CCR divers run two information streams in parallel: the unit’s onboard handset, which manages the loop and shows setpoint and PO2, and an external dive computer that runs the decompression model and stages the ascent. Both matter. Neither is optional on a real technical profile. The decision that often gets dragged into pre-dive at the last minute is how the external computer is actually configured for a CCR dive, because a wrist computer programmed like an open-circuit deco brain will produce a deco plan that fights the unit instead of supporting it.
The configuration choices that matter — algorithm, gradient factors, diluent and bailout gases, PO2 monitoring, alarms — all reflect operational decisions you have already made about the dive. The computer doesn’t make those decisions for you. It enforces them. Get the inputs wrong and the schedule the computer hands you on the descent has very little to do with the dive you actually planned.
This is a configuration walkthrough, not a gear review. The same principles apply to every modern trimix-capable dive computer used in CCR mode.
What Does an External Dive Computer Add to a CCR?
The onboard handset on an AP Diving Inspiration or Evolution runs the solenoid, manages setpoint switching, and displays cell voltages and oxygen partial pressure. It does not run a decompression model in the way a wrist computer does. That separation is intentional and useful: the unit handles the loop while the computer handles the ascent. But it means the deco plan only exists on the external computer, and any failure in the wrist unit silently removes the deco brain from the dive.
That is the redundancy argument for an external computer, and it generalizes to every modern CCR. The unit’s controllers track PO2 and command the solenoid; the external computer holds the gradient-factor model, the gas list, and the schedule. On a deep technical dive, divers usually run two external computers in parallel — a primary and a backup — and treat the unit’s onboard handset as a third reference for PO2 only. The redundancy logic is the same logic behind the dual-controller design at the heart of an Inspiration: critical information has to live in more than one place on the dive.
What the external computer does not directly know is what the loop is doing. It reads ambient depth, runs its own clock, and accepts whatever PO2 you tell it the dive is running at. Some computers can integrate with the unit’s data bus and read live PO2. Most don’t. Either way, the operational decision is the same: program the computer with the setpoints you actually plan to use, and check the computer’s PO2 against the unit’s handset every couple of minutes underwater. They should agree. When they don’t, that disagreement is itself a piece of information worth investigating.
How Should You Pick Your Decompression Algorithm?
Most trimix-capable wrist computers run a Buhlmann ZHL-16C variant with gradient factors. A smaller set still ship VPM-B as an option. The choice between Buhlmann and VPM is a separate conversation; for CCR divers, the more practical decision is the gradient factor settings inside whichever algorithm the team is using.
Gradient factors set the conservatism of the model. GF Low pulls the first decompression stop deeper; GF High shapes how aggressively the final off-gassing window closes near the surface. Common starting ranges for technical CCR dives sit around 30/70 to 50/85 depending on water temperature, fitness, and the team’s historical profiles. There is no single correct pair. There is a team standard and a personal track record, and the gradient factor pair on your computer should match what the team has agreed to and what you have actually dived without problems.
Two configuration mistakes are common. The first is leaving a wrist computer at the manufacturer’s default gradient factors — often something around 40/85 — without realizing the default was tuned for open-circuit recreational diving, not for the long shallow-stop loading that comes with CCR ascents. The second is using aggressive gradient factors borrowed from a single source you read once, without applying them to your own profile or temperature. Conservative settings cost minutes; aggressive settings cost margin. Either way, the schedule comes out of the inert gas accounting that drives your ascent, and the gradient factors are the lever that controls how that accounting plays out in the water.
For most technical CCR profiles, the team should agree on a fixed gradient factor pair across all primary and backup computers in the group, so a diver can read another team member’s schedule on a stop and see numbers that match their own. A team where every computer is set differently is a team running five separate dives in the same water.
What Gases Should You Program Into the Computer?
A CCR dive needs at least two gas categories programmed into the computer: the diluent and the bailout. Both have to be entered accurately, in the right depth window, before the unit leaves the surface. A computer that does not know what gas it is calculating against will produce a schedule that does not match the dive.
