Decompression theory  neoHaldane models
ABSTRACT
This section describes the Haldane or neoHaldane decompression theories.
On each dive the divers body takes up inert gasses, like Nitrogen. After
the dive the divers body is 'supersaturated' with inert gas and has to get
rid of this excess gas (decompression). Decompression theories predict the
inert gas uptake by the body (divided in hypothetical tissue compartments).
Furthermore, they define limits (Mvalues) which apply to the
supersaturation of each tissue compartment. If supersaturation values
exceed these limits, decompression sickness (DCS) symptoms develop. The
modeling of the gas uptake and these limits enable calculation of diving
tables, decompression profiles and simulation by diving computers.
History
In modern diving, tables and schedules are common for estimating
nodecompression limits, decompression profiles
and saturation levels. Use of a diving computer during the dive is most
common nowadays. Tables and computers are based on decompression theory,
which describes inert gas uptake and saturation of bodily tissue when
breathing compressed air (or other gas mixtures). The development of this
theory was started in 1908 by John Scott Haldane c.s. Haldane, an English
physiologist, described the Nitrogen saturation process by using a body
model which comprises several hypothetical tissue 'compartments'. A
compartment can be characterized by a variable called 'halftime', which is
a measure for inert gas uptake. Theory was further developed during the
years '50 and '60 by U.S. Navy. The concept of 'Mvalues' was developed by
Robert D. Workman of the U.S. Navy Experimental Diving Unit (NEDU). In the
early '70s Schreiner applied the theory to changing pressure
(ascending/descending). Recently Bühlmann improved the theory and
developed ZH12L and ZH16L model, which are quite popular in current
diving computers. At this moment a lot is still unknown about
the exact processes which take place during saturation and decompression.
Most of the theory presented has been found empirically, i.e. by performing
tests on human subjects in decompression chambers and from decompression
accident statistics. Recently, a more physical approach resulted in bubble theories. These theories physically
describe what is happening during decompression.
Inert gas saturation and supersaturation
When you breathe a breathing gas that contains an inert gas (gasses which do
not take part in the oxidative metabolism and are not 'used' by the body)
like Nitrogen (N_{2}) or Helium (He_{2}), this gas is
dissolved in the blood by means of gas exchange in the lungs. Blood takes
the dissolved gas to the rest of the bodily tissue. Tissue takes up
dissolved gas from the blood. Gas keeps on dissolving in blood and tissue
until the partial pressure of the dissolved gas is equal to the partial
pressure of the gas breathed in, throughout the entire body. This is called
saturation. Rates of saturation vary with different parts of the
body. The nervous system and spine get saturated very fast (fast tissues),
whereas fat and bones saturate very slowly (slow tissues).
When staying at sea level for a long time, like most of us do, and breathing
air, again like most of us do, the entire body is saturated with Nitrogen,
which makes up the air for 78%. Since at sea level air pressure is roughly
1 bar (we can neglect barometric air pressure variations, which are
expressed in millibar), the partial pressure of the dissolved Nitrogen
thoughout the entire body is 1 bar * 78%=0.78 bar (actually it is a bit
less, as we will see later, but for the moment this will do). If
a diver dives to 20 meter, he breathes air at 3 bar. Partial Nitrogen
pressure in the air he breathes is 3 bar * 78% = 2.34 bar. If the diver
sits down and wait for quite a long time, the diver's body gets saturated
with Nitrogen at a partial pressure of 2.34 bar (the fastest tissues
saturate in 25 minutes, the slowest take two and a half day to saturate). So
far, so good (at least if our diver has enough air supply). If the diver
goes back to the surface, however, he arrives with his body saturated with
Nitrogen at a partial 2.34 bar pressure, whereas the air he breathes at the
surface has a partial Nitrogen pressure of 0.78 bar. The body is
supersaturated. Dissolved Nitrogen in the tissue and blood will go
back to the free gas phase, in order to equalize the pressures. The
Nitrogen forms micro bubbles, which are transported by the blood and
removed from the body by the respiratory system. However, if to much
Nitrogen goes back to the free phase, micro bubbles grow and form bubbles
that may block veins and arteries. The diver gets bent and will develop
decompression sickness (DCS) symptoms.
A certain amount of supersaturation is allowed, without getting bent
(at least with low risk of getting bent). In fact, supersaturation (a
pressure gradient) is needed in order to decompress (get rid of the
excess Nitrogen). The amount of allowed supersaturation is different for
various types of tissue. This is the reason the body is divided in
hypothetical tissue 'compartments' in most decompression models. Each
compartment is characterized by its halftime. This is the period the
tissue takes a partial inert gas pressure which is half way between the
partial pressure before and after a pressure change of the environment.
