Most closed-circuit rebreather divers think about diluent twice in their training: first when they pick the blend that will go in the bottle, and again when they bench-test the unit before a dive. After that, the gas tends to fade into the background. The automatic diluent valve handles routine injection on descent, the manual valve sits within arm’s reach for the occasional flush, and the diver focuses on cells, scrubber, and the dive itself. That works on a calm dive where nothing surprises the loop. It does not work on a profile that involves real depth changes, a long bottom time, an unexpected leak, or a deliberate setpoint shift in the middle of the ascent.
The CCR divers who run the cleanest dives are the ones who treat diluent addition as an active part of how they fly the loop, not a passive system that happens around them. They know what the automatic diluent valve is supposed to do on every meter of a descent, when to override it with the manual valve, what its firing pattern is telling them about loop integrity, and when the onboard 3-litre cylinder has stopped being enough for the dive in front of them. Here is the diluent management plan that experienced AP Diving Inspiration and Evolution divers run, and the reasoning behind each decision.
Why Does Diluent Management Matter on a CCR Dive?
Diluent does three jobs in a closed-circuit rebreather. It keeps the loop volume topped up as ambient pressure compresses the gas inside it, it dilutes pure oxygen to keep the inhaled PO2 inside the diver’s chosen range, and it sits in reserve as the open-circuit bailout supply if the loop has to be abandoned. The first job runs constantly. The second runs every time the unit injects oxygen to hold setpoint. The third runs only on an emergency that ends the loop dive. All three matter, and they share a single supply.
That shared role is why diluent management is more than just “press the button if the loop feels small.” Every cubic foot of diluent the diver injects has to be planned against the bailout reserve. Every flush on the descent affects the gas budget on the bailout cylinder, because the same supply is doing both jobs unless the diver has rigged an offboard bottle. A diver who treats the ADV as a free auto-fill is silently eating into the bailout margin every time the valve fires. The diver who watches the diluent gauge as carefully as the oxygen gauge keeps the math honest across the whole dive.
The choice of diluent mix shapes the rest of this conversation. A 12/65 trimix diluent for a 60-metre wreck dive behaves very differently from an air diluent on a 30-metre reef dive, and the diluent’s maximum operating depth dictates when a flush is safe and when it is hyperoxic. the helium-or-air diluent choice that anchors every CCR dive plan drives every downstream decision about ADV behavior, flush timing, and offboard staging that this article walks through. If the blend in the bottle is wrong for the dive, no amount of in-water diluent craft will rescue the profile.
How Does the Automatic Diluent Valve Behave Underwater?
The automatic diluent valve sits in the loop and opens whenever loop pressure falls a small amount below ambient. The exact threshold varies by unit and by how the valve is tuned, but the principle is the same across every Inspiration and Evolution platform: when the diver inhales hard enough or the loop volume compresses enough that the internal pressure dips below ambient by roughly 10 to 15 millibar, the diaphragm cracks the seat open and diluent flows until the loop pressure equalizes again. The valve then closes itself. The diver does nothing.
On a normal descent that means the ADV fires repeatedly. As the diver goes from 10 metres to 30 metres, the absolute pressure roughly doubles and the loop volume tries to halve. The ADV adds gas through the descent in small bursts to keep the breathing volume comfortable. Once the diver settles at a working depth and stops descending, the firings stop. The valve should sit silent unless the diver vents the counterlungs to adjust buoyancy, changes body position in a way that shifts the loop’s volume distribution, or breathes hard during a current swim. Those are all normal triggers.
Verifying that this behavior is intact starts on the bench, not in the water. A clean ADV cycle belongs in the pre-dive workflow that already includes a real ADV bench test so the valve has been proven to crack at the right threshold and reseat cleanly before the unit goes in the water. Two or three controlled cycles – breathe the loop down, let the valve refill it, listen for a clean soft click rather than a sticky hesitation – is enough to confirm the valve, the diaphragm, and the supply hose are all behaving. A diver who skips that bench check is gambling that nothing has changed since the last dive, which is a poor habit on a life-support unit.
When Should You Override the ADV With Manual Addition?
The manual diluent valve exists for the moments when the ADV’s threshold is the wrong tool. The most common is a deliberate diluent flush to verify cell behavior. After a long deep portion of a dive, cells can drift, get slow, or disagree. A flush at a known PO2 drops the loop’s oxygen fraction back to the diluent’s fraction, which gives the diver a check against the cell readings: if the cells should read 0.65 in the loop after the flush and one of them reads 0.85, that cell is suspect. The manual valve is the right tool because the ADV will not fire unless the loop pressure has dropped, and a flush is about resetting the gas mix, not topping up volume.
