellow salt residue, and run the handset through a self-test if the unit’s electronics manual supports it. The cell readings in the first twenty-four hours after a wet event are diagnostic. Cell warnings in the days that follow can tell you whether the sensors took damage in the flood or whether they are continuing on a normal aging curve. If any cell is drifting beyond its expected millivolt envelope under known PO2 conditions, replace it before the next dive rather than waiting for the routine service interval to come around.
How Does Silent Diving Help After a Flood Incident?
Silent Diving is the authorized AP Diving service distributor for North, Central, and South America, and any post-flood inspection on an Inspiration or Evolution unit benefits from a technician who has worked on hundreds of these chassis. The electronics, sensor harness, counterlung connections, mushroom valves, and dump valves all have failure modes that look identical to routine wear when inspected by the owner and obvious when inspected by a technician with the unit on the bench. Silent Diving’s authorized service team handles full chassis and electronics inspection, sensor swap and calibration, and absorbent canister condition checks as part of a focused post-flood service visit.
Whether the event was a single-cup wet breath or a full loop flood, the unit should not return to deep diving without an inspection that confirms the seals, the cells, and the electronics are sound. A short service visit now is the cheapest possible insurance against a follow-on failure on the next deep dive, and it gives the diver a clean baseline to plan future profiles from.
Frequently Asked Questions
What is the difference between a wet breath and a flooded loop?
A wet breath is a small amount of water that enters the mouthpiece and is cleared by the loop’s drainage or a quick switch to the open-circuit dive surface valve. A flooded loop is sustained water that has reached the scrubber canister or the counterlungs and changes the loop’s chemistry or breathing resistance. The first is routine and recovered without ending the dive. The second is a dive-ending event in most cases.
Does a flooded loop always create caustic cocktail?
No. Caustic cocktail requires enough water to liquefy the surface of the CO2 absorbent and produce a hot caustic slurry. A small wet breath that the mouthpiece drainage handles will rarely reach the canister at all. A sustained flood that wets the absorbent for more than a few seconds is the scenario where caustic cocktail becomes a real risk, and that is the reason any taste change on inhale is treated as a bailout signal.
Should you always bail off the loop the moment it floods?
Not for a single wet breath that clears cleanly. Yes for any sustained taste change, breathing resistance, or wetness that the loop cannot self-drain in two or three attempts. The general rule is that an unnecessary bailout is cheap and a delayed bailout is expensive. When in doubt, switch to open circuit, hold depth, breathe normally for thirty to sixty seconds, and decide whether the loop is recoverable from that stable position.
How do you clear water from a CCR loop underwater?
Most modern CCR mouthpieces include a drain that purges water from the lowest point of the loop when the diver tilts the unit and exhales gently into the open-circuit side. The exact procedure is unit-specific and must be drilled in training before it is needed in the water. If the loop drainage cannot clear the water in two or three attempts, the loop is no longer minor-wet and the dive should finish on bailout while the loop drains on the ascent.
Will a flooded loop damage my oxygen cells?
Sometimes. Galvanic oxygen sensors are rated to handle the moisture from normal breathing, but salt water on the sensor face can deposit salts on the PTFE membrane and shift the cell’s millivolt output. The damage is not always immediate. Drift over the next few dives is the more common picture, which is why post-flood calibration and tracking the cells closely for several dives are the standard response after any wet event.
Do you have to send the unit in for service after a flood?
For anything beyond a single wet breath that cleared cleanly, yes. Even when the dive completes and the diver feels nothing unusual, the chassis seals, the cell connectors, the dump valves, and the counterlung fittings should be inspected by a technician who can confirm that nothing took hidden damage. For a unit serviced by Silent Diving, a post-flood inspection is a focused short visit rather than a full annual service cycle.
Can you prevent a loop flood completely?
You cannot reduce the risk to zero, but you can drive it close. The most common cause is operator habit. Leaving the dive surface valve open when removing the mouthpiece, packing the unit roughly between dives, or skipping a positive-negative pressure pre-dive test are all addressed by discipline rather than by a hardware upgrade. The pre-dive pressure checks in particular are the single most effective defense against a slow loop leak that turns into a real flood at depth.
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