CCR Setpoints

A crucial concept for CCR divers is the relationship between loop volume, loop PO2, and buoyancy. All three work together. Our loop is what we’re breathing and is an airspace. This airspace effects our buoyancy control. As we descend, the loop volume decreases. We need to add gas to keep a comfortable volume, but that addition will dictate not only our breathing comfort, but also buoyancy and loop PO2. The opposite is true as we ascend.

New CCR divers can struggle with this trifecta relationship. Balancing the 3 is a requirement for comfort, safety, and hovering.

CCR computers are mentioned throughout. To understand these interfaces better, please reference the relevant computer manual from Shearwater.

https://shearwater.com/pages/petrel-3-support

https://shearwater.com/pages/nerd-2-support

Dil PO2 – This is the partial pressure of oxygen (PO2) in our diluent gas. When we add diluent to the loop, the PO2 changes based on the gas mix and the depth. Since diluent is usually a lower-oxygen mix (like air or trimix), adding it will typically drop the loop PO2. This is super important to understand for doing diluent flushes. Since Dil PO2 is a Partial Pressure, it’s depth dependent. This value is visible on our computer by using the right button from the home screen.

FiO2 – This is the fraction of oxygen (FiO2) in the loop at any given time. Unlike open-circuit diving, where FiO2 is dictated by the gas in your tank, a CCR constantly adjusts the loop gas mix. FiO2 changes as we add oxygen, metabolize it, or flush the loop. It’s also why we can’t think about FiO2 the same way as open-circuit divers—our mix is dynamic, not fixed. This value is visible on our computer by using the right button from the home screen.

Loop PO2 – This is the actual PO2 inside our breathing loop. It’s what we really care about. Our setpoints help control this, but things like metabolism, depth changes, and manual adds all affect it. The goal in CCR diving is to keep the loop PO2 where we want it for the dive plan. There are some situations where we will “turn the solenoid off” by putting it on low setpoint but manually adding oxygen to keep the loop PO2 at high setpoint. Knowing our loop PO2 is the #1 rule of CCR diving.

Surface Mode – This is a feature that prevents the solenoid from constantly injecting oxygen when the unit is on but not in the water which will waste O2, batteries and cause the OPV to release gas. The CCR allows you to swap to a low PO2  (0.19) when on the surface. That way, if you’re setting up, or testing the solenoid doesn’t go nuts trying to reach a dive setpoint that doesn’t make sense topside. We should never enter the water in surface mode. We should drive the PO2 above low setpoint, and then swap the controller to low setpoint. that makes it ready to go when we splash.

Low Setpoint – This is the lower PO2 setting we use on descents and during shallow portions of the dive, like decompression stops. Typically, it’s around 0.7. The reason? At lower PO2, the solenoid fires less often, which means fewer unnecessary buoyancy changes. It also keeps oxygen levels conservative when we don’t need high PO2, like during descent when our PO2 will be increasing do to increasing pressure. Low setpoint is the equivalent of “turning the solenoid off” but we still maintain a loop PO2 that is equal to the high setpoint during all dive portions.

High Setpoint – This is our “ideal” PO2 setting for the dive. Usually, it’s something like 1.0 to 1.3 , providing a good balance of oxygen exposure and decompression efficiency. Once we hit our target depth we will ensure the loop PO2 is at high setpoint value and then swap the controller. Manually increasing the loop Po2 before telling the controller to prevent excessive solenoid firing. This concept follows the approach of “the solenoid follows us, we don’t follow the solenoid”. The unit will add oxygen as needed to maintain this setpoint automatically after we swap. The high setpoint is where we spend our time because it maximizes the benefits of diving a CCR.

Parachute Mode – This is when a diver sets the unit to a low setpoint (like 0.7) but manually adds oxygen to maintain a higher PO2 throughout the dive. It comes from old-school concerns about solenoid reliability, but it doesn’t really make sense as a protocol for the whole dive. If you don’t trust the solenoid to manage a high PO2, why would you trust it to save you at a low PO2? If something fails, you’re still in trouble. Instead of overthinking it and increasing task loading, it’s better to use the system, keep up with maintenance, and watch your PO2.

Loop PO2, setpoint, and dil PO2 are all connected, but it’s important to remember that we control loop PO2—not the solenoid. The setpoint is just a target that tells the unit when to add oxygen, but the solenoid isn’t some magical autopilot. It only fires when the loop PO2 drops below the setpoint, and it has no idea what’s actually happening in our dive—depth changes, metabolism, manual adds, etc. Dil PO2 is the oxygen partial pressure in our diluent gas. this number is useful for doing cell validations with diluent. But at the end of the day, loop PO2 is what we’re actually breathing, and that’s what matters. That’s why experienced divers manage their PO2 manually in shallow water where the solenoid can be too aggressive and interfere with buoyancy control. This is also why backup computers should always be set to high setpoint—they calculate deco based on actual loop PO2, not what the solenoid is trying to do. If your backup is stuck on low setpoint, it’s not tracking your real deco exposure, which could throw off your decompression in the event of a bailout – which makes the backup overly conservative. Bottom line? The solenoid is a tool, not a decision-maker. We drive the PO2, and our backups should reflect what we’re actually breathing, not necessarily the controller’s setpoint.