The second common case is setpoint craft on the ascent. As the diver rises through stops, the unit will keep injecting oxygen to hold setpoint, but the diver can speed up offgassing by deliberately running through the manual flushes that keep the loop honest through long deco stops rather than letting the unit handle gas exchange in the background. The flush gets the loop closer to a clean diluent fraction so the cells, the controller, and the deco profile are all working from the same baseline.
The third case is buoyancy and trim adjustment that the ADV is not designed to handle. If the diver wants to deliberately overfill the loop slightly to push the counterlungs into a specific position, or to add a bubble of diluent to balance a drysuit squeeze on a fast descent, the manual valve is the precise tool. The ADV will only add what the loop pressure has lost. The manual valve adds what the diver asks it to add. Both behaviors are useful, but they are not interchangeable, and a diver who treats the manual valve as an emergency-only control is leaving real craft on the table.
What Do ADV Firing Patterns Tell You About the Loop?
The ADV is also a diagnostic instrument. Once a diver learns the normal firing pattern for a given dive profile, deviations from that pattern become an early warning system. A valve that fires every few minutes at constant depth is almost always a leak somewhere in the breathing loop. The leak is small enough that the diver does not feel a change in the loop’s stiffness, but it is constant enough that the loop pressure keeps dropping below ambient until the ADV catches up. The likely sources are the mouthpiece seal, the hose connections at the counterlungs, a counterlung seam that is starting to perish, or the scrubber cap o-ring. None of those will fix themselves, and all of them get worse with depth.
A valve that does not fire at all during a controlled descent is the opposite warning. Either the ADV is stuck closed, the threshold is set too low, or the diluent supply is restricted somewhere upstream. The diver who notices a quiet descent on a unit that normally fires every few metres should switch to manual injection to keep the loop comfortable, watch the diluent gauge to confirm the supply is actually flowing on manual demand, and end the dive at a planned turn point rather than push through. A unit that is suppressing its own ADV is a unit that needs the bench.
Continuous firing – a valve that is essentially open the whole time – is the worst version. That usually means the seat is fouled or the diaphragm has failed, and gas is flowing through the ADV with no closing pressure. The loop overpressurizes, the overpressure valve dumps gas continuously, and the diluent cylinder empties at a startling rate. The correct response is to shut the manual diluent isolator, switch to open-circuit bailout, and ascend on a planned schedule rather than try to diagnose the valve at depth. The cylinder may not last to the surface if the valve continues to flow.
When Does an Onboard Diluent Cylinder Stop Being Enough?
The onboard 3-litre diluent cylinder is sized for an entry-level technical CCR dive with a modest descent, a working bottom time, and a deco obligation that the same cylinder can support on a bailout to the surface. That math holds on a single-mix dive in the 30-to-45-metre range with a tolerant deco profile. Once any of those variables stretches, the onboard supply gets thin. A diver doing a series of fast descents to 60 metres with helium in the diluent will burn through onboard gas faster than the ADV cadence suggests. A diver running a long deco hang with the unit holding setpoint will draw down the same cylinder more slowly but for longer. A diver who is also using the diluent as drysuit gas in a cold-water profile is splitting one supply across two jobs.
The cleanest answer to all three is an offboard diluent cylinder rigged on the diver’s side, with its own first-stage regulator and a swap fitting that connects to the unit’s diluent inlet. The offboard supply takes over the diluent role and frees the onboard cylinder to act purely as bailout. On a long expedition profile that is the standard rig. Drysuit divers commonly add a separate inflation cylinder on the same logic – one supply, one job – rather than splitting onboard gas between the loop and the drysuit. The decision is rooted in the deep-trimix planning math that drives those offboard decisions, and the diver who can recite the diluent budget for a planned dive at the start of trip planning is the diver who arrives at the dive site already knowing which cylinders go in which positions.
Two depth bands with different mixes is the other common trigger for offboard staging. A wreck dive that has a 50-metre bottom phase and a 6-metre clean-up phase wants a denser trimix at depth and a leaner mix for the shallow work. Switching diluent mid-dive used to be a clumsy operation. With a proper offboard rig and a dedicated swap protocol, it becomes a routine team manoeuvre, and the diver who plans for it benefits from cleaner gas at every depth instead of compromising on a single onboard fill that has to work for both ends of the profile.
How Does Silent Diving Support Your Diluent Plan?
