A CCR pre-dive check is not a paperwork ritual. It is the only thing standing between a known unit and an unknown unit, and on a closed-circuit rebreather the unknown is not survivable in the same way it is on open circuit. Open-circuit gear that fails on the surface gets caught at the surface. A rebreather that has a slow leak, a tired oxygen cell, an unflushed scrubber, or a battery that did not fully charge overnight will hide that condition until you are at depth, and by then the answer to the problem is bailout, not diagnosis.
The check exists because every variable that matters on a CCR dive has to be confirmed in a sequence the diver controls. The lab tested the unit. The technician verified the build. The diver verifies the mission. This post walks through how a careful pre-dive check actually runs from the moment you set the unit on the bench through the final breath at the surface before you splash.
Why Does Every CCR Dive Start With a Full Verification?
The simple answer is that yesterday’s unit is not today’s unit. Cells continue to age in the case overnight. Batteries self-discharge. Sealed scrubbers gain humidity from air swaps every time the loop is opened. The diluent cylinder you closed yesterday at 3,000 psi might read 2,940 today because of temperature change in the dive locker. Each of those drifts is small. None of them disqualify the unit on its own. Together, on a dive that gets cold, deep, or longer than planned, they can move the operating envelope in ways that a shortened check would not catch.
The CCR is also a system of dependencies, not a stack of independent components. The cells feed the controller. The controller drives the solenoid. The solenoid pulls from the oxygen cylinder. The scrubber processes the loop the controller is keeping alive. If any one of those is wrong at the start of the dive, the others compound the problem instead of catching it. A check that runs in a fixed order verifies the dependencies in the right direction so a fault gets caught at the cheapest stage, on the dock, with the unit dry, the diver dry, and the dive still optional.
What Should You Check on the Unit Before You Build It?
The bench check happens with the unit not yet assembled for diving. This is where you confirm the chassis, the electronics, and the hardware before any breathable surface is closed up. On an AP Diving Inspiration or Evolution, this stage is built around the Vision electronics doing their startup self-test while you visually inspect the parts you are about to put together.
Battery, Power, And Electronics
Power up both controllers. Confirm full battery state on each handset. Both should be charged to the level you would expect for the planned dive duration with reserve. If one handset shows a different battery state than expected, that is the moment to investigate, not after the loop is closed. Confirm the date and time on the handset, since the Vision logs every dive event with a timestamp that has to match the unit’s actual configuration. Run through the menu items you depend on during the dive: setpoint, decompression algorithm, conservatism, and gas list, all confirmed for the dive you are actually about to do.
Cells, Connections, And Display
Look at the cell readings at atmospheric pressure with the loop open. All three cells should track within a tight band of each other in air. Numbers that drift apart at this stage tell you the cell stack needs attention before the unit ever sees water. Inspect the mouthpiece, the breathing hoses, the T-piece, and the counter-lung exteriors for visible damage. Check the wiring harness routing for kinks. Confirm the scrubber canister has the correct sorb load logged, and that the unit’s accumulated time on that canister matches what your DiveLog records say. None of this takes long when nothing is wrong. When something is wrong, this is where it shows up first.
How Do You Test the Loop Once It’s Assembled?
Once the loop is built and closed, the bench check becomes a leak test and a chemistry test. The order matters. You confirm the loop is sealed before you confirm the chemistry, because a chemistry check on a leaky loop tells you nothing useful.
Positive Pressure Test
The positive pressure test confirms the loop holds gas against an outward pressure gradient. Close the mouthpiece. Add gas to the loop until the counter-lung is firm but not strained, and the over-pressure relief valve is just shy of venting. Set a timer for at least three minutes, ideally five. The loop should hold the pressure with no visible counter-lung relaxation. A loop that softens during the hold has a leak somewhere in the seals, the hose connections, the mouthpiece, or the over-pressure valve itself. A leak found here is a five-minute fix on the dock. The same leak found at depth is a different conversation.
Negative Pressure Test
The negative pressure test confirms the loop holds against an inward gradient. Evacuate the loop until the counter-lung is fully drawn down, close the mouthpiece, and again hold for at least three minutes. The counter-lung should stay drawn. A loop that fills back up during the hold is letting outside water or surface air in past a seal, which on a wet dive becomes water in the loop. Both tests have to pass cleanly. One pass is not a substitute for the other, since some seals leak in only one direction.
Prebreathe And Cell Verification
With the loop sealed and the gases plumbed in, the prebreathe at the surface is the chemistry confirmation. Sit still and breathe the loop for at least three minutes, longer if conditions allow. The cells should track each other through the prebreathe, the controller should hold the chosen setpoint, and the breathing should feel normal with no unusual resistance. A prebreathe that produces a headache, a feeling of breathlessness, or a divergent cell reading is the moment to stop and rebuild rather than chase the symptom into the water. Cell verification at this stage is also a chance to think about how O2 cells age between twelve and eighteen months, since a cell that linearizes cleanly today can still be quietly current limited next month.
What Gas Checks Need to Happen Before You Splash?
The gas side of the check is the easiest to skip and one of the most common places for a CCR dive to start out wrong. Pressure gauges read what the cylinder has, but they do not tell you what is inside. The diver verifies both.