Haldane suggested two compartments, recent theories (like ZH16L) use up to
16 compartments. Decompression theory deals with two items:
 Modeling the inert gas absorption in the bodily tissue
 Estimate limits of supersaturation for each tissue, beyond which
decompression sickness (DCS) symptoms develop
If these items are known, one can fill in tables, estimate nodecompression
times, calculate decompression profiles, plan
dives, etc.
Calculating inert gas absorption by the tissue
In this section we will derive the equations which describe inert gas
uptake by bodily tissue. If you get frightened by a bit of mathematics,
please skip the derivation, but have a look at the end result in the
high lighted boxes (equation (6a/b) and (11)). The rate a particular tissue
(compartment) takes up inert gas (i.e. the rate of change in partial
pressure of that gas in the tissue) is proportional to the partial pressure
difference between the gas in the lungs and the dissolved gas in the
tissue. We can express this mathematically by:
  = k [ P _{alv} ( t )  P _{t} ( t ) ]   (1a) 
 P_{t}(t)
 Partial pressure of the gas in the particular tissue (bar)
 P_{alv}(t)
 Partial pressure of the gas in the breathing mix. To be
precise: Gas exchange takes place in the lungs
(alveoli). Hence, we have to consider
the gas alveolar partial pressure. This pressure may be changing
with time, if the diver changes depth (bar)
 k
 A constant depending on the type of tissue (min^{1})
 t
 Time (min)
This is a differential equation which is quite familiar in physics and
apply to many processes like diffusion and heat transfer. Solving this
equation requires following steps:
Step 1: Write the equation to the familiar form (a.k.a. the
inhomogenous differential equation):
  + k P _{t} ( t ) = k P _{alv} ( t )   (1b) 
Step 2: Solve the homogenous equation (2) by trying
P_{th}(t) = C_{0}e^{λt} and solve for
λ.
  + k P _{th} ( t ) = 0   (2) 
The 'h' in P_{th} denotes that we are dealing with
the homogenous equation. Substituting the solution in (2) results in
λ=k. So the homogenous solution of the equation (2) is:
 P _{th} ( t ) = C _{0} e ^{  kt}   (3) 
Step 3: Find the particular solution for the inhomogenous equation
(1) and solve constants using boundary conditions. In order to solve this
equation, we have to know more about the partial gas pressure
P_{alv}(t). Two useful situations described in literature are
a situation in which P_{alv}(t) is constant
(corresponding to remaining at a certain depth) and a situation in
which P_{alv}(t) varies linearly with time
(corresponding to ascending/descending with constant speed). We will have a
look at both situations.
Situation 1: constant ambient pressure
We look at the situation in which the alveolar partial pressure of the
gas remains constant: P_{alv}(t)=P_{alv0} . This
corresponds to a diving situation in which the diver remains at a
certain depth. Equation (1b) becomes:
  + k P _{t} ( t ) = k P _{alv0}   (4) 
We 'try' the solution:
 P _{t} ( t ) = C _{0} e ^{  kt} + C _{1}   (5) 
If we subsitute solution (5) in equation (4) the e's cancel out and we
are left with C_{1}=P_{alv0}. Now we have to think of a
boundary condition, in order to find C_{0}. We assume some
partial pressure in the tissue P_{t}(0) = P_{t0} at t=0. If
we substitute this into equation (5) we find that
C_{0}=[P_{t0}  P_{alv0}]. So we are left with
the following equation for the partial pressure in a specific type of
tissue (characterized by the constant k):
 P _{t} ( t ) = P _{alv0} + [ P _{t0}  P _{alv0} ] e ^{  kt}   (6a) 
 P_{t}(t)
 Partial pressure of the gas in the tissue (bar)
 P_{t0}
 Initial partial pressure of the gas in the tissue at t=0 (bar)
 P_{alv0}
 Constant partial pressure of the gas in the breathing mix in the
alveoli (bar)
 k
 A constant depending on the type of tissue (min^{1})
 t
 Time (min)
This equation is known in literature as the Haldane equation. We can
rewrite it a bit so that it corresponds to a form which is familiar in
decompression theory literature:
 P _{t} ( t ) = P _{t0} + [ P _{alv0}  P _{t0} ] [ 1  e ^{  kt} ]   (6b) 
Situation 2: linearly varying ambient pressure
Very few divers plunge into the deep and remain at a certain depth for
a long time. For that reason we will look at the situation in which the
diver ascends or descends with constant speed. This means the partial
