A diver who has only ever ascended through deco on open circuit can be surprised the first time they run the same depth on a closed-circuit rebreather. The clock on the wrist computer counts down, the depth gauge ticks up in slow increments, the bubbles overhead are missing, and the plan that came out of the deco software does not look like the one open-circuit divers around them are running. CCR decompression is not magic. It is the same set of physical laws applied to a loop that holds oxygen at a constant partial pressure across changing depth, which gives the planner different inputs to work with. This article walks through what changes during a CCR ascent, where gradient factors fit, how each stop is actually executed, and what happens when one of those stops will not let go on schedule.
Why Are CCR Deco Profiles Built Differently?
The single largest difference between a CCR decompression profile and an open-circuit decompression profile is what the loop does for the diver while they are ascending. On open circuit, a diver hits a deco stop breathing whatever percentage of oxygen sits in the cylinder they are currently using. That gas was tuned for a depth window. At the shallowest stops the diver carries deco gas, often 50 percent oxygen or pure oxygen at 20 feet, and switches manually when the depth window opens. The breathing mix and the planned profile are coupled through hardware. Change either one and the math behind the dive changes.
On closed circuit the diver is breathing whatever blend of inert gas and oxygen the loop is currently mixing to hit the diver’s chosen setpoint. As depth decreases, the loop’s diluent contribution shrinks and the oxygen fraction climbs. By the time a CCR diver is at a 20-foot deco stop holding a 1.3 setpoint, the loop is roughly 88 percent oxygen without the diver lifting a finger. There is no gas switch and no carried deco cylinder for the loop’s own ascent. The deco gas is built into the closed system.
That single fact reshapes the deco profile in three ways. First, the diver is on the ideal mix at every depth, not just at the bottom and the planned stops. Second, the inert gas pressure driving on-gassing and off-gassing is lower at every depth than it would be on open circuit, because the loop is constantly pulling the oxygen fraction up to the chosen setpoint. Third, the diver carries open-circuit bailout cylinders sized for failure recovery, not for normal ascent gas. The plan that emerges from CCR-aware deco software is shorter, smoother, and built on different assumptions than the open-circuit equivalent. A CCR diver who reads their first computer-generated deco plan against an open-circuit dive of the same depth and bottom time sees a noticeably shorter total runtime that still respects the slow tissue compartments the deco model cares about.
How Does Constant PO2 Reshape Tissue Loading?
Decompression theory tracks how much inert gas saturates a diver’s tissues at depth and how fast it leaves on ascent. On open circuit the diver’s tissues are loaded by whatever inert gas fraction sits in the cylinder. Air at 130 feet loads tissues with nitrogen at roughly 4.0 atmospheres of nitrogen partial pressure. A nitrox 32 mix at the same depth loads them with nitrogen at roughly 3.4 atmospheres. The lower the inert gas fraction in the breathing gas, the lower the loading rate.
A CCR loop running at a 1.3 setpoint at 130 feet has an inert gas partial pressure close to 3.7 atmospheres if the diluent is air, but the planner can choose to run a lower setpoint at depth for central nervous system reasons, which changes that math again. More importantly, the same loop at 90 feet still holds 1.3 atmospheres of oxygen, so the inert gas portion is only roughly 2.4 atmospheres rather than the 2.8 atmospheres an air diver would be carrying. Multiply that across every depth on the ascent and the tissue compartments are off-gassing into a smaller inert pressure than they would on open circuit, which accelerates real off-gassing across the dive.
The setpoint is the lever the diver pulls to control that effect. A higher setpoint on ascent and through decompression maximizes off-gassing speed but pushes the diver’s central nervous system clock harder. A lower setpoint stretches the deco time but keeps the CNS exposure conservative. Most CCR divers run a low or moderate setpoint at depth to protect against central nervous system oxygen toxicity and then lift to a higher setpoint once the dive transitions to deco. The setpoint a diver picks for the bottom phase and the setpoint they lift to for the ascent are coupled decisions that affect both safety margin and total runtime, which is why CCR planning software accepts both numbers and not just one.
Where Do Gradient Factors Fit on a CCR Plan?
Gradient factors are the conservatism dial CCR planners set on their deco software, expressed as a paired GF-low and GF-high. The GF-low number controls how aggressively the diver leaves the bottom and how soon they begin their first stop. The GF-high number controls how shallow the final stop sits and how conservatively the diver clears their last tissue compartment before surfacing. Lower numbers mean more conservative deco, deeper first stops, longer total runtime, and a wider safety margin against decompression sickness. Higher numbers mean a shorter total runtime and tighter tolerances.
