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.

Full text

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On the reliability of
dive computer generated run-times
01.07.2022, Part VII:
Altitude Test
Miri Rosenblat, TAU
Nurit Vered, Technion Haifa
Yael Eisenstein & Albi Salm,
SubMarineConsulting
DOI: t.b.d.

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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.

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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.

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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.

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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

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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.

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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

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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!

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Results (3):
DCIEM table: stop depth [m] / stop times [min.] TTS
15 12 9 6 3
54 m / 25 min 5 5 7 9 39 65 + 4
Shearwater PERDIX:

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Results (4):
ZH-86 table:
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

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Results (5):
ZH-86 table:
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
RATIO iX3M - 3 5 8 20

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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

14
Results (7):
actual depth [m] depth correction
[m] corrected
depth [m] corrected
NDL [min]
24 + 9 33 12
27 + 9 36 10
30 + 9 39 8
33 + 12 45 7
36 + 12 48 6
39 + 12 51 6
42 + 12 54 5
45 + 15 60 5

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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

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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!

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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

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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

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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

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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.

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Source: [248], p. 361

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Source: [248], p. 365