Diluent And Oxygen Cylinder Pressures
Confirm the oxygen cylinder is fully charged, the regulator is open, and the supply line shows pressure at the controller. Confirm the diluent cylinder is at the pressure you expect for the planned depth profile, with reserve for ADV use through the dive. If your diluent is anything other than air, confirm it is the actual mix you planned, with the analyzer reading on a freshly calibrated meter. The cylinder label is a help, not a verification. Mixed-gas mistakes happen at the fill station, not at the dive site, and the only correction is for the diver to confirm the contents independently.
Bailout Cylinders And Drop Stages
Bailout is the part of the check that protects the dive from everything the rest of the check missed. Confirm cylinder pressure, regulator function, mouthpiece condition, and gas analysis on every bailout cylinder you are taking. Confirm that the cylinder volumes still match the bailout volume math for the worst-case loss point on the planned profile, especially if conditions changed since the plan was built. A bailout cylinder filled for a sixty-minute dive does not cover an eighty-minute dive, and a stage drop you are no longer using is dead weight that just makes the swim harder. Verify, do not assume.
When Should You Stop and Reset the Check?
The check is only useful if a failed step actually stops the build. The temptation, especially on a busy dive boat or a long-planned trip day, is to note the issue, finish the build, and assume the issue is small enough to dive around. That is how rebreather divers get hurt. The discipline is to treat any unresolved item as a stop condition until it is actually resolved.
Anything That Doesn’t Match The Plan
A pressure test that softens, a cell that reads off-band, a battery that did not show full at startup, a scrubber that is closer to its planning threshold than expected, or a gas mix that does not match the analyzer is a stop condition. Reset the relevant section, fix the cause, and re-run the affected step from the beginning rather than spot-patching. The same logic applies to scrubber duration variables: if the canister log says ninety minutes used and the dive plan calls for one-fifty more, the canister fails the check before the loop ever closes, no matter how clean the rest of the unit looks.
Equipment That Was Touched Recently
Anything that was serviced, swapped, or repacked since the last verified dive deserves an extra pass. New cells need a calibration check before they get trusted in the cell stack. A new mouthpiece needs both pressure tests run twice. A serviced solenoid needs a full breathing prebreathe under load. The check is a search for surprises, and recently touched components are where surprises hide. Slow down on those steps. The minutes you spend at the dock are the minutes you do not have to spend dealing with a problem at thirty meters.
How Do You Make the Pre-Dive Check Stick?
The pre-dive check is the one part of the dive nobody else can run for you. It is the diver’s contract with the unit, repeated for every dive, every day, regardless of how routine the conditions look. Build the routine the same way every time, in the same order, with the same pauses. The minute the order starts changing dive by dive is the minute steps start getting skipped. When something in the check does not pass, the dive does not start. When the check raises a question that has not gone away after rebuilds, that is the moment to talk to Silent Diving’s authorized service team and let the people who know the unit at the chassis level help you figure out what the diagnostics are pointing at.
Frequently Asked Questions
How long should a full CCR pre-dive check actually take?
For an experienced AP Diving owner with a unit that was packed and prepped the night before, a full pre-dive check including bench, loop pressure tests, gas verification, and a three-minute prebreathe runs in roughly twenty to thirty minutes. Less than that usually means a step was rushed. More than that on a regular basis suggests the night-before prep is not complete, which is a different fix than rushing on the dive boat.
Can the prebreathe replace the positive and negative pressure tests?
No. The prebreathe checks loop chemistry under live conditions. The pressure tests check loop integrity against gradients you do not produce while breathing normally. A small leak that the pressure test catches in three minutes can be invisible during a relaxed prebreathe and only present at depth, where ambient pressure changes the gradient. Run all three.
Do I need to redo the full check between dives on the same day?
You do not redo the full bench check, but you do redo the loop integrity step, the gas pressure verification, and a short prebreathe. The unit was disassembled or partially serviced between dives in many cases, and gases were touched. Each subsequent dive of the day deserves a verification of the parts that changed since the previous one closed.
What does a failed positive pressure test usually point to?
The most common causes are a counter-lung connection that did not seat fully, a hose clamp that was not tightened to spec after the last service, a mouthpiece that was reassembled with a debris-fouled o-ring, or an over-pressure relief valve that has aged into a soft seal. Inspect each of those before assuming the test was wrong.
Should I run the check alone or with a buddy watching?
The diver runs the check, but a CCR-trained buddy or instructor watching the prebreathe and the cell readings catches things the diver might rationalize past. The buddy’s role is not to verify the steps. It is to be a second set of eyes on the diver’s behavior during the prebreathe, which is the part of the check most vulnerable to early CO2 retention symptoms the diver might not notice on themselves.
What if the dive boat is rushing me and there isn’t time to run a full check?
The dive boat is not the authority on whether your unit is dive-ready. Polite, firm communication and an unwillingness to splash on a half-checked unit is the only correct answer. A skipped pressure test does not catch up to you on the surface. It catches up at depth, and there are no boat operators who would prefer you skipped it.
When does the pre-dive check escalate into a service call?
Repeated pressure test failures, cell readings that drift across multiple checks even after fresh sensors, intermittent solenoid behavior, or any electronics fault that resets a configuration on its own are signals to stop diving the unit and book a service appointment. The pre-dive check is a diagnostic. When the diagnostic keeps reporting the same fault, the unit is telling you something the diver cannot fix on the dock.
Need help applying this to your own CCR setup?
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