There is no universal pair every CCR diver should run. Open-circuit divers were historically taught around the 30/85 default that emerged from Erik Baker’s gradient factor approach, then a wave of literature pushed toward 50/85 or 30/70 depending on the school of thought. CCR divers face the same conservatism question with one practical wrinkle: the loop already delivers a faster real off-gassing rate at every depth than an equivalent open-circuit profile would, so a CCR diver running the same gradient factor pair as an open-circuit diver is functionally more aggressive. Many experienced CCR divers compensate by tightening their GF-high by 5 to 10 points compared to their open-circuit defaults, which lengthens the final shallow stop and clears slow tissues more cleanly.
The dive computer’s CCR planning mode is the tool that makes this choice operational. A computer set to OC mode while the diver actually runs CCR will produce a plan that ignores the loop’s contribution and prescribes deco as if the diver were breathing back-gas the whole way up. That is conservative, because the diver will surface cleaner than necessary, but it is also useless for runtime planning and for any dive where the bailout volume math depends on a realistic ascent profile. Most CCR divers run their deco software in CCR mode with their actual setpoint pair and their chosen gradient factor pair entered, and then verify the resulting plan against a backup independent computer before splash. Two computers that agree to within a minute or two on total runtime are a quiet sign that the plan is sound.
How Do You Execute Each Stop on a CCR Ascent?
Executing a CCR deco stop is more deliberate than executing an open-circuit deco stop, because the diver has more inputs to monitor and more levers to pull. Most stops break down into four small tasks: hold depth, monitor the loop, manage the ventilation rhythm, and watch the clock against the computer’s evolving call.
Depth control on a CCR is precise because the loop’s volume can be adjusted in small increments through the diluent and oxygen valves rather than depending on a buoyancy compensator that pushes the diver up and down with each inhale. A skilled CCR diver can hold a stop within plus-or-minus six inches of the planned depth, which matters because the deco model assumes the diver actually sits at the depth on the plan. Drifting up two feet through a 30-foot stop shortens the effective stop and short-changes the slow tissue compartments. Drifting down two feet does the opposite. Stable trim and a clean hover are not cosmetic skills; they are deco discipline.
Loop monitoring through a stop is a constant background task. The diver scans the handset for setpoint accuracy, cross-checks the cell readouts for agreement, and listens for the solenoid metering rhythm. Most CCR planners build the inert gas math your planning tools rely on around the assumption that the loop holds setpoint to within plus-or-minus 0.05 atmospheres. A cell that is starting to drift can shift the diver’s real PO2 away from planned without showing an obvious failure, which is why a CCR diver runs three independent cells in a voting circuit and treats any cell that votes outside the cluster as suspect for the rest of the dive.
Ventilation through a stop has a small but real effect on inert gas elimination. Breathing on a CCR through deco should be calm and full, not shallow. A diver who slips into a shallow breathing pattern through a long stop can reduce the rate of inert gas movement out of the lungs, which slowly degrades the stop’s real value. Calm, full breathing also keeps carbon dioxide moving through the scrubber at the rate the scrubber chemistry was designed for, which keeps the diver alert and reading the handset clearly through a long ascent.
What Should You Do If a Stop Stretches Out?
Not every CCR deco profile runs the way the plan said it would. A diver might hit a stop and watch the time-to-surface counter sit longer than expected, or run a long stop and discover the computer is adding minutes rather than clearing the obligation. Three common causes drive most of those moments: a sensor that has drifted from the rest of the cluster, a setpoint that was set too low for the ascent phase, or a gradient factor pair set more conservatively than the diver remembered at the planning table.
A drifted sensor is the most insidious because it shifts the loop’s real PO2 away from the assumed value without the diver noticing. If the deco computer reads two healthy cells and one drifting cell, the voting average can still produce a slightly low real PO2, which slows off-gassing and stretches the stop. The fix is to verify cell agreement actively. Flush the loop briefly with diluent to confirm all three cells respond together, and consider running the rest of the dive on the two cells that agree if the third has clearly fallen out.
A setpoint that is set too low through deco extends the stops without any sensor problem at all. A CCR diver who forgets to lift the setpoint from a low descent value to a higher ascent value will sit at the lower PO2 through deco, which slows tissue clearance. The remedy is to switch the setpoint up on the handset, confirm the solenoid responds, and ride the new value out to surface. A more conservative GF pair than the diver remembered is the simplest fix and the most embarrassing. The deco computer is doing exactly what the diver told it to do at setup time. The lesson is to verify the pair before splash, not to second-guess the computer mid-ascent.
When the diver expected a short stop but the computer is calling a long one, the right move is to follow the computer, not to lobby for shorter stops based on memory of the plan. Cumulative gas reserves should always include enough margin to accept a longer-than-planned ascent without burning into the bailout volume you would need from the deepest required stop. A bailout plan that assumes the deco runs to the minute on a good day and offers no extra margin is a plan that assumes nothing will go wrong, and CCR dives that go wrong tend to go wrong at exactly the moment the diver is least able to absorb the surprise.
