Following the scenario of the arterial bubble assumption, the critical case is defined as the arrival of an arterial bubble in a tissue compartment (figure A2.1); it is assumed that:
• The bubble was formed elsewhere. Its growth did not modify the local tissue gas load.
• The bubble is reputed to be small when compared to the tissue gas capacity, at least at the beginning of the decompression process. It does not change the tissue perfusion time response.
• Stuck in place, the bubble exchanges gases with both blood and the adjacent tissue.
• However, the bubble is stable and keeps a critical volume.
The Tissue Gas Exchange Model
Tissue compartments are just an historical approach and their identification is not important. The use of a series of compartments avoids the difficulty of accurately defining the process of the gas exchanges, whether perfusion, diffusion, or combined perfusion and diffusion. Thus, in this model, the exponential compartments are considered as harmonics of a complex mathematical solution that are control the decompression one after the other. For this reason, we used the general classic expression for compartment gas exchanges:
) 693(
. 0
gas gas
gas Pa Ptis
T dt
dPtis
−
=
Where T is the compartment half-time as defined in the perfusion equation, Pa and Ptis, the arterial and tissue inert gas tensions.
The modern trend in table computation is to consider all the possible compartments and treat their half-times as a continuous variable. The difficulty then is to express the safe ascent criteria in terms of the compartment half-time. Because modern computers are fast, we decided to treat tissue compartments individually but express them with a geometrical series to remove any subjectivity in their selection. We used the Renard’s series, named after a French admiral who faced the standardization of ropes, sails, planks, etc. in navy arsenals, and elegantly solved the problem with a geometric progression
Tissue
Blood capilary
Arterial Bubble Blood gas exchanges Tissue gas
exchanges Tissue
Blood capilary
Arterial Bubble Blood gas exchanges Tissue gas
exchanges
Figure A2.1. Definition of the critical case.
based on a square root of 10. For instance, with 10 values per decade (1010 ), the series gives the following values:
10 - 12.5 – 16 – 20 – 25 – 32 – 40 – 50 – 63 – 80 – 100 minutes Experimentally, we found that the computation becomes stable when the number of compartments is set in between 15 to 20 values per decade. This way, the description of the tissue gas exchange model only requires defining the boundaries. The fastest
compartment obviously corresponds to instant equilibration and does not need to be specified. The slowest compartment is defined as the one used in saturation
decompressions. Based on Comex saturation experience, these values were set at 270 minutes for heliox and 360 minutes for nitrox saturation. Finally, the tissue gas exchange model only requires one parameter to be defined, corresponding to the half-time of the slowest compartment.
The Bubble Gas Exchange Model
To cope with the complexity of the inert gas exchanges in the bubble, we decided to simplify the process by considering two extreme situations (figure A2.2).
In one case, the bubble is purely vascular and remains in place. The blood flows around it and exchanges gas by convection so efficiently that there is no laminar layer and no diffusion delay at the bubble interface. In these conditions, we adopted for the bubble gas exchanges a formula similar to the classic tissue perfusion equation. We further assumed that the blood flow draining the bubble is a small fraction of the tissue perfusion and that the blood leaves the bubble equilibrated with its gas pressure. This permits an arbitrary expression of the quantity of inert gas molecules transiting through the bubble interface into the blood as:
) 693(
. , 0
gas gas
gas Pa Pb
C T dt
blood
dn = −
Where dn,bloodgas is the number of molecules of inert gas passed from the bubble into the blood, Pagas the arterial inert gas tension, Pbgas the bubble inert gas pressure, T the
compartment half-time and C a coefficient that accounts for the fraction of the tissue blood perfusion that governs these exchanges, the relative capacity of the bubble to the surrounding tissue, etc.
Vascular bubble
Arterial side
Venous side Tissue Bubble
Blood
Tissue bubble
Arterial side
Venous side Tissue Bubble
Blood
Interface bubble
Arterial side
Venous side Tissue Bubble
Blood
Vascular bubble
Arterial side
Venous side Tissue Bubble
Blood
Tissue bubble
Arterial side
Venous side Tissue Bubble
Blood
Interface bubble
Arterial side
Venous side Tissue Bubble
Blood
Figure A2.2. Possible bubble gas exchange situations.
In the second case, the bubble is purely extravascular. The bubble exchanges gas with the surrounding tissue by diffusion. We used the classic assumption of a linear gradient in a surrounding shell and obtained a second general expression for the number of inert gas molecules diffusing through the bubble interface from the tissue.
