by Bruce R. Wienke
LANL C & C Dive Team Leader
Los Alamos, New Mexico
Timothy R. O’Leary
NAUI Technical Operations
South Padre Island, Texas
ebreathers, as we know, hold oxygen partial pressures, ppO2, or oxygen fractions, fO2, constant in the breathing mix. Open circuit regulators operate at constant breathing mix fractions of oxygen, helium, and nitrogen, that is, fO2, fHe, and fN2. Rebreathers have decompression advantages over open circuit devices because oxygen fractions or partial pressures can be held at higher levels, while forcing inert gas levels of nitrogen and helium lower. Not withstanding oxtox concerns at higher oxygen partial pressures, divers on rebreather systems can optimize dive time by minimizing decompression requirements. This is certainly well known in the tech, scientific, military, and commercial sectors.

Open circuit divers also optimize their dive time, while minimizing decompression requirements, by making gas switches at various depths. Obviously, gas switching can be dialed to hold gas fractions constant on ascent; or, we should say, relatively constant on ascent, with constancy increasing with the number of switches. The limit to the number of switches is logistic, of course, depending on depth, time, and the capacity of divers to carry switch gases, and/or tie switch gases off on an ascent line. The former limitation more likely confronts technical divers operating without support teams able to string decompression tank lines. The latter limitation is likely one of cylinder availability. Or bodies. Not discounting logistics problems, it might be interesting to compare rigging open circuit diving to mimic rebreather diving, particularly as far decompression debt versus gas switching at relatively constant ppO2.
Comparative Dive Profiles

Let’s compare a test dive to 250 fsw for 15 min, using a CCR with set point of ppO2 = 1.2 atm, and a set of doubles (triples) filled with 14/56 trimix. The diluent is also taken to be 14/56 trimix. At 250 fsw, ppO2 = 1.2 atm, and ppN2 = 85 fsw = 2.6 atm for both open circuit and rebreather divers. With that said, we first compare raw RGBM decompression profiles for the exposure; that is, no gas switches on open circuit. This gives the rebreather baseline, too. Descent and ascent rates are 90 fsw/min and 30 fsw/min, respectively. Both central nervous system and full body oxtox are not concerns here, just minimization of decompression time. The comparative decompression schedules are given in Table 1. These are nominal RGBM profiles, consistent with released RGBM Tables and commercially licensed software.
Next, let’s make a switch on open circuit at 125 fsw, the midpoint of the dive, to 25/45 trimix. Here ppO2 = 1.2 atm, as required, while ppN2 = 47.5 fsw = 1.44 atm.

Table 2 tabulates the rebreather and open circuit decompression schedules for quick comparison. The single switch on open circuit reduces decompression time by roughly 35% compared to the previous schedule.

Finally, let’s make gas switches on open circuit at 200, 150, 100, and 50 fsw to 17/53, 22/48, 30/40, and 48/22 trimix, respectively. Note that the nitrogen fraction stays constant, fN2 = 0.30, as we increase oxygen and decrease helium, keeping ppO2 = 1.2 atm. We come back to this point at the end of the comparison.

Table 3 now clocks rebreather and open circuit decompression schedules with the four gas switches indicated. Further reduction in decompression time on open circuit is clearly evident. Decompression time has been cut in half, only 20 minutes longer than the rebreather decompression time.

Apart from purposes of illustration, holding the nitrogen fraction to 0.30 also eliminates ICD problems on all open circuit switches. More complex rules take a generic form, as discussed in previous articles, and can be summarized in broad hierarchical fashion. Holding fN2 to 0.30 induces a range for ppN2 of 85 fsw at the bottom to 20 fsw in the shallow zone. But what’s more important, there are no large (ingassing) countercurrents of nitrogen on top of outgassing helium, possibly leading to super saturation and isobaric counter diffusion (ICD) problems and bubbles. The simple cases described above are consistent with a zeroth order rule for ICD problem amelioration.

Mixed Gas Rules Of Thumb

The consideration of both oxygen toxicity and decompression sickness in mixed gas diving play off against the need for fast and efficient decompression for arbitrary exposures. Detailed analyses of all mixed gas diving variables, mix switches, highest oxygen fractions, and couplings require desktop computing power at the least—plus experience. From field experience in the technical and commercial diving sectors, a few broad rules of thumb have emerged over recent times. And some tests have been performed in the chamber and underwater.

1. Zeroth Order Rule – No N2 Switches
Early work by Lambertsen suggested that the best rule for safety, especially in the saturation realm, was no switches off helium to nitrogen. Therefore, both the partial pressure of nitrogen, ppN2, and nitrogen gradient, £G ppN2, produce zero ingassing. This fail-safe rule pervades much of commercial, military, and scientific diving. For very deep and saturation diving, violation of this rule often resulted in helium inner ear DCS. The same can be said for heliox and trimix divers violating the following prescriptions. Additionally, as depths increase beyond the 400 fsw range, problems with helium to nitrogen switches compound at alarming rates of DCS, as reported in many quarters.
2. First Order Rule – No N2 Switches Below 100 fsw
Backing off the limit point of no N2 switches, world navies had terrible experiences on deep air and nitrox switches off heliox, as did the technical diving community. So a modified rule is no N2 switches off heliox and trimix below 100 fsw.
3. Second Order Rule – No N2 Switches Below 70 fsw, and END Less Than 50 fsw
Over the past 15 years or so, with extensive diving in the 200 fsw depth range, a modified rule has emerged, coming from all quarters of the technical diving community worldwide. Switching to EAN50 at 70 fsw, keeping the ingassing gradients for N2 as small as possible off bottom mixed rich in helium for switches, and maintaining END in the 50 fsw range, has become popular for trimix diving. For deeper dives, HPNS needs to be factored into the bottom trimix.
Actually, in the above scenarios with switches to nitrogen based mixes, recent protocols use a helium based switch mix in the same proportion as the nitrogen based mix. For instance, in the Second Order Rule, EAH50 (helitrox) is efficiently substituted for EAN50 with rather minimal increases in overall decompression hang time from the 70 fsw level up to the shallow zone with a switch to pure O2. A switch to pure O2 in the shallow zone is standard these days for mixed gas decompression diving.

After plowing through all of the above, do you get the impression that diving rebreathers is perhaps best overall from a vantage of efficiency and dive time management? Maybe so….