BIOMEDICAL AND OPERATIONAL CONSIDERATIONS
FOR SURFACE-SUPPLIED MIXED-GAS DIVING TO 300 FSW
Michael L. Gernhardt
Astronaut Office Code CB
NASA Johnson Space Center
Houston, TEXAS 77058 U.S.A.
Introduction
Surfaced-supplied mixed-gas diving to 300 fsw would significantly extend the depth
capabilities of the scientific diving community beyond the limitations of air and nitrox
diving. Closed-circuit mixed-gas rebreathers offer some unique advantages including,
but not limited to, the ability to perform constant oxygen partial pressure dives with
subsequent decompression advantage and minimal deck space requirements for the
support vessel. However, there are disadvantages with rebreathers including safety,
equipment maintenance, and diver proficiency levels necessary to conduct safe
operations. Surface-supplied mixed gas offers some unique capabilities that may be
useful for a range of scientific diving operations.
The ability to efficiently train scuba divers to use surface-supplied diving techniques
(Fig. 1) under the supervision of an experienced support team has recently been
demonstrated with the NASA NEEMO 8 (NASA Extreme Environment Mission
Operations) mission conducted in March of 2005.
Figure 1. Surface-supplied diving techniques from air saturation.
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On this mission, three divers with no previous experience in surface-supplied diving
techniques were able to undergo a short training program and safely use surface-supplied
diving techniques to make excursion dives from air saturation at 50 fsw in the NOAA
Aquarius Habitat.
Physiological and Operational Considerations
The following is a brief overview of physiological considerations associated with
surface-supplied mixed-gas diving, which include, but are not limited to:
Oxygen Toxicity
The potential for both acute and chronic oxygen toxicity requires careful attention to
the selection, mixing, and monitoring of bottom, in-water, and chamber decompression
gases. Control of oxygen partial pressure is necessary for the individual dives as well as
control of multi-day oxygen Unit Pulmonary Toxicity Doses (UPTDs). Standard and
field-proven techniques are well established for these practices.
Thermal Stresses
The increased thermal conductivity of helium can increase respiratory heat loss and
depending on water temperature can drive the need for dry or hot water suits and even
respiratory gas heaters under extreme cold water temperature conditions.
Speech
Helium is well known to cause speech distortion. Use of helium unscrambler radios
is a requirement to maintain clear communications between the dive supervisor and the
diver.
Isobaric Counter Diffusion
Depending on the depth and bottom time, up to four different breathing gases can be
used on the same dive. A typical dive profile might utilize a 12% heliox mix on the
bottom at 300 fsw, followed by switches to air at 150 fsw, to 50/50 nitrox at 50 fsw, and
to 100 % oxygen in a deck decompression chamber during the surface decompression
portion of the dive. When inert gases are switched there is always the potential of isobaric
counter diffusion in localized tissue areas due to the asymmetry between the mass
transfer coefficients of the two different inert gases (Harvey and Lambertsen, 1976).
Generally, isobaric counter diffusion becomes a problem on very deep and long dives that
require long decompressions (such as commercial bell-bounce dives) and would not
generally be considered a problem for the bottom times and depth ranges considered for
scientific diving.
Work of Breathing and CO2 control
Proper pulmonary ventilation is required to eliminate CO2 and provide sufficient
tissue oxygenation to meet the metabolic needs of the working diver. The demands of the
gas delivery system increase with depth. Most commercial diving helmets or band masks
provide both a demand and free-flow gas delivery and have been well proven in the depth
ranges and workloads associated with scientific diving to 300 fsw.
37
Narcosis
Nitrogen narcosis is a consideration for surface-supplied mixed gas diving. Most of
the widely used decompression tables utilize a gas switch from heliox to air as deep as
150 fsw.
Decompression
The decompression requirements associated with 300 fsw mixed gas dives are
considerable, even with short bottom times of 20 minutes. Helium has faster uptake and
elimination kinetics than nitrogen (for most tissue types) and, therefore, direct ascent to
the surface in response to an equipment malfunction is not a viable option.
Decompression techniques are varied and include numerous options: breathing gases
combined with diver deployment and recovery (open and closed bells) and in-water
versus surface decompression.
Operational Considerations for Surface-Supplied Mixed Gas Diving
Table 1 provides an overview of the many operational considerations associated with
surface-supplied mixed gas diving.
Table 1. Operational considerations.