) 1 (
,
gas gas
gas Ptis Pb
K dt
tis
dn = −
Where dn,tisgas is the number of molecules of inert gas diffusing from the tissue into the bubble, Ptisgas the tissue inert gas tension, Pbgas the bubble inert gas pressure, K a coefficient that accounts for the diffusibility of the gas, the thickness of the layer, the surface of the bubble, etc.
Finally, we imagined an intermediate situation where the bubble is at the interface between the blood and the tissue and exchanges gas through the two above mechanisms.
The importance of the exchange varies with the relative area of the bubble exposed to each medium. The ratio between the two exposed areas of the bubble is called α and varies from 0 to 1. The inert gas mass balance of the bubble becomes:
, ) ) 1 , (
) ( (
dt blood dn dt
tis R dn
dt PbVb
d = τ α gas + −α gas
Where R is the gas constant, τ the absolute temperature and Vb the volume of the bubble.
The Safe Ascent Criteria
The ascent criteria simply seeks the stability of an arterial bubble, with a critical size, stuck at the interface of the blood vessel and exchanging gas with both the blood and the tissue. We translated this statement by specifying that the overall mass balance of the arterial bubble remains unchanged in these conditions:
) 0
( = + =
dt VbdPb dt
PbdVb dt
PbVb d
This last condition means that the sum of all the internal gas pressures equals the external ambient pressure plus the stabilization pressures (surface tension, skin elasticity, tissue compliance). This is written as:
Pbstab Pamb
Pb Pb
Pb
Pbgas+ O2+ H2O+ CO2 ≤ +
Where Pbgas, PbO2, PbH2O, PbCO2 are respectively the pressures of the inert gas, oxygen, water vapor and CO2 inside the bubble, Pamb the ambient pressure and Pbstab the sum of the various stabilization pressures.
Assuming PbO2 is constant and equal to the tissue oxygen tension and introducing B, a coefficient of obvious definition, we obtained a simpler form of the criteria:
B Pamb Pbgas ≤ +
In these conditions, the total of gas transfers between the bubble and its surroundings are balanced. For each gas, the same amount of molecules enters and leaves the bubble during a unit of time. There is no gas accumulation inside the bubble.
dt tis dn dt
blood
dn, gas , gas
) 1
( −α =−α , and yields:
) 693(
. ). 0 1 ( )
( gas gas Pagas Pbgas
C T Pb
K Ptis − =− −α −
α
Finally, the two equations above are combined to eliminate Pbgas. After defining another coefficient A, the final expression of the safe ascent criterion becomes:
gas
gas Pa
T B A T Pamb
Ptis ≤(1+ A)( + )−
This last equation sets the condition for a safe ascent to the next stop according to the initial hypothesis: an arterial bubble exchanging gas with blood and tissue that keeps a critical size during the ascent. It is a function similar to an M-value. With the tissue compartment tension perfusion equation, it permits the classic computation of a decompression stop time. The rate of ascent to the first stop is not part of the model control and is set arbitrarily to 9 m/min. The AB Model-2 provides deeper stops than for a classic decompression model.
A COMPARISON OF SURFACE-SUPPLIED DIVING SYSTEMS FOR SCIENTIFIC DIVERS Michael F. Ward Dive Lab, Inc.
1415 Moylan Road Panama City Beach, FLORIDA 32407 U.S.A.
Lightweight Surface-Supplied Diving Modes
Lightweight surface-supplied diving uses demand mode full-face masks (FFM) and/or free flow masks (Jack Brown). The typical lightweight (¼” internal diameter) umbilical supplies air to depths of 25 meters (80 FSW) or less. Below are five of the most widely used masks, as well as hookah, which employ a scuba half-mask with second stage regulator.
a. Jack Brown lightweight, free-flow full-face mask.
b. AGA c. EXO-26
d. M-48 SuperMask e. KMB band masks f. Hookah
Advantages Disadvantages Safer than scuba Limited to shallow depths < 25 msw (80 fsw)
Rapid deployment Marginal Communications Light weight, highly mobile Minimal Redundancy
Multiple air supply sources Poor Safety Record Small package/footprint
Minimal investment, maintenance and operational costs
Minimal personnel required
Lightweight surface-supplied diving incorporates a tether and communications, which makes this mode safer than scuba. It has a small footprint, can be easily transported by pick up truck or small boat, and is rapidly deployed. Multiple air supply sources can be used such as a small LP compressor or HP console fed from scuba cylinders. There is minimal investment, maintenance, and operational costs and it can be operated with a minimum of personnel.
Although limited to shallow depths (<25 msw - 80 fsw) with standard ¼” umbilicals, these full-face masks can be used well beyond this range when employed with umbilicals of 3/8” internal diameter, or specially designed intermediate systems for use with higher pressure ¼” umbilicals. These FFMs can, and do, employ communications. Most FFMs can be mated to a diver-worn manifold with a backup emergency gas supply (EGS).