pressure of the gas he breathes varies linearly with time. Going back
to equation (1) this means P_{alv} can be writen as
P_{alv}=P_{alv0} + Rt. P_{alv0} is the initial
partial pressure of the gas in the breathing mixture at t=0, and R is
the change rate (in bar/minute) of the partial pressure of this gas in the
alveoli. Note: R is positive for descending (pressure increase) and
negative for ascending (pressure decrease). Substituting this in (1b) gives
us:
  + k P _{t} ( t ) = k P _{alv0} + k R t   (7) 
We 'try' the solution:
 P _{t} ( t ) = C _{0} e ^{  kt} + C _{1} t + C _{2}   (8) 
Substituting solution (8) in equation (7) leaves us with:
 [ k C _{1}  k R ] t + [ C _{1} + k C _{2}  k P _{alv0} ] = 0   (9) 
To find a solution for C_{1} and C_{2} that hold for
every t we have to make both parts between the square brackets in (9) equal
to 0. This results in C_{1} = R and C_{2} =
P_{alv0}  R/k. In this way we find:

P_{t}(t) = C_{0}e^{kt} +
R t + P_{alv0}  R/k

(10) 
 P _{t} ( t ) = C _{0} e ^{  kt} + R t + P _{alv0}     (10) 
Again we use as boundary condition P_{t}(0) = P_{t0} at
t=0 in order to find C_{0}. Substituting this in (10) we find
C_{0} = P_{t0}  P_{alv0} + R/k. So for the
ultimate solution we find:
 P _{t} ( t ) = P _{alv0} + R   t        P _{alv0}  P _{t0}     e ^{  kt}   (11) 
 P_{t}(t)
 Partial pressure of the gas in the tissue (bar)
 P_{t0}
 Initial partial pressure of the gas in the tissue at t=0 (bar)
 P_{alv0}
 Initial (alveolar) partial pressure of the gas in the breathing mix at
t=0 (bar)
 k
 A constant depending on the type of tissue
 R
 Rate of change of the partial inert gas pressure in the breathing
mix in the alveoli (bar/min) R=Q R_{amb}, in which Q is the
fraction of the inert gas and R_{amb} is the rate of change of the
ambient pressure.
 t
 Time (min)
This solution was first proposed by Schreiner and hence known as the
Schreiner equation. If we set the rate of change R to 0 (remaining at
constant depth), the equation transforms in the Haldane equation (6a). The
Schreiner equation is excellent for application in a simulation as used in
diving computers. With the same frequency of measuring of the environment
pressure and performing the calculation, applying the Schreiner equation
gives a more precise approximation of the actual pressure profile in the
bodily tissue than the Haldane equation.
Halftimes
So we see an exponential behavior. When we look at the first situation
(constant depth) we have a tissue with in initial partial pressure
P_{t0}. Eventually the partial pressure of gas in the tissue will
reach the partial pressure of the gas in the breathing mixture
P_{alv0}. We can calculate how long it takes for the partial
pressure to get half way in between, i.e. e^{kτ} = 1/2. The
variable τ (tau) is called the 'halftime' and is usually used for
characterizing tissue (compartments). Rewriting: kτ = ln(1/2) =
ln(2). So the relation between k and the halftime τ is:
 τ =    (12a) 
 k =    (12b) 
The alveolar partial pressure
So far we did not worry about the values of P_{alv}. We will have a
closer look at this alveolar partial pressure of the inert gas and how it
is related to the ambient pressure. The pressure of the air (or gas
mixture) the diver breathes is equal to the ambient pressure
P_{amb} surrounding the diver. The ambient pressure depends
on the water depth and the atmospheric pressure at the water surface. To
be precise: it is equal to the atmospheric pressure (1 bar at sea level)
increased with 1 bar for every ten meters depth. The partial pressure of
the inert gas in the alveoli depends on several factors:
 The partial pressure (fraction Q) of the inert gas in the air or gas
mixture breathed in
 The water vapor pressure. The dry air breathed in is humidified
completely by the upper airways (nose, larynx, trachea). Water vapor dilutes
the breathing gas. A constant vapor pressure at 37 degrees Celsius of
0.0627 bar (47 mm Hg) has to be subtracted from the ambient pressure
 Oxygen O_{2} is removed from the breathing gas by respiratory
gas exchange in the lungs
 Carbon Dioxide CO_{2} is added
to the breathing gas by gas exchange in the lungs. Since the partial
pressure of CO_{2} in dry air (and in common breathing mixtures) is
negligible, the partial pressure of the CO_{2} in the lungs will be
equal to the arterial partial pressure. This pressure is 0.0534 bar (40 mm
Hg).