Computer Interface

Our computer (whether a NERD or a PETREL) is often called our “controller”. This is kind of a misnomer. Typically the CCR brain is inside the unit and the computer is just an interface, but I’ll continue that misnomer here by using the blanket term “controller”. Although it’s not the brain, neither is our TV remote or computer keyboard, but they still control things.

Setpoints

Setpoints can be established in 2 places through the shearwater interface – system setup and dive setup. Either works. We can use dive setup to change setpoints during a dive, we can only use system setup at the surface.

Gases

There are 2 gas categories in the shearwater system. Closed Circuit (CC) and Bailout (BO). These are totally separate and both need to be set properly before the dive. We can add gases underwater, which is a good interface to know, but we should never plan on doing that. This is a bit goofy when using a dilout approach, because we’ll set the same gas in two different places, but that is what it is. It’s important to only set gases you are actually taking on the dive because the computer will use all gases set to “ON” during decompression calculations.

Dive setup allows you to add/edit gases of your current mode. For example, if you’re in CC mode you can edit the CC gases but not BO. This is the only place to edit gases when underwater. To make using the gas system in dive setup easier, change to the “new gas selector” detailed in the petrel 3 manual on page 60.

The superior place to edit gases is system setup. There you can access both CC and BO gas menus. this menus is only available at the surface but is the easiest location for setting gases predive.

Cell Validations

These are tests we do to make sure our oxygen sensors are reading correctly. Both tests are using a known gas at a known PO2 (depth dependent) to see expected values and give us confidence that the cells are reading correctly. These checks are key to making sure we’re not diving with bad data—because trusting faulty sensor readings can lead to big problems.

• Dil Flush – We flush the loop with diluent to see if the actual value matches the expected value (dil PO2). If the loop Po2 does not match the Dil PO2 after a flush, we might have a sensor issue. Example: We are at a loop PO2 and High setpoint of 1.2. We change the controller to low setpoint (to turn off the solenoid) and empty the loop. Then we fill the loop with diluent gas. We might vent and fill two times. If we have air as diluent the expected PO2 would be ~0.84. Our cells should drop from 1.2 to ~0.84. If they don’t drop, or if they drop below that value, something is wrong and we need to bailout and end the dive. It’s important to note that Dil flushes are for cell validation – they are not a good solution for rapid Po2 lowering in a hyperoxic situation. If the loop PO2 is hyperoxic (anything flashing red) the protocol is bailout and then problem solve.

• O2 Flush – This is the same as a diluent flush but with oxygen. This flush is inappropriate at bottom depth, and can only be performed at depths shallower than 20ft. Example: We are at 10ft. If we vent and then flush the loop with oxygen, we would expect a PO2 ~1.2. A Po2 higher or lower than this would indicate our cell data is unreliable.

Automatic Diluent Valve (ADV).

The ADV is a second stage regulator. It is not smart. It “fires” (puts diluent gas into the loop) when there is negative pressure in the loop. This design allows us to add gas to the system hands free to replace loop volume. Unfortunately, if the ADV is left on it can fire when we don’t want it to – for example, if we turn sideways. If the ADV is too active it will drive the loop PO2 down and cause the solenoid to fire. Allowing the ADV to battle the solenoid will cause excessive positive buoyancy and loss of buoyancy control. It will also make the loop volume uncomfortable and the loop PO2 will be unstable.

To prevent a runaway ADV and an ADV vs. solenoid battle the ADV is equipped with a shutoff. The ADV should be turned on and left on in 4 scenarios.

1. We are descending.

2. We are flushing the loop to preform a cell validation drill.

3. We are replacing volume from clearing our mask.

4. We are using diluent to dilute the loop PO2 for in range high PO2s (any PO2 out of range requires a bailout).

Outside of those scenarios, the ADV should be left off. We need to be able to quickly access the ADV on/off switch, but we should generally have the ADV off. This gives us the most control of the diluent injection.

Diluent Selection

Picking the right diluent in a dilout (integrated diluent/bailout) setup is all about balancing two things: a workable dil PO2 and a solid bailout gas. Since dil is also your bailout, you don’t just want “whatever works for the loop”—you need something breathable if things go sideways. The trick is finding a mix where the dil PO2 is below your setpoint (so adding dil doesn’t spike your oxygen and mess with your loop control) but still rich enough to be a safe open-circuit bailout at depth.

For short bottom times, a leaner dil (lower oxygen) makes sense. It allows for an obvious drop of PO2 during a dil flush/cell validation, and since you’re not hanging out forever, you don’t need a super-efficient bailout gas. But for dives where we have a significant bottom time to return to the surface, like cave dives, a higher dil PO2 is better. That way, if you have to bail, you’re not stuck sucking down a lean gas while kicking out for an hour or more. At the end of the day, it’s about thinking beyond the rebreather—your dil choice isn’t just about making the solenoid happy; it’s about having a gas you can actually survive on if you need to. Choosing a lean dilout for a hyperoxic situation is not a logical choice. If the loop PO2 is hyperoxic and in the red (aka out-of-range) we should bailout. A rapid flush to drop the PO2 during a hyperoxic situation breaks basic rules of CCR diving and is prioritizing breathing a toxic gas over a simple solution-bailout.