2.2 Operations During the Dive
Dive Planning & Staging Logic
This text is not intended as a replacement for proper closed circuit rebreather training, nor does it present the full body of knowledge required to dive any specific unit, in any particular environment, nor for any specific purpose. However, several fundamental points for dive planning and staging a dive warrant highlighting. In my opinion, these are among the types of considerations that should become standardized within the community, particularly when moving towards adopting this technology for working or mission-oriented dives.
First, the diver should have an immediately breathable open-circuit gas supply, appropriate for the depths being dived, at all times. This point is particularly critical when diving at or near the surface and when incorporating hypoxic diluents into the dive plan. When a BOV is utilized, a normoxic or hyperoxic diluent must feed the BOV when at or near the surface. By always using air or nitrox as an onboard diluent routed to a 3-way ball valve which permits a simple switch to the deeper hypoxic diluent required, this gas can be selected and verified on ascent such that any open-circuit bailout is certain to provide a breathable gas, thus mitigating risks of hypoxia in the shallows. This of course is a contentious debate related to system configuration, though additional context and technical details will be reviewed.
Second, bailout considerations for all dives must provide for sufficient volumes of open-circuit breathing gas to egress the furthest point (depth and/or duration) of the dive while following the ‘rule of thirds’ at a minimum and should account for a too deep/too long contingency, with the contingency planning based on the more probable emergency in the given diving environment.
Lastly, when multiple diluent supplies are utilized, gas selection must be clear and intuitive for the diver, as well as any support personnel who may interface with the diver in-water or topside. Cylinder placement following the ‘rich on right, lean on left’ mantra and both tactile and color cues for visual identification should be utilized.
Five Rules of 1
For planning working or technical dives which tend to be more ‘aggressive’ from a labor and psychological demand perspective, the diver should consider a simple personal operations management strategy and ensure that is reflected throughout the team's understanding of the operation. Simple is always good, complex can be paralytic.
I employ ‘five rules of 1’ almost exclusively on all dives where:
| Five Rules of 1 | |
| Rule 1 | Target maintaining a pO2 of 1.0 throughout the dive (provides for 5 hours of time for NOAA CNS clock, which matches 'standard' scrubber capacity and oxygen supply well). |
| Rule 2 | Supply O2 starting at approximately 1 liter per minute. |
| Rule 3 | Use only 100% oxygen near the surface (<20 fsw). |
| Rule 4 | Incorporate ONE diluent supply to all auxiliary unit functions and bailout at a time. |
| Rule 5 | Make ONE dive per day. |
This rule set makes on-the-fly decision making for long duration rebreather dive planning incredibly intuitive. These rules do not invoke optimal considerations for all circumstances that may be extracted from profile software or dive computers; however, they provide a benchmark to simply get in the water routinely and easily, which is a necessity when stakes are high in stressful working environments. I, like many, have likely been accused of too much tinkering with my rebreather at the surface. This is half the fun for many of us involved with the technology, though I must say that this should not be viewed by newbies as any such requirement. Rebreather diving can be, and frankly is, as simple as these rules guide it to be. Important to note is that these rules are all keyed to the NOAA CNS charts for oxygen REPEX. I will emphasize throughout - 'what we dive' should and must be keyed to 'how we dive', and that includes physiological exposure limits. It works out fortuitously and beautifully, that a five-hour dive (shift) can be readily accommodated, and repeated over multiple days, from an oxygen exposure standpoint when considering lowered pO2s than conventionally considered for bounce dives. As a working diver, it's all about time on the bottom - this is how to get that time, without question. With the gift of time on our side, decompression becomes a separate issue, which we will discuss in subsequent sections.
| NOAA REPEX exposure table. Note in green that a pO2 of 1.0 bar allows for 5 hours of single day and multi-day repetitive exposures. This conveniently aligns with a work shift, and was further used as the capacity benchmark for the RD1 Rebreather design. |
Sensor Validation & In-water Systems Checks
A systematic and methodical approach to the entire immersion provides for effective personal life support management and awareness of the environment. This awareness is critical as now you, the diver, is ultimately responsible for atmospheric management. Life support and atmospheric management then require one critical and fundamental exercise: knowing ‘what you are breathing’.