The diluent gets entered as the inspired gas while the computer is in CCR mode and tracking PO2 setpoint. Most computers want the diluent composition as fraction of oxygen and fraction of helium, and they let you toggle between low and high setpoints. The setpoint values you enter into the computer must match the setpoints the unit is actually targeting underwater. If your unit is set to switch from 0.7 ATA to 1.3 ATA at 6 meters and your computer is still calculating at 0.7 from 0 to 30 meters, your decompression schedule on the ascent will be wrong in a direction that costs you minutes of off-gassing.
Bailout gases get entered separately, as open-circuit gases the computer can switch to mid-dive. The bailout list usually mirrors the cylinders staged on the unit and any deco gases planned for the ascent — for example, a trimix bottom bailout plus a 50% nitrox at 21 meters and 100% oxygen at 6 meters on a longer profile. Each gas needs the correct depth range so the computer can apply the right inspired gas to the model when a switch happens. The volume side of that planning is a related question entirely, and how bailout cylinder volumes are sized on technical profiles is the math that should be done before any of those gases ever get programmed into a computer.
If you change a gas plan during the surface interval — a different stage, a different mix from the fill station, a swap to a higher-helium diluent because the dive grew — the computer has to be updated before the dive starts, not after. This is one of the few moments where a quick stop and a re-check at the surface saves a meaningful problem at depth.
How Should Your Computer Handle PO2 on a CCR?
PO2 monitoring is where the configuration choice has the most direct impact on the deco model. Three approaches are common, and the right one depends on what the computer can actually read.
The first is a fixed-PO2 mode where the computer assumes you are holding a constant setpoint throughout the dive. This is the simplest configuration. It works well when the unit is reliably holding the setpoint and the cells are recent and healthy. It does not catch a real-world PO2 drift caused by a slow solenoid, a flooded counterlung, or cells that are heading out of calibration. That last failure mode is the one most worth understanding before settling on this configuration, because how oxygen cells age in service and what that means at depth is the same drift the computer will silently miss if it is told to assume a flat setpoint.
The second is a manual setpoint switch on the computer that mirrors what the unit is doing. When you switch from low setpoint to high setpoint on descent, you also switch the computer. This adds one task per descent and one per ascent, but it keeps the computer’s gas calculations honest against the loop. Most CCR teams use this approach because it works on every modern trimix computer without depending on the unit and computer talking to each other electronically.
The third is live PO2 integration over the unit’s data bus, where the computer reads the unit’s actual measured PO2 and applies it to the deco model in real time. This is the most accurate configuration when it works, but it depends on the unit and computer combination supporting it, the cable being correctly connected, and the backup computer still being configured manually in case the data feed drops. Many teams running live integration on the primary computer treat the integration as a bonus and configure the backup as if it didn’t exist.
The configuration question — which approach to use — should be answered before the unit goes in the water, not improvised on the descent. Switching modes between dives is fine. Switching modes mid-dive because the data bus disconnected is a different problem entirely.
Which Alarms Actually Help You Underwater?
A wrist computer can throw a long list of audible and visual alarms at the diver during a dive. Most of them are useful in some situation and noise in another. The configuration decision is which ones to keep enabled, which to silence, and which thresholds to set against your actual plan.
Three categories are worth thinking about separately. Depth and ascent rate alarms are routine and should stay enabled; an ascent that creeps faster than 10 meters per minute is a problem worth catching. Gas switch alarms help when a planned switch is approaching, and most divers leave the prompt enabled and confirm the switch manually rather than letting the computer auto-advance. PO2 ceiling and floor alarms are critical and should be set to the values your actual dive plan allows, not the defaults. A device shipped from the factory with a PO2 ceiling alarm at 1.6 is not appropriate for a CCR dive run at a 1.3 high setpoint, because the alarm will only fire after the loop is already in a place no plan put it.
What does not help is alarm density. A computer that beeps every 30 seconds about an approaching deco stop, a battery warning, a gas switch coming up in 6 minutes, and a slow ascent at the same time creates a noise floor that hides the alarm that actually matters. On a long dive, the diver stops reacting to alarms with the urgency the system was designed for, and the next real alarm gets ignored along with the noise.