How Does Silent Diving Help You Run Cleaner Ascents?
Silent Diving is the exclusive distributor of AP Diving rebreathers across the Americas, which means the Inspiration and Evolution chassis CCR divers buy from us come paired with the same service, support, and parts network AP Diving has built over decades. For divers planning a serious decompression season, that support is concrete. Pre-trip service includes cell rotations, solenoid bench tests, and harness fit so the platform holds setpoint and trim through the deco profile the diver actually plans to run. Cell-life tracking against expected scrubber budgets, dive computer firmware updates, and battery-runtime checks all happen on the bench rather than on the boat.
The dealer network across the Americas means a working CCR is rarely more than a day from a local technician, which matters when a diver discovers a setpoint anomaly the week before a trip. Talk to Silent Diving’s authorized AP Diving service team before the season starts, not after a problem surfaces during a deco stop. We will work the pre-season checklist with you, confirm cell life against the bottom-time and ascent profiles you plan to run, and confirm the platform is dialed for the dives you intend.
Frequently Asked Questions
Are CCR ascents really shorter than open circuit?
In most cases yes, though the size of the savings depends on the dive. A CCR’s constant setpoint delivers a higher effective oxygen fraction at every depth shallower than the bottom, which lowers inert gas loading and accelerates off-gassing. For a 150-foot, 30-minute dive the savings can be 10 to 20 minutes of total runtime versus the same dive on open-circuit nitrox. For shallow recreational dives the savings shrink because the loading was already mild. For multi-hour decompression dives the savings compound dramatically and a CCR ascent can run an hour or more shorter than the equivalent open-circuit ascent.
What gradient factors do most CCR divers run?
There is no single right answer, but ranges cluster around 30/70 to 40/85 for serious CCR decompression dives. Lower numbers mean more conservative deco. Most experienced CCR divers tighten the GF-high by 5 to 10 points compared to their open-circuit defaults because the loop’s faster real off-gassing already makes any given gradient factor pair functionally more aggressive than the same numbers on open circuit. The right pair depends on the diver’s fitness, hydration, history of decompression sickness, and the specific dive profile being planned.
Should you change your setpoint during decompression?
Most CCR divers run a higher setpoint through decompression than they ran on the bottom, typically 1.3 to 1.4 atmospheres. The higher PO2 accelerates inert gas elimination. The trade is that central nervous system oxygen exposure accumulates faster, which matters on long total runtimes. The setpoint switch is usually done once at the start of the ascent rather than tweaked at each stop. Some computers automate the switch through depth-triggered logic; others require the diver to bump the setpoint manually on the handset and confirm the change.
What happens if your CCR fails during a deco stop?
The diver bails to open-circuit gas and completes the remaining decompression breathing from a cylinder rather than from the loop. The bailout plan should already have calculated the open-circuit volume required to surface from the worst-case loss point, including all required stops. A deco-phase bailout is less stressful than a bottom-phase bailout because the diver is already shallow, but it still demands the bailout regulator to be rigged, accessible, and tested before the dive. Practiced bailout drills make the transition reflexive.
How long should a CCR ascent take overall?
Total ascent time depends on the dive profile, the deco software, the gradient factor pair, the setpoint plan, and the diver’s chosen conservatism. A typical 130-foot, 60-minute CCR dive with moderate gradient factors and a 1.3 ascent setpoint runs roughly 30 to 45 minutes of total ascent including all stops and the safety-margin shallow stop. Deeper trimix profiles push that number into hours. The plan that comes out of the deco software is the right reference, not a memorized rule of thumb pulled from another dive on another day.
Do you still need backup deco gas on a CCR?
Yes. The loop is the primary decompression tool, but bailout open-circuit cylinders cover the failure case. For deep deco dives, the bailout plan should include a deco-gas mix appropriate for the shallow stops so a bailed-out diver does not have to surface on a bottom mix that is wrong for the shallow stops. Most CCR deep divers carry at least one open-circuit deco cylinder of nitrox 50 or pure oxygen alongside their bottom-mix bailout. The redundancy is the point of CCR safety planning, and well-trained CCR divers practice the bailout swap on every training dive even when the loop is healthy.
What dive computer modes work best for CCR deco?
Modern technical dive computers offer a CCR-specific mode that accepts the diver’s setpoint pair, diluent mix, and gradient factors. That mode produces a deco plan based on the loop’s real gas behavior rather than the open-circuit math. CCR divers typically run two independent computers, both in CCR mode, with matching settings, so a failure of one does not strand the diver without a viable ascent plan. Some divers run a backup in OC mode as a worst-case fallback for total CCR failure that forces the diver to bail out and complete the dive on open circuit alone.
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