However, many divers do not employ an EGS citing shallow depth diving. Some full- face demand masks allow for free-flow capability in addition to demand mode. Because of low start up costs, many divers with only scuba experience use lightweight equipment without proper training and therefore do not realize the potential hazards of surface- supplied diving.
Surface-Supplied Deep-Sea Helmet Demand Mode
Surface-supplied deep-sea demand helmet use is limited to a maximum depth of 200 fsw on air and 300 fsw on HeO2.
Advantages Disadvantages Moderate weight High supply pressure requirements for deep diving
Current compatible Large gas storage or compressor system Physical protection Moderately heavy support system
In-water mobility Deep mixed gas economically not feasible/ practical without reclaim
Contaminated water protection Gas-reclaim capability
Various EGS options Doffing/donning ease Good communications
Minimal open-circuit volume requirements Minimal gas usage in SCR
Surface-Supplied Deep-Sea Helmet Free-Flow Mode
Surface-supplied deep-sea free-flow helmet use is limited to a maximum depth of 200 fsw on air and 300 fsw on HeO2.
Advantages Disadvantages Heavy weight, good in currents Large compressor/gas storage system
Physical protection Large gas consumption Contaminated water diving Heavy, large footprint
Large transport craft required Loud back ground noise
Marginal communications
Poor in-water mobility
Limited EGS capability
Surface-Supported Alternative Modes
Surface-supported alternative methods use a specially configured, semi-closed or fully closed-circuit rebreather that is tethered from the surface or open bell. The rebreather systems use specially configured full-face masks or helmets equipped with an open-circuit demand regulator system for emergency use and shallow
decompression. This method can use ¼” ultra-lightweight umbilicals weighing as much as 4 times less than conventional umbilicals. The system can be used as deep as 300 fsw from the surface and deeper if deployed from an open bell. This method
enjoys the conservative gas use of rebreathers with the safety of umbilical support, including topside communications and monitoring. Monitoring can include loop PO2, temperature, onboard pressures, and depth. The system also allows for topside
intervention of components. This method has been used by the military as well as the commercial industry, but is not readily publicized.
Advantages Disadvantages
Lightweight Specialized training in equipment and procedures Small gas use and storage requirements Regular team training
Small footprint Routine proficiency practice required Real-time diver monitoring and override via the
umbilical Chamber support required Open-circuit back up capability
SCIENTIFIC DIVING OPERATIONS WITH UNTETHERED, OPEN-CIRCUIT MIXED GAS SCUBA
Douglas E. Kesling Henry J. Styron, III NOAA Undersea Research Center University of North Carolina Wilmington 5600 Marvin K. Moss Lane Wilmington, NORTH CAROLINA 28409 U.S.A.
In 1993, the NOAA Undersea Research Center at the University of North Carolina Wilmington (NURC/UNCW) began exploring the possibility of offering a technical diving program to visiting investigators for scientific research applications. The need for a technical diving capability was realized after a review of the attempts by the NOAA Diving Program (NDP) to support deep, mixed gas diving operations for a team of underwater archaeologists and scientific divers exploring the USS Monitor. The discovery of the wreck of USS Monitor established the first designated National Marine Sanctuary by NOAA and lies at a depth 240 fsw. Access to this site is considered by some beyond the reach of conventional, open-circuit compressed air scuba diving techniques. The review of this initial NOAA tethered, scuba diving effort lead NURC/UNCW to establishing a new diving program to support the scientific community wanting to conduct in-situ research beyond a depth of 130 fsw while safely exceeding the no-decompression limits using specialized techniques and equipment. NURC/UNCW currently possesses an in-house capability of supporting scientific research diving up to 300 fsw using untethered, open-circuit scuba technology. Each year NURC/UNCW supports at least one technical diving operation.
Introduction
The NOAA Undersea Research Center at the University of North Carolina Wilmington (NURC/UNCW) is one of the East Coast Centers funded by a grant from the National Oceanic and Atmospheric Administration’s National Undersea Research Program (NURP) to support undersea research using divers, ROVs, submersibles and an undersea habitat. As a diving technology leader in the scientific diving community, the Center at UNCW constantly strives for ways to make scientific diving safer, more productive, and cost-effective. After an initial attempt by the NOAA Diving Program to conduct in-situ research at the USS Monitor deep-water archaeological site, NURC/UNCW began a planned progression towards developing an in-house capability to
support technical diving operations, with the notion that advanced diving technology could be applied to other forms of marine science investigations.