The process of Oxygen consumption and Carbon Dioxide production is
characterized by the respiratory quotient RQ, the volume ratio of
Carbon Dioxide production to the Oxygen consumption. Under normal steady
state conditions the lungs take up about 250 ml of Oxygen, while
producing about 200 ml of Carbon Dioxide per minute, resulting in
an RQ value of about 200/250=0.8. Depending on physical exertion and
nutrition RQ values range from 0.7 to 1.0. Schreiner uses RQ=0.8 , US Navy
uses RQ=0.9 and Bühlmann uses 1.0.
The alveolar ventilation equation gives us the partial pressure
of the inert gas with respect to the ambient pressure:
 P _{alv} = [ P _{amb}  P _{H2O}  P _{CO2} + Δ P _{O2} ] Q   (13a) 
 P _{alv} =   P _{amb}  P _{H2O} +   P _{CO2}   Q   (13b) 
 P_{alv}
 Partial pressure of the gas in the alveoli (bar)
 P_{amb}
 Ambient pressure, i.e. the pressure of the breathing gas(bar)
 P_{H2O}
 Water vapor pressure, at 37 degrees Celsius 0.0627 bar (47 mm Hg)
 P_{CO2}
 Carbon Dioxide pressure, we can use 0.0534 bar (40 mm Hg)
 ΔP_{O2}
 Decrease in partial Oxygen pressure due to gas exchange in the lungs
 RQ
 Respiratory quotient: ratio of Carbon Dioxide production to Oxygen
consumption
 Q
 Fraction of inert gas in the breathing gas. For example N_{2}
fraction in dry air is 0.78
The Schreiner RQ value is the most conservative of the three RQ values.
Under equal circumstances using the Schreiner value results in the highest
calculated partial alveolar pressure and hence the highest partial pressure
in the tissue compartments. This leads to shorter no decompression times
and hence to less risk for DCS.
Examples
We will have a look at our diver who plunges to 30 m and stays there for a
while. The diver breathes compressed air and did not dive for
quite a while before this dive. So at the start of the dive, all his tissue
is saturated with Nitrogen at a level that corresponds to sea level. We
neglect the period of descending. In particular, we will look at two types
of tissue in his body with a half time of 4 minutes (the fastest tissue)
resp. 30 minutes (medium fast tissue). The ambient pressure at 30 meters is
4 bar. Equation (13) gives us a partial alveolar N_{2} pressure of
3.08 bar at 30 meters and 0.736 bar at sea level, using the RQ=0.9 value of
the US Navy. Substituting these values in equation (6) result in (14),
predicting the partial pressure in the tissues. This pressure is shown in
figure 1:
 P _{t4} ( t ) = 3.08 + [ 0.736  3.08 ] e     t 
  (14a) 
 P _{t30} ( t ) = 3.08 + [ 0.736  3.08 ] e     t 
  (14b) 
Figure 1: Partial Nitrogen pressure in tissue with halftimes 4 and 30
minutes
Apparently, the faster tissue saturates much faster than the medium
fast tissue. Usually, after 6 halftimes the tissue is called
saturated.
After 20 minutes at 30 meter our diver decides to head back to
the surface at a very slow speed of 3 meter per minute (negative, since he
is ascending). It takes 10 minutes to swim to the surface. The rate of
change of partial alveolar pressure R is related to the change in ambient
pressure R_{amb} and the fraction Q of the inertial gas by:
In our example the ambient pressure drops (41)=3 bar in 10 minutes. This
corresponds to R_{amb}=0.3 bar/min. The partial pressure change of
the alveolar N_{2} R = 0.3 * 0.78 = 0.234 bar/min. After 20
minutes at 30 meter, the partial N_{2} pressure is given by (14)
and is equal to 3.00 bar in the 4 min tissue and 1.60 bar in the 30 min
tissue. Substituting this in equation (11) gives us equation (16) for the
partial pressure of the N_{2} in the tissues:
 P _{t4} ( t ) = 3.08  0.234   t        3.08  3.00 + 0.234    e     t 
  (16a) 
 P _{t30} ( t ) = 3.08  0.234   t        3.08  1.60 + 0.234    e     t 
  (16b) 
Figure 2: Partial Nitrogen pressure during and after surfacing
Time t is with respect to the start of the ascent.