ƒ Type of vessel and mooring system
ƒ Deck space, crew support
ƒ Stable platform for diver deployment
and recovery
Consumables and logistics
ƒ Demand/open circuit heliox versus gas reclaim
ƒ Closed and semi-closed breathing systems
ƒ Significant logistics considerations
ƒ Diver thermal protection
ƒ Wet suit, dry suit, hot water suit
ƒ Gas heating
ƒ
ƒ Diver deployment/recovery systems
ƒ Stage, open-bottom bell, closed bell
ƒ Scientific equipment deployment and
recovery
Umbilical management
ƒ Diver and/or diving bell
ƒ Location of vessel versus worksite
ƒ Interactions of umbilicals with marine
environment and scientific equipment
ƒ Deck decompression chamber system
ƒ Surface decompression, treatment,
number of chambers
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
Training level
ƒ Diving supervisors, rack operators, life support
techs
ƒ Training and proficiency for scientific divers
Bail out system
Air
Mixed gas
Potential for misusing can result in
hypoxia or acute O2 toxicity
38
Gas supplies/mixing vs. premix, minimum 02
concentration limits
Oxygen cleaning/compatibility
High pressure versus cryogenic storage
Boost and transfer pumps
ƒ Standard operating procedures; diving
safety manuals; dive recording and
reporting
ƒ Decompression procedures
Contingency planning/operations
ƒ Emergency decompression
ƒ Omitted decompression
Surface-supplied mixed gas diving is an order of magnitude more complex than air or
nitrox scuba diving, and a professional dive support team will be a requirement for safe
operations.
Surface-Supplied Mixed-Gas Decompression Tables
Commercial diving companies use a variety of surface mixed-gas decompression
tables, most of which are based on some variation of the U.S. Navy (USN) helium partial
pressure tables. Typically, the diving companies do not use 100% oxygen for the 40 fsw
in-water decompression stop. There are various modifications that include doubling the
40 fsw stop time on air for both the 50 fsw and 40 fsw stops, while some companies
utilize a 50/50 nitrox decompression gas for the 50 and 40 fsw stops to increase
conservatism.
The USN partial pressure tables and the majority of the commercial diving mixed-gas
decompression tables are based on variations of the Haldane/Workman decompression
model. This model incorporates the same perfusion-limited exponential inert gas
exchange model as the original Haldane model. However, instead of using the Haldane
pressure reduction ratio as a measure of decompression stress, Workman utilized a
critical pressure difference between the calculated tissue nitrogen partial pressure and the
surrounding hydrostatic pressure as a criterion for safe decompression stress.
This tolerable supersaturation is referred to as an M-value. In Workman’s model,
each half-time tissue has its own M-value, with the M-values decreasing as the half time
increases. Each M-value is allowed to increase with depth at a linear rate defined by a
delta M-value. Most of the models used in commercial diving incorporate twelve tissue
half times, each with its own M-value and delta M-value. There are different M-values
for nitrogen and helium, with helium M-values allowing greater supersaturation. On
mixed gas dives that incorporate air decompression the effective M-value is calculated
based on the proportion of tissue helium and nitrogen tension. Because the
Haldane/Workman model has multiple degrees of freedom, it is very adaptable at
incorporating the results of diving experience and new laboratory trials by changing the
parameters within the model to make it “fit” the diving data. Even though the DCS
incidence associated with tables based on this model is generally low (well under 5%) for
most of the commercial diving companies (Lambertsen, 1991), there is a pattern of
increasing DCS incidence with increasing time and depth of the dives. This pattern was
shown in an epidemiological study of DCS incidence in North Sea diving (Figure 2).
39
Figure 2. Distribution of DCS incidents associated with North Sea commercial
diving operations (Shields et al, 1989).
One of the limitations of the Haldane/Workman model is that it does not directly
model separated gas phase, but assumes that as long as the supersaturation is controlled
to the M-value limits there will be no gas phase separation and no DCS. There is an
abundance of data that indicates gas phase separation and growth occurs prior to
symptoms of DCS. Decompression stress would therefore be better managed by
controlling the size or volume of gas bubbles, rather than the supersaturation. In practice,
the parameters of the Haldane/Workman model have empirically evolved to approximate
gas bubble control. However, since the model does not directly model bubble growth, it
is very limited in its ability to extrapolate to longer and deeper depths or new dive
profiles that are outside the database on which the model was calibrated.