Diluent management lives at the intersection of chassis condition, valve service, and the right rig for the dive in front of you. Silent Diving’s authorized AP Diving service team overhauls the ADV and the manual diluent valve as part of a normal Inspiration or Evolution chassis service, confirms intermediate pressure on the diluent regulator, pressure-tests the supply hose, and runs the unit through a series of bench cycles so that the valve cracks at the right threshold and reseats cleanly under load. A unit that has had a recent service window is a unit that the diver can trust to behave the way the rest of this article describes.
For divers who are stepping up to dives where offboard staging is the cleaner answer, the service team can also walk through the cylinder, valve, and regulator choices that match the trip’s profile, and confirm that the diluent inlet fittings on the unit are compatible with the planned offboard rig. The simplest pre-trip rhythm is a phone call eight to ten weeks before sailing, a service appointment six weeks out if anything needs attention, and a parts order four weeks out so the offboard regulator, hoses, and any replacement valve internals arrive before the unit is packed for travel. That timeline has held for hundreds of customer trips and is the calm version of the work, rather than the panic version that happens when the gear gets checked the night before departure.
Frequently Asked Questions
Can you dive a CCR without using the ADV?
Technically yes, but it is not a normal practice. Most AP Diving Inspiration and Evolution divers keep the ADV armed on every dive because the valve handles routine descent-volume loss automatically and frees the diver to focus on trim, instruments, and the environment. A diver who chooses to dive with the ADV disabled has to add diluent manually on every depth change, which is workable on a short bench-test dive but is not how the unit is designed to be flown on a working profile. If your ADV is acting up, the right answer is to get it serviced, not to dive around the problem.
How often should the ADV fire during a normal descent?
Expect the valve to fire roughly every few meters during a controlled descent, because Boyle’s law squeezes the loop volume the deeper you go. The exact cadence depends on your lung volume, your trim, how much you are venting from the counterlungs, and your descent rate. What matters more than a specific count is the pattern: regular short firings during descent, no firings once you settle at depth, and an occasional firing if you change body orientation or vent the loop on purpose. Continuous firing at constant depth is not normal and points at a leak or a stuck valve.
Is it safe to manually flush diluent at depth?
Yes, and on many dives it is the right thing to do. A deliberate diluent flush lowers the loop’s oxygen fraction back toward the diluent’s mix, which is useful for resetting a PO2 spike, verifying cell behavior, or running a planned setpoint transition. The technique matters: vent the loop down first, then add the flush in controlled bursts rather than one long press. Doing this at the wrong depth on a hyperoxic mix would be dangerous, so the diluent’s maximum operating depth has to support a flush at your current depth before you touch the valve.
Do you need offboard diluent for every deep dive?
Not for every deep dive, but it gets harder to avoid as bottom time stretches and as the diluent mix gets richer in helium. A small onboard 3-litre diluent cylinder will run an entry-level technical CCR dive comfortably, but a long expedition profile that demands a denser fill, a hot drysuit gas, or two depth bands with different mixes is the place where staging offboard diluent (or a second drysuit-gas cylinder) becomes the simpler answer. If you are planning a trip where the math is tight, the dive plan should be built around the offboard rig from the start, not added as a workaround.
What should you do if the ADV sticks open mid-dive?
A stuck-open ADV will overpressurize the loop and dump gas continuously, which usually shows up as constant freeflow from the overpressure valve and a rapidly emptying diluent cylinder. The correct response is to shut the manual diluent isolator valve to stop the gas flow, switch to bailout on open circuit, and end the dive in a controlled ascent. Do not try to diagnose the ADV in the water. Once you are on the surface, the unit needs a service appointment before the next dive.
How do you check ADV function before a dive?
Build the unit, open the diluent supply, hold the ADV body closed manually, and breathe the loop down. Release the ADV body. The valve should open and refill the loop with a clean soft click rather than a sticky hesitation or a continuous hiss. Doing this in two or three cycles confirms both that the valve seats are clean and that the activation diaphragm is responding at the right threshold. The diluent pressure gauge should drop slightly with each cycle, which confirms the supply is actually flowing through the ADV and not around it.
Does Silent Diving service ADV and manual diluent valve components?
Yes. ADV service, manual diluent valve overhauls, intermediate-pressure regulator service, and offboard regulator setup are part of the normal AP Diving Inspiration and Evolution chassis service the Silent Diving team handles. If you are noticing sticky ADV behavior, slow opening, or an intermediate pressure that drifts between dives, the right call is to book a service appointment before your next trip rather than diving around the symptom.
Need help applying this to your own CCR setup?
Talk with Silent Diving before your next dive.
Get practical guidance on AP Diving products, rebreather service, parts, training, and planning support from the Silent Diving team.