Figure 2 depicts the situation: the solid lines represent the period the
diver is at 30 m depth. The lightcolored parts in the middle of the graph
(2030 min) represent the period of ascending. The darkercoloured parts at
the right represent the period after ascending, when the diver is at
sea level. Using equation (6) this part has been calculated, using a
N_{2} level in the tissues of P_{t0_4}=1.83 and
P_{t0_30}=1.66 bar for the 4 minutes resp. 30 minutes tissue at the
moment of arriving at the surface. As we can see the faster tissue
saturates faster than the slower tissue. However, it desaturates faster as
well, even during ascending. Since the diver ascended slowly, there is no
much difference in Nitrogen levels between the tissues at the moment of
arriving at the surface.
Supersaturation limits and MValues
So we are now able to calculate inert gas levels and the amount of
supersaturation in all tissue compartments of the diver. As we stated a
certain amount of supersaturation is allowed, without developing DCS
symptoms. In this section we will define limits applying to
supersaturation levels. As we will see these limits depend on:
 Type (halftime) of the tissue
 Ambient pressure, i.e. the pressure of the breathing gas
(depending on depth and atmospheric pressure)
Limits according to Haldane
In 1908 Haldane presented the first model for decompression. He noticed
that divers could surface from a depth of 10 meter, without developing DCS.
He concluded that the pressure in the tissue can exceed the ambient
pressure by a factor of 2. (Actually the factor the partial pressure of
the Nitrogen in the body exceeds the ambient pressure is
0.78*2=1.56, as Workman concluded)
Haldane used this ratio to construct the first decompression tables. Up
to 1960 ratio's were used. Different ratio's were defined by various
scientists. In that period most of the US Navy decompression tables
were calculated using this method.
Workman Mvalues
At longer and deeper dives, the ratio limits did not provide
enough safety. Further research into supersaturation limits was
performed by Robert D. Workman around 1965. Workman performed research
for the U.S. Navy Experimental Diving Unit (NEDU). He found that each
tissue compartment had a different partial pressure limit, above which
DCS symptoms develop. He called this limiting pressure M. He found a
linear relationship between this Mvalue and depth. Hence he defined
this relationship as:
 M
 Partial pressure limit, for each tissue compartment (bar)
 M_{0}
 The partial pressure limit at sea level (zero depth), defined for each
tissue compartment (bar)
 ΔM
 Increase of M per meter depth, defined for each compartment (bar/m)
 d
 Depth (m)
The actual Workman Mvalues are shown in the
MValues tables. Workman found that M values decrease with
increasing halftime of the tissue compartment, indicating fast tissues
can tolerate a higher supersaturation level. Using (17) we can
calculate (for each tissue compartment) the minimum tolerated depth
d_{min} the diver should stay at during a decompression stop,
depending on the amount of supersaturation:
 d _{min} =    (18) 
In order to estimate the actual depth the diver should stay below, we have
to calculate the depth for each compartment, and take the largest depth as
the limiting depth.
The Bühlmann models
Bühlmann performed research to decompression from 1959 up to 1993.
Like Workman he suggested a linear relationship between supersaturation
limits and ambient pressure. However, his definition is somewhat
different:
 P _{t.tol.} ig = M =   + a   (19) 
 P_{t.tol.}ig
 Partial pressure limit, for each tissue compartment, equals M (bar)
 P_{amb}
 The ambient pressure, i.e. the pressure of the breathing gas (bar)
 b
 1/b is the increase of the limit per unit ambient pressure
(dimensionless)
 a
 The limit value at (theoretical) absolute 0 ambient pressure
(bar)
The big difference (actually, the minor difference) between the Workman
definition and the Bühlmann definition is that Workman relates
M to ambient depth pressure (diving from sea level), whereas
Bühlmann relates to absolute zero ambient pressure. However, in both
cases, the partial pressure limit is related to ambient pressure by a
linear relationship. Conversions between both definitions can be easily
made, resulting in the following relationships:
 ΔM =    (19) 
 M _{0} = a +   
In 1985 Bühlmann proposed the ZHL12 model (ZH stands for Zürich,
L for 'limits' or 'linear', and 12 for 12 pairs of Mvalues). In 1993 book
he proposed the ZHL16 model, which is quite popular as basis for diving
computers. The coefficients of both models are presented in the tables in Mvalue style.