Tissue Bubble Dynamics Model
In order to address the limitations of the Haldane/Workman model, the Tissue Bubble
Dynamics Model (Gernhardt, 1988; 1991) was developed based on first principles to
provide a model to control decompression stresses based on the tissue bubble dynamics.
A graphic description of the model is show below in Figure 3.
The TBDM incorporates a perfusion-limited gas exchange between the lungs and the
tissue, combined with a diffusion-limited gas exchange through a diffusion barrier
between the bubble and the well stirred tissue. The model accounts for gas solubility and
diffusivity in various tissues as well as the surface tension and tissue elasticity. The
model was retrospectively validated by statistical analysis of 6457 laboratory dives which
resulted in 430 cases of DCS (Lambertsen et al, 1991) using the logistic regression
method (Lee, 1980). The decompression data (provided by the International Diving,
Hyperbaric Therapy and Aerospace Data Center) included a wide range of
40
decompression techniques. Data sets were combined based on the likelihood ratio test.
The results of the statistical analysis are shown below in Table 2.
Figure 3. Tissue Bubble Dynamics Model (TBDM)
Table 2. Results of logistic regression analysis of 6547 laboratory decompression
dives. Goodness of fit was calculated using the Hosmer and Lemeshow Goodness-ofFit Test.
Data set: In-water decompression
on air
Test for
improvement
Test for
goodness of fit
x2
p-value
x2
p-value/
Df*
Index
Loglikelihood
Null set
-529
Bubble growth
Index
-498
62.8
.000
4.8
0.77/8
Relative
supersaturation
-524
10.8
.001
19.4
0.08/12
Exposure
index
.505
47.9
.000
30.5
0.00/9
The Bubble Growth Index provided a significant and better prediction of the data than
either supersaturation or the Hempleman exposure phase index (Hempleman, 1952). It
also provided the very good fit of the data based on the Hosmer and Lemeshow goodness
of fit test (significance p>.05). The DCS incidence data associated with different degrees
of theoretical bubble growth were plotted as a histogram in Figure 4. The x-axis denotes
the bubble growth index (the maximum bubble radius in any tissue compartment divided
by the initial radius) and the y-axis denotes the associated DCS incidence. The number
of dives associated with each interval is shown at the top of each bar.
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Figure 4. Histogram of DCS incidence versus decompression stress index (Bubble
Growth Index, Supersaturation, and Exposure Index (pressure x square root of
time.)
The TBDM was used to generate new surface decompression tables. These were
tested in a limited laboratory trial, followed by sea trials with time/depth recorders and
post-dive Doppler VGE measurement, followed by routine operations.
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Table 3. Results of laboratory trials of the Bubble Dynamics Tables
Profile
fsw/min
n
DCS
VGE
(Grade 3, 4)
90/80
6
0
0
120/40
6
0
1: Grade 4
130/40
3
0
0
150/40
9
0
3: Grade 3
Total
24
0
4 (16%)
Table 4. Results of sea trials and operational use (phase IV) of the Bubble Dynamics Tables.
Many of these dives were in the USN extreme exposure range.
Phase
Decompression
procedure
Offshore
dives
DCS
incidents
DCS
%
III
*No
Decompression*
20,000
0
0%
III
1993-5
Air sur-D-O2
With N2O2
4,000
500
9
.2%
IV (ops)
Air sur-D-O2
and multi-depth
2,500
1
.04%
The final operational implementation resulted in less than .1 % DCS on dives with
depth and bottom times in the extreme exposure range of the USN standard air tables.
These decompression tables were based on controlling the bubble growth index to < 2.8.
.
Bubble Growth Index (BGI)
8
7
6
Max BGI
5
Max BGI
4
3
Mod 10 mins O2
50-40 ft and 20
mins O2 40-0’
2
1
0
0
100
200
300
400
Time ( minutes)
Figure 5. Bubble Growth Index for Oceaneering Alpha Table 300 fsw/15 minutes
on 90% helium, 10% oxygen bottom mix, with and without conservative field
modifications to chamber sur-D-O2 decompression.
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500
The TBDM was used to analyze commercial diving surface-based mixed gas
decompression tables. The mixed gas decompression tables that have the best reputation
for low DCS incidence are the Oceaneering Alpha tablesMany of the published
decompression tables are conservatively modified for field operations. One common
modification was to increase the ascent time from 50 to 40 fsw during chamber O2
breathing from 2 to 10 minutes and to increase the ascent from 40 fsw to the surface from
10 to 20 minutes. Figure 5 shows the bubble growth index for the published and fieldmodified table.