Prior to 1993, the NOAA Diving Program had limited diving involvement on the USS Monitor. In that same year, NDP attempted to use a Class II, open-bottom bell and position a research vessel overhead in a 4-point moor for staging facilities and a recompression chamber to conduct the planned dives to the Monitor. The concept was that the bell would support two tethered, open circuit scuba divers and allow decompression to be conducted in the water, safely inside the bell. With the ship being held in a fixed, moored position, heavy seas and strong currents prevented dive operations during most of this 17-day expedition. Only three dives were completed. It was reported that divers were hampered by difficulties associated with controlling the bell and umbilicals in unpredictable seas and currents. The conclusion was that the operation was expensive, logistically complex, at times potentially hazardous, and ultimately unproductive. (Dinsmore and Broadwater, 1999). After review of this operation, NURC/UNCW submitted a plan to NDP to obtain a decompression diving capability for visiting investigators. NURC was impressed by its findings of the technical diving community efforts in refining open-circuit, mixed-gas scuba diving techniques, which eventually led both NURC/UNCW and NOAA program divers to request special technical trimix dive training from an outside vendor (Newell, 1995).
Technical Diving
Technical Diving is defined as “the use of advanced and specialized equipment and techniques to enable the diver to gain access to depth, dive time and specific underwater environments more safely than might otherwise be possible” (Palmer, 1994).
Specifically, technical diving occurs beyond a working depth of 130 fsw and incorporates mixed gases, although compressed air is still used operationally up to 150 fsw. The equipment used in technical diving is most always self-contained. Either an open-circuit scuba apparatus or rebreather is worn by the diver. Scuba is preferred by the research community because it is most commonly used for entry-level diving, it is relatively inexpensive, light weight and highly mobile, requires minimal support and maintenance, and is readily available off the shelf (Phoel, 2003).
To view technical diving from the proper perspective, it was developed to avoid having to use air for deep dives (Hamilton and Silverstein, 2000). Helium is added to the breathing mixture to reduce both oxygen percent and nitrogen percent, to help operate within safe oxygen exposure limits, and reduce nitrogen narcosis. This trimix combination, though costly, has numerous advantages for conducting technical diving operations. Dr. Morgan Wells developed a special mix of 18/50 (18% Oxygen/50%
Helium – balance nitrogen), which became known as NOAA Trimix I or the “Monitor Mix”. This mix was conceived more for operational flexibility, than for physiological reasons. Filling the cylinder half full of helium and then topping off with Enriched Air Nitrox (NOAA EANx 36), easily prepared the balanced dive gas. Decompression Tables
developed for NOAA by Hamilton Research, Ltd. and were used for subsequent years on NOAA and NURC combined Monitor projects (Hamilton, 1993).
Gaining Experience
Investigation of the training requirements and components for technical diving was obtained directly from a technical diving leader, Captain Billy Deans of Key West Divers, Inc. Deans was contracted by the Monitor National Marine Sanctuary to provide initial dive training for the NOAA Divers and to help support the next NOAA field expedition to the Monitor conducted in 1995 (Kesling and Shepard, 1997).
After this first NOAA/NURC expedition in 1995, NURC contracted Benthic Technologies, Inc., a technical dive training agency to get additional NURC staff divers trained and qualified for technical diving and to obtain instructor credentials for its leadership staff to conduct in-house technical diver training and certification programs for future visiting investigators requesting this new technology.
Equipment
After initial training efforts commenced, it became apparent for NURC to establish a new dive locker with enhanced diving equipment and capabilities. As the Center gained more experience with field operations, the dive locker was expanded with the necessary equipment to support technical diving. One of the benefits of technical diving is that the diving equipment used is similar to what scientific divers are already familiar with.
Though packaged in a new configuration, the equipment consists of double scuba cylinders, redundant two stage scuba regulators and wing-style buoyancy compensators, a back plate with harness, mask, fins and either a wet or dry suit for diver thermal protection. Additionally, small cylinders of either steel or aluminum are configured with a two-stage regulator and carried by the divers. Much of this equipment is obtainable off the shelf. Most diving programs have the capability of maintaining the equipment in- house to keep it in good working order. Divers can also utilize this equipment for other routine dives, like those not requiring decompression or mixed gases to maintain proficiency.
There is an up front investment $3,500 to fully outfit one diver with a complete set of technical diving equipment. NURC helps defray the initial cost for this purchase by the visiting investigator by maintaining a dive equipment inventory for six divers to be used on an as-needed basis, or until such time that the research team can acquire their own personal dive equipment. The equipment is configured and standardized for this research diving team training by NURC. Technical dive equipment is relatively compact and is easily transported to the research sites or loaded aboard research vessels.