For the ZHL16 model Bühlmann used a emperical relation for the
a and b coefficient as function of the halftime τ for
Nitrogen N_{2}:
 a = 2 bar τ    
  (20a) 
 b = 1.005  τ    
  (20b) 
This results in the Aseries coefficients. However, these coefficients were
not conservative enough, as was empirically established. So he developed
the B and Cseries of coefficients for table calculations and computer
calculations respectively. All three sets are presented in the tables.
Other models
DCAP (Decompression and Analysis Program) uses the M11F6 Mvalues,
established by Bill Hamilton for the Swedish Navy. This set of Mvalues
is used in many decompression tables used in trimix and technical
diving.
The PADI Recreational Dive Planner ^{TM} uses the set of
Mvalues developed by Raymond E. Rogers and Michael R. Powell from
Diving Science and Technology Corp (DSAT). These values were extensively
tested and verified, using diving experience and Dopplermonitoring (a way
to detect silent bubbles in tissue).
The Recreational Dive Planner ^{TM} is a table used for
nodecompression dives only. This means that for the calculation of table
values no decompression stops are included: the diver can return
to the surface any time. The only relevant limit M is the limit at sea
level, M_{0}. For this model ΔM is not needed.
A comparison between the models
In the graphs below the limit M for the partial pressure of Nitrogen is
plotted as function of the halftime for the different models. As we can
see the limits according to the different models are comparable to each
other.
Figure 3: Partial Nitrogen pressure limit M vs. τ at sea level
Figure 4: Partial Nitrogen pressure limit M vs. τ at 30 m
The ambient partial Nitrogen pressure is shown as a dashed line in the
graphs. In fact, if the partial pressure of a tissue compartment is
somewhere between the dashed line and the limit, the compartment
decompresses safely. This means, the tissue gets rid of the excess
Nitrogen in a controlled way. In decompression dives the diver should be as
close to the limit during decompression stops in order to decompress
most efficient (fast). Bühlmann expresses the position of the tissue
pressure with respect to the limit as a percentage: 0% if the
partial pressure of the compartment equals the ambient pressure, 100% if it
equals it limiting M value.
Nodecompression times
A number of tables, like the PADI RDP express
nodecompression times. These are maximum times a diver can stay at a
certain depth being able to go to the surface without the need for
decompression stops. Based on equation (6) we can calculate this time for a
particular tissue compartment:
Of coarse we have to calculate this time for every tissue and take the
minimum value as limit for the diver to remain at the depth. So what about
P_{no_deco}. If we neglect the time the diver takes swimming to
the surface, it would simply be M_{0}. However, as we have seen,
the period the diver swims to the surface is important for decompressing as
well. So we can use equation (11) to calculate M_{no_deco}. We
assume a ascending speed of v (m/min) and we use the fact that we want
to arrive at the surface with the tissue compartment partial pressure of
M_{0}. We like to know the pressure
P_{no_deco}=P_{t0} at which we have to start ascending.
 P _{no_deco} =   M _{0}  P _{alv0}  R   t_asc      e ^{k t_asc} + P _{alv0}     (22) 
In equation (21) en (22) we have:
 M_{0}
 Partial pressure limit (Mvalue) at sea level (bar)
 t_asc
 Time needed for ascending, t_asc=depth/v (min)
 P_{no_deco}
 Partial pressure at which ascending has to be started (bar)
 d
 Depth (m)
When we use the DSAT RDP values, we find for example the smallest nodeco
time of 53.9 min for the 30 minutes halftime compartment in case of a
maximum depth of 20 m. The ascending speed v=18 m/min, t_asc=1.11 min.
More conservative limits
The limits discussed so far are actually not absolute. It merely is a solid
line in a gray area. If one stays within limits, there is no guarantee
that one never would develop DCS. Actual calculations can be made more
conservative (include more safety) by adding extra depth, simulating
asymmetrical tissue behaviour (a longer halftime for desaturation than for
saturation), adding surplus of Nitrogen, assuming a higher ascending speed,
etc. Uwatec uses a Bühlmann model ZHL8 ADT (ADT stands for adaptive),
which uses 8 tissue compartment and takes into account the water
temperature and the amount of work the diver performs (measured from the
amount of air he uses). If the water is cold the halftime for desaturation
is longer than the halftime for saturation.
Last modified on March 13 2005 22:51:32.Copyright 19992016 Deep Ocean
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