The pragmatic setup most experienced divers settle on is to enable the alarms that signal a deviation from the plan, set the thresholds to the plan, and rely on the schedule on the display for the routine prompts. Anything that beeps should mean “look up and decide.” Anything that beeps and means “acknowledge and continue” should be turned off.
How Does Silent Diving Support Your Computer Setup?
Silent Diving is the AP Diving service and dealer headquarters for the Americas. Customers running an AP Inspiration, Inspiration EVP, or Evolution Plus often have specific questions about how their unit’s onboard handset interacts with the external dive computer they are planning to dive — what the data bus exposes, what the recommended setpoint switch points are for a given profile, and how the cell architecture should inform what the wrist computer is told to track.
Those are configuration conversations that benefit from talking to the dealer that knows the unit and the courses that train on it. If you are configuring a new wrist computer or revisiting your gradient factors before a trip, Silent Diving’s dealer and service desk can walk through the setpoint logic, cylinder configuration, and bailout plan that should be reflected in the computer before it sees water. The computer is the part of the system that catches the mistake on the ascent that nobody planned to make, and getting it programmed correctly for the unit, the dive, and the team is one of the cheapest forms of margin a CCR diver can carry.
Frequently Asked Questions
Do you actually need an external dive computer if your CCR already has an onboard handset?
Yes for any dive that includes mandatory decompression. The onboard handset on most CCRs displays setpoint, PO2, cell voltages, and depth, but it is not running a decompression model in the way a wrist computer is. A real technical CCR dive needs a deco brain external to the unit, and most teams run two external computers in parallel so a single failure does not leave the diver without a schedule on the ascent.
Can you use the same wrist computer for open-circuit and CCR diving?
Most modern trimix-capable computers support both modes and the switch is in the device’s menus. The configuration changes between modes, though. In CCR mode you enter setpoints and let the device track inspired PO2 against the diluent composition; in OC mode it tracks the breathing gas directly. Switching modes between dives requires walking through the gas list and gradient factors each time, because they are usually saved separately for each mode on most devices.
What gradient factors are most CCR divers using on technical dives?
There is no universal pair. Common starting ranges for technical CCR dives sit around 30/70 to 50/85, with team standards and personal track record driving the final choice. Colder water and longer bottom times tend to pull the pair toward the conservative end. The pair you carry should match what your team has agreed to and what you have personally dived on real profiles without symptoms or problems.
Should you run live PO2 integration or manual setpoint switching on the computer?
Live integration is the most accurate configuration when the unit and computer combination supports it and the cable is intact, because the deco model is reading what the loop is actually delivering rather than what the dive plan assumed. Manual setpoint switching is more universal and less dependent on hardware compatibility, so it remains the dominant approach among CCR divers. Many teams running live integration on the primary computer still configure the backup for manual switching so a data-bus failure does not take both computers offline at the same time.
What happens if your external computer dies mid-dive?
You stay on the schedule from the second computer, which is one of the reasons most technical CCR divers wear two. If the unit’s onboard handset is still running and showing setpoint and PO2, the loop side of the dive is intact and the surviving computer manages the deco. A single-computer dive that loses the computer is a bailout-to-conservative scenario, not a continuation, and the procedure should be in the dive plan before the dive starts rather than improvised at depth.
How often should you check the wrist computer’s PO2 reading against the unit’s handset?
Often enough to catch a drift early. Most divers cross-check on every meaningful change — descent, ascent, gas switch, level changes — and at least every couple of minutes during stable bottom time. A persistent disagreement between the two values is one of the earliest signals that either cells are drifting, the solenoid is misbehaving, or the setpoint on one device is not what you think it is.
Can you skip programming bailout gases into the computer if you never expect to bail out?
No. Bailout gases need to be programmed so the computer has a schedule to follow if you do bail out, even on a dive where bailing out is not the plan. The whole point of carrying bailout is for the unplanned scenarios, and the deco brain has to know what gas you are breathing during the ascent so the model produces a usable schedule. Bailout that is not in the computer is bailout you cannot use mid-dive without burning time you might not have.
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.