Work Efficiency Index
For this 15 minute bottom time the decompression requirement was 67 minutes of inwater decompression followed by 69 minutes of chamber decompression. The work
efficiency index (Gernhardt, 1991) of this dive defined as bottom time/total
decompression time was .11. It is clear that the work efficiency of deep mixed gas diving
is not very high. Table 5 below compares the work efficiency indices of various forms of
diving.
Table 5. Diving method and depth versus Work Efficiency Index (WEI)
Dive type
Depth range (fsw)
Work efficiency index
(WEI) = bottom time/deco time
sur-D-O2(single depth)
70-170
.5-.65
Repet-up
40-190
.8-1.0
sur-D-O2 (multi-depth)
30-190
1.75- 2.0
sur-D-O2 (HeO2)
200-300
.1- .4
Multi-depth, multi-gas
30-300
1.0 -3.5
HeO2 saturation
300-1000
3-10
(10-30 Days)
Air Saturation (Aquarius)
50
3.8-4.7
Multi-Depth/Multi-Gas Decompression Tables
There are significant decompression advantages associated with multi-depth diving
that have been well utilized by the sport, scientific, and commercial diving industries. It
is also well documented that appropriately switching inert gases can result in a
decompression advantage. The USN Helium Partial Pressure Tables and virtually all of
the commercial diving mixed gas tables incorporate a switch from HeO2 to air at various
depths. The off-gassing gradients of an individual gas species are determined by the
difference between the partial pressure of the inspired inert gas and the tension of that gas
dissolved in the tissues. Each gas will diffuse into or out of the tissue under its own
electrochemical potential gradient. Switching from helium- oxygen to nitrox results in a
44
net decompression advantage as the helium will be eliminated faster than the nitrogen is
absorbed (in the majority of body tissues). Combining the decompression advantages of
multiple depth diving with inert gas switches can significantly improve the work
efficiency index of mixed gas diving. Since some, if not many, scientific diving
operations would involve study of marine life along a wall, this method of diving would
be well suited to optimizing the science return from a given dive. Figure 6 shows the
bubble growth index of a multi-gas/multi-depth dive to 300 fsw.
8
Bubble Growth Index (BGI)
7
6
5
4
3
2
1
0
0
50
100
150
200
250
300
350
400
450
500
Time ( minutes)
Figure 6. Bubble Growth Index associated with a 300 fsw multi-depth/multi-gas
decompression: 40'/45 mins-air; 300'/15 mins-HeO2; 150'/15 mins-air; 90'/15
mins-air; 60'/45 mins-50/50 nitrox.
This dive profile would start with a 45 minute air dive at 40 fsw, a switch to HeO2 for
a 15 minute exposure at 300 fsw, followed by a switch back to air for working at 150 and
90 fsw with a final switch to a 50/50 nitrox mix at 60 fsw. This type of dive profile
provides for significant bottom time across a depth range from 40 - 300 fsw and results in
a total bottom time of 105 minutes with no in-water decompression and an 80-minute
surface decompression on oxygen. The resulting work efficiency index (WEI) is 1.3
versus .11 for the equivalent single-depth dive of 15 minutes at 300 fsw. The bubble
growth index is controlled at less than the 2.8 level used for the successful Bubble
Dynamics Tables that resulted in less than .2% DCS on over 7000 operational dives,
many of which were USN extreme exposure profiles.
This analysis suggests significant operational and safety advantages associated with
this type of diving. There have been limited, but successful, field experiences with this
type of dive profile in commercial diving operations. Development of multi-gas/multidepth decompression tables for scientific diving would require careful analysis of the
mission profiles, followed by appropriate laboratory testing and controlled sea trials.
Literature Cited
45
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decompression. Advances in Underwater Technology, Ocean Science and Offshore
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Gernhardt, M.L. 1991. Development and evaluation of a decompression stress index
based on tissue bubble dynamics. Ph.D. dissertation, University of Pennsylvania,
UMI #9211935.
Harvey, C. and C.J. Lambertsen. 1976. Deep tissue isobaric inert gas exchange:
predictions during normoxic helium, neon, and nitrogen breathing by men at 1200
feet of sea water. Proc. 6th Underwater Physiology Symposium, Shilling, C.W. and
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