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2
On the reliability of dive computer
generated run-times, Part VII:
Altitude Test
Abstract:
Here, in Part VII, we performed an altitude test, i.e. the simulation of diving
in a mountain lake.
During the previous parts I to VI ([1] to [6] and all the references therein), we
observed by some of the dive computer manufacturers deviations from
documented algorithms/decompression models with simulated dives on sea-
level (SL), whereas Part VII covers a test at reduced ambient pressure of ca.
0.8 Bar, i.e. a mountain lake at a ca. altitude of 2.000 m above SL.
Introduction: slides # 3 & 4
Methods: slides # 5 7
Results: slides # 8 14
Discussion & Conclusion: slides # 15 & 16
References: slides # 17 19
Attachment: Historical Perspective, slides # 20 22.
3
Introduction (1):
Scuba diving at altitude, i.e. at reduced ambient pressure,
requires adapted diving-procedures and –tables. These procedures & tables
are regularly derived from those at sea-level (SL), that is, at an altitude of
ca. 0 m above SL.
There are many of those procedures available: these differ in the methods of
extrapolation of the maximal allowed or tolerated compartment (tissue-)
inertgas partialpressure from SL, i.e. at Pamb (ambient pressure) of ca. 1 Bar,
to the reduced Pamb at altitude.
The outcome is usually a reduced NDL in comparison to SL procedures resp.
longer stop-times at the various stop-depths and, depending on Pamb, a shift
of the last 2 deco-stop depths from 6 & 3 m to 4 & 2 m, respectively.
One such method is called the „Linear Extrapolation Method” (LEM), used
by Bühlmann, Hahn, and others and is usually implemented in the ZH-L16
type of dive computers.
4
Introduction (2):
However, the most prominent one is the so-called
“Cross Correction“ named after Ellis Royal Cross (1913 – 2000). It is a
Constant Ratio Translation (CRT), and implemented in the USN, DCIEM
and NOAA diving manuals.
As there are not enough reliable and
reproducible data available, we can presently
not decide which procedure is better / safer.
Thus we decided to compare only the outcomes
from the dive computers with the respective
procedure / table to check, how reliable
these are implemented.
So this is no decision / recommendation
in favor of one method or another.
5
Methods (1):
We simulated our notorious test-profile (42 m bottom depth,
25 min bottom time) at the reduced ambient pressure in the passengers
cabin during various intercontinental flights from Zürich to Tel Aviv with
commercial, civilian air-planes. The measured cabin-pressures were read
by the dive computers after ca. 1.5 h into the flight, that is ca. 60 min after
reaching the final, cruising height of ca. 32.000 feet:
The readings for this topical test were ca. 781 +/- 5 mbar, equivalent to an
approximate altitude of ca. 2.100 to 2.500 m above SL.
6
Methods (2):
The used dive computers were, from left to right:
Scubapro (UWATEC) Galileo „G2“, Hardware: 0.0, Software Version: 1.6
RATIO (Dive System) iX3M DEEP, Software Version: 4.0.81.1 / 016
SHEARWATER PERDIX, Firmware v87 / BT10, Hardware: SA-02A,
Deco Model: DCIEM
Scubapro (UWATEC) ALADIN TEC 2G, Software Version: 40 20 72 73 25
7
Methods (3):
The profile of the simulated test dive was 42 m bottom depth,
25 min bottom time with air as breathing gas and water density set to:
“Fresh“. No user adjustable conservativism factors have been used.
These parameters have been fed into the planning mode /
dive simulation interfaces of the computers, with the results
on slides # 8 14.
The results were then compared to the appropriate dive tables
for mountain-lake / altitude diving resp. the procedures for diving
at reduced ambient pressure.
For the Scubapro / UWATEC type of computers and the RATIO
products this is the mountain-lake table ZH-86 from
A. A. Bühlmann [65], pp. 229 235.
For the Shearwater computers with the DCIEM software-option
the procedures are described in the DCIEM Diving Manual [28],
pp.1-29 1-32 and table 5 on p. 1B-59.
8
Results (1):
The displayed pressures / calculated altitudes
(pls. cf. slide # 5) are well within the limited precision of the dive
computers pressure (piezo-)sensors, the daily variations and the
cabin-pressures, announced from the cockpit;
pls. cf. Ref. [7], slides # 2 & 4.
With these readings, the following tables / procedures have to be
used for comparison:
[65], Table 32 for 701 – 2.500 m above SL on p. 230, with an
adaption of 60 min. + required.
This table yields the following run-times:
stop depth [m] / stop times [min.] TTS [min]
12 9 6 4 2
42 m / 24 min -- 4 4 7 18 36
42 m / 27 min 1 5 5 9 21 44
9
Results (2):
The DCIEM procedures from [28], table 5, 1B-59 require
for the target altitude of 2.100 2.399 m above SL
a depth correction of + 12 m for the planned,
actual diving depth of 42 m @ SL.
Thus the altitude corrected DCIEM run-time yields for
42 + 12 = 54 m on page 1B-16:
stop depth [m] / stop times [min.] TTS [min]
15 12 9 6 3
54 m / 25 min 5 5 7 9 39 65 + 4 =
ca. 70
For this target altitude group (2.100 2.399 m) the stop depths
have to be corrected accordingly, i.e:
15 12 ; 12 9.5 ; 9 7 ; 6 5 ; 3 2.5!