Sensor Validation
Diving a rebreather requires that you monitor the oxygen sensor behavior in an active and supervisory role, rather than just a passive checking role. For example, if your sensors get damaged, are wet, or are at the end of their useable life, they may appear to calibrate fine to .95/.98 bar, but then are actually not capable of producing the current required for proper loop monitoring in high partial pressures of oxygen – this is referred to as a ‘current limited’ oxygen cell. When operating under pressure, a current limited cell will not produce adequate current to be translated for display as a higher pO2. The inability to gauge high pO2 could result in the diver adding an extraneous amount of oxygen to the breathing loop – possibly resulting in a toxic breathing media and resulting in oxygen toxicity or even death.
In a manual system, with three sensors, the logic is that the diver can observe cell responsiveness to change and can validate cell accuracy with consistency across three cells. One may argue that one cell is more likely to fail than two simultaneously, and this would be detected by the diver with one cell standing as an outlier. If two of the three sensors (or all three) failed simultaneously, it would be possible to be incorrectly gauging pO2 altogether, which may lead to inappropriate display readings and inappropriate loop content management – this could be fatal.
In reality, as we’ve presented, only one oxygen sensor is truly needed; however, this is with the strong presumption that it is working properly, and its function can be constantly validated. Some systems have incorporated this logic within their computer controller systems. For manual rebreathers, sensor validation can be achieved by checking pO2 against known diluents and at fixed depths and only requires a little bit of quick math.
Considering this potentially complicated interpretation of the sensor displays, it is highly advantageous to validate sensors throughout the dive. One simple check is to verify that the display is responsive to the one-hundredth (0.0X) bar which can be determined with a short burst of diluent over the sensor face with a properly placed MAV inlet in proximity to the sensors. If the sensor is not responsive, it may be as simple as a wet sensor face or an indication of a bad sensor.
|
Note on table above: The above table should also be considered for appropriate diluent selection. Diluents should be selected such that when the loop is flushed at the maximum operating depth, pO2 should drop below the desired working ppO2. This is an important safety consideration in the event a rapid descent results in a ppO2 spike, or in the event acute oxygen toxicity symptoms are present and need to be alleviated. Diluents should dilute! In the absence of the ability to drive pO2 down, the only option is to shut off passive oxygen addition and metabolize down the breathing loop. The significant downside to this is a corresponding increase in inert gas partial pressure, which may be cause for increased nitrogen narcosis. For example, a diluent flush using air at 190fsw will result in a pO2 of 1.4 bar (pN2 5.35 bar). Breathing the oxygen ppO2 down to 1.0 bar would result in a pN2 of 5.75 bar, which is an exceptional narcotic exposure. Thus, while air may be suitable for OC diving to 190 fsw in optimum circumstances, it is not a practical choice for CCR diving to 190 fsw.
The practical depth for air diluent use is more reasonably 132fsw, though still under optimum conditions. At this depth, an air diluent flush would result in a pO2 of 1.0 bar (pN2 4.0 bar, roughly the narcotic equivalent of the recreational air diving limit of 130fsw.) However, note that this recreational diving limit is still under optimum conditions. If breathing this 1.0 bar pO2 down, narcosis is increasing - more practically speaking, particularly if under stress, the air diluent limit would be 100fsw, 0.8 bar O2, 3.2 bar N2. At 100fsw, an air diluent flush can push a target pO2 of 1.0 bar 'down' for sensor validation purposes and would not compromise a reasonable narcotic threshold. |
Throughout the dive, purging the loop with either pure oxygen (up to 20 fsw only!) or diluent of a known oxygen fraction and at a fixed depth can provide for a simple means to validate oxygen sensor responsiveness. For example, purging the breathing loop with pure oxygen at 20 fsw should result in a pO2 display of 1.4 to 1.6 bar (assume slight leftover diluent). This is good practice both on descent (to ensure cells are not current limited at start of dive) and again on ascent which will double to ensure that the breathing loop contents is indeed breathable during the final ascent (remember, bump up to 1.0!), where pO2 will drop.
In-Water System Checks
In addition to sensor validation, discussed first given its life critical nature of this exercise, a series of very simple yet methodical steps can be taken to perform in-water system checks throughout the duration of lengthy dives. These should become habitual.