13
Results (6):
The DCIEM altitude procedures could be used as well for
creating an NDL („no decompression limit“) table for the target altitude of
2.100 2.399 m above SL. For each planned, actual depth, there is then
the depth correction:
The corrected tables NDL could then
be compared with the NDL planner
tool from the Shearwater Perdix
computer
actual depth [m] depth correction
[m] corrected
depth [m] corrected
NDL [min]
12 + 6 18 50
15 + 6 21 35
18 + 6 24 25
21 + 9 30 15
15
Discussion / Conclusion (1):
Synopsis of results for 42 m / 25 min @ 2.100 2.399 m above SL:
RATIO iX3M DEEP TTS = 20 min (*)
ZH-86 table 42 m / 24‘ TTS = 36 min
ZH-86 table 42 m / 27‘ TTS = 44 min
Scubapro ALADIN TEC 2G @ MB Level = 0: TTS = 48 min
Scubapro Galileo „G2“@ MB Level = 0: TTS = 53 min
PERDIX with DCIEM option TTS = 60 min
DCIEM table 54 m / 25‘ TTS = 70 min
MB Level (= Micro Bubble Level) are proprietary modifications of the
tolerated compartment inertgaspressures, here they are set = 0. This
implies that the original ZH-L16 C values should be used.
(*): despite the used Gradientfaktors, default: GF Hi = GF Lo = 0.93
16
Discussion / Conclusion (2):
The deviation of the Scubapro/ UWATEC computers
using the ZH-L16 C system from the ZH-86 table with proprietary,
undocumented factors are yielding increased TTS than the original tables, for
altitude and as well for SL diving [1]. So both of the computers err, but on the
conservative side.
The Shearwater DCIEM option is close to the original procedure, but with an
error of a slightly decreased TTS. This is reflected as well with the
NDL analysis (pls. cf. slides # 13 & 14), where the Perdix NDL are longer .
Within the limited precision of the dive computers depth sensors, the
agreement between the original tables / procedures is quite sensible. The
deviations fall quickly within the error margins of civilian / recreational
diver behaviour.
The RATIO iX3m DEEP fails completely: the TTS is too short!
The altitude TTS is even shorter than the TTS for the test-profile @ SL:
this is around ca. 30 min, pls.cf. [1]. As this computer constantly fails to
produce sound TTS / run-times, pls. cf. all the Refs. in [8] & Ref. [9],
its usage is strongly discouraged!
17
On the reliability of dive computer
generated run-times, Part VII
References (1):
[1] Rosenblat M., Vered N., Eisenstein Y., Salm A. (26.07.2021)
On the reliability of dive computer generated run-times, Part I;
DOI: 10.13140/RG.2.2.16260.65929
[2] Rosenblat M., Vered N., Eisenstein Y., Salm A. (11.01.2022)
On the reliability of dive computer generated run-times, Part II;
DOI: 10.13140/RG.2.2.11343.41126
[3] Rosenblat M., Vered N., Eisenstein Y., Salm A. (02.02.2022)
On the reliability of dive computer generated run-times, Part III;
DOI: 10.13140/RG.2.2.21973.50405
[4] Rosenblat M., Vered N., Eisenstein Y., Salm A. (22.02.2022)
On the reliability of dive computer generated run-times, Part IV;
DOI: 10.13140/RG.2.2.11469.72169
[5] Rosenblat M., Vered N., Eisenstein Y., Salm A. (07.02.2022)
On the reliability of dive computer generated run-times, Part V;
DOI: 10.13140/RG.2.2.18129.81763
18
On the reliability of dive computer
generated run-times, Part VII
References (2):
[6] Rosenblat M., Vered N., Eisenstein Y., Salm A. (23.02.2022)
On the reliability of dive computer generated run-times, Part VI;
DOI: 10.13140/RG.2.2.36242.32969
[7] Salm, A. (2019) Altitude Diving, the 3rd.
https://www.divetable.info/skripte/Altitude_Diving_III.pdf
[8] Salm, A.: the SNAFU series from 2017 to 2022 @:
the little virtual dive computer museum
https://www.divetable.info/museum_e.htm
[9] https://www.divetable.info/kap16_e.htm
[10] Salm, A. (04/2020) Historical dive tables:
on overview on ca. 110 years of dive tables development
https://dx.doi.org/10.13140/RG.2.2.32813.03042
19
On the reliability of dive computer
generated run-times, Part VII
References (3):
[28] DCIEM Diving Manual, DCIEM No. 86-R-35 (1992): Part 1 Air Diving
Tables and Procedures; http://www.divetable.eu/p125936.pdf
[65] Albert A. Bühlmann, Ernst B. Völlm (Mitarbeiter), P. Nussberger
(2002) Tauchmedizin, Springer, ISBN 3-540-42979-4
http://www.divetable.eu/BOOKS/65.pdf
[248] Strauss, R.H. (ed.)(1976) Diving Medicine,
Grune & Stratton, Inc., N.Y., ISBN 0-8089-0699-2
20
On the reliability of dive computer
generated run-times, Part VII
Attachment: Historical Perspective
An overview of ca. „110 Years of dive tables development“ you will find in [10]
@ Researchgate.
Here, we compare with a table for altitude diving, developed by Bühlmann et
al. in 1973, for eg. in [248], pp. 361. Back in these days, divers were divers
and men were still men: the attitude towards DCS has changed since; pls. cf.
the next 2 slides with the SL table and one for 2.001 – 2.500 m above SL.
As there is no entry for 42 m, we have to take the next deeper level, i.e.:
45 m / 25 min; TTS @ SL: 25 + 4 min
TTS @ altitude: 63 + 4 min
This could be contrasted with the table of TTS from slide # 15.