Lombardi and Godfrey (2011) described a strategy to partition dive profiles into four distinct phases.
- at/near the surface and initial descent to 20fsw, final ascent from 20fsw
- the precipitous descent to working depth
- working phase of dive and ascent to first decompression stop
- lengthy decompression
This partitioning of the dive into digestible phases, rather than the mass of its entirety, can greatly reduce diver stress (personal observation), improve psychological performance, and allow for considering vastly extended range excursions where each dive phase can be managed with its own contingencies, and possibly even enhanced with their own unique technologies or systems.
At each phase, specific but simple protocols should be developed for systems checks, personal wellness checks, and work performance checks. These should be established by the Dive Supervisor or Lead Diver in concert with the dive team and will vary based on the environment and tasks to be completed. These checks can be recorded by the in-water lead diver or topside dive supervisor and serve as a partial record for the dive event.
At a minimum, the diver should stop at 20 fsw and complete the BOSS mnemonic where:
● B – bubble check
● O – flush loop with pure oxygen
● S – sensor validation
● S – self-check/safety drills (briefly actuate all auxiliary items and rehearse OC bailout)
Upon leaving 20 fsw and arriving at each subsequent dive phase, a series of basic checks should continue to be carried out including, at minimum:
● Tactile and visual verification of proper diluent gas selection
● Diluent flush to validate sensors (and make any necessary gas switches)
● Self-check/safety drills (actuate all auxiliary items and rehearse OC bailout)
The notion of frequently stopping to self-assess and/or check a dive partner sounds tedious and distracting, however, this is not the case. These routine checks are done frequently, and often subconsciously, which is how they should be ingrained through proper training and through the achievement of rebreather diving proficiency. Here, however, I am advocating the little bit of extra effort to stop, take pause, and consciously perform self and partner checks on a structured basis. Considering time is not a pressing factor when rebreather diving, allocating 30 seconds or less in between each defined dive phase is hardly a burden. It may in fact save someone’s life or at least significantly reduce stress in continuing through to the next dive phase.
Stop, Breathe, Think, Breathe, Act, Keep Breathing!
Don’t wait until it’s too late and do it like your life depends on it - because it does!
Diluent Gas Switching to Avoid Hypoxia
From an operational standpoint, making a diluent gas switch serves a couple purposes. One, typically onboard diluent supplies are small cylinders and do not offer substantive bailout at depth. This gas should be reserved for the most demanding part of the dive - at or near the surface where waves, currents, the weight of bulky gear, and ingress/egress can become complicated. This small cylinder should be available to you for immediate open-circuit bailout. That said, using an offboard gas supply as 'drive gas' preserves this small onboard volume. Second, for deeper dives, mixed gasses are likely used. Maintaining this as a separate supply from the rebreather keeps a potentially hypoxic mix clear and away, and also preserves this offboard supply as your go-to deep bailout. Accessing both onboard or offboard gases should be simple and intuitive. I advocate for a three-way ball valve.
Should you plan on making a [diluent] gas switch, it is recommended to make the male to female quick-disconnect connection prior to the dive. This will prevent any water from entering your hoses and contaminating critical orifices (oxygen side in particular). Should you make a wet connection, slight water will intrude through the male side of most fittings. This is not detrimental; however, care should be taken to clean any critical orifices that have been exposed to seawater, and ensure hoses dry out completely.
To make a gas switch using a ball valve, simply turn the valve switch towards the desired input gas. Perform a strong diluent flush if this is a diluent switch. Done. To switch back to onboard, reverse the process; select the desired gas, Flush if necessary. DONE.
A critical note: if making a diluent side gas switch, it is good practice to be sure to turn the gas not being used ‘off’. Otherwise, should the ball valve/isolator be open inadvertently or switched to the other gas, the rig will be supplied with the incorrect gas supply. This is possible given that the supply regulator with the higher intermediate pressure will be drawn from first. All systems should be designed such that intentional checks and balances are made during the gas switch process. A ball valve allows for tactile reference to the gas selected and can easily be verified by feel. A ball or quarter turn isolator valve should be positioned such that it is readily accessible. My preference is positioning at the left chest (lean on left). The pictured ball valve switch block acts as a diluent manifold - all diluent peripherals are supplied from the gas that is selected 'on'.