Nerites
Underwater Monitoring System of Diver’s Respiration and Regulator Performance
using Intermediate Pressure Signal
Corentin Altepe
1
, S. Murat Egi
2,3
, Tamer Ozyigit
2
and Paola Pierleoni
4
1
Bogazici Underwater Research Center, Yavuzturk Sk. 32/1 Altiyol 34716, Istanbul, Turkey
2
Galatasaray University, Comp. Engineering Dpt., Ciragan Cad. 36 Ortakoy 34349, Istanbul, Turkey
3
DAN Europe Research Division, 12 Via Basilicata, Roseto degli Abruzzi TE, Italy
4
Marche Polytechnic University, Information Engineering Dpt., Via Brecce Bianche 12, 60131 Ancona, Italy
Keywords:
Breathing Monitor, Breathing Frequency, First Stage Regulator, Two-Stage Regulator, SCUBA Diving,
Breathing Detection.
Abstract:
This study is about a system for monitoring the breathing cycles of divers and the functioning of regulators
underwater. It warns the diver and other surrounding divers in case of long time cessation of respiration or if
the regulator’s intermediate pressure is out of predefined limits, enabling immediate intervention to regulator
and breathing problems.
The system is equipped with two pressure sensors and a microprocessor. It can be quickly mounted on the
scuba diver’s equipment to sense distortions in the intermediate pressure and to sense the depth while under-
water.
Generally all data, including dive profile, are logged and transferable to PC for post-dive analysis. The low
and high level alarms enable immediate intervention in case of breathing or regulator problems.
Major benefits of the system are rapid detection of respiration and regulator problems and ease of locating
an unconscious or deceased diver. Additionally, this system aims at contributing to decompression illness
research and at helping regulator manufacturers with the data it will collect while underwater.
While this study focuses on the hardware development of the system, breathing detection algorithms are cur-
rently being studied and optimized off-line using data collected by the system.
1 INTRODUCTION
1.1 Nerites
The system developed in this study is called Nerites,
named after a sea deity from the Greek mythology,
son of Nereus and Doris.
1.2 Scuba Diving Accidents History
In 2010, Divers Alert Network (DAN) Europe and
DAN America reported 70 percent of the scuba fa-
talities of their members were caused by drowning,
for a total of 112 fatalities of DAN Europe members
and 814 fatalities of DAN America members. 14 per-
cent of the DAN America fatalities and 13 percent of
DAN Europe fatalities were reported to be caused by
cardiac events (P. J. Denoble and Vann, 2010). The
Nerites system addresses these causes for the major-
ity of scuba diving fatalities as a method of prevention
from drowning and an early alert in case of drowning
or cardiac event.
1.3 Two-Stage Regulator
The regulator type used in open-circuit scuba diving is
called two-stage regulator. The breathing gas is stored
in a tank under a pressure of 5 to 300 bar. The first
stage of the regulator drops the gas from the tank’s
pressure (High Pressure, or HP) to an Intermediate
Pressure (IP) of typically 9 to 11 bar above the Low
Pressure (LP). The second stage of regulator is then
used to drop the gas pressure from IP to LP, which
corresponds to the ambient pressure. Nerites system
addresses only two-stage regulators for application in
open-circuit scuba diving.
Figure 1 displays a simplified mechanism of a reg-
Altepe, C., Egi, S., Ozyigit, T. and Pierleoni, P.
Nerites - Underwater Monitoring System of Diver’s Respiration and Regulator Performance using Intermediate Pressure Signal.
DOI: 10.5220/0005646601190124
In Proceedings of the 9th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2016) - Volume 1: BIODEVICES, pages 119-124
ISBN: 978-989-758-170-0
Copyright
c
2016 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
119
Figure 1: Simple representation of gas pressure regulator
mechanism.
ulator, which can be used to understand the mecha-
nisms of both the first and the second stages of a scuba
diving regulator. The fluid at ambient pressure en-
ters the right chamber and applies pressure on the di-
aphragm (2). The ambient pressure and the spring (1)
move the diaphragm which in turns moves the valve
(3). Once opened, the valve (3) lets the input gas (at
higher pressure) enter the main chamber of the regu-
lator. This higher pressure pushes the diaphragm back
to its initial position and closes the valve (3). It is then
output from the left part of the regulator, at a lower
pressure than the input. The mechanical design of the
diaphragm (2) and the spring (1) define the pressure
of the output gas.
The second stage of the regulator is equipped with
the valve (4), which allows the diver to exhale through
the regulator. The excess of pressure opens the valve
(4) which is lets the gas exit to the ambient air or wa-
ter. This valve is absent in the first stage regulator.
1.4 Breathing Mechanics
While O
2
consumption of the body is directly linked
to the workload, it remains similar for an equivalent
activity underwater for scuba divers. Gas pressure
in the lungs is normally equal to ambient pressure.
At higher depth, gas pressure in the lungs is higher.
However, the body needs in O
2
and its CO
2
produc-
tion remaining the same as dry land, surface equiva-
lent activity, the respiratory gas exchange rate desig-
nated with the symbol R, remains the same as surface,
dry land conditions:
R = V
CO
2
/V
O
2
(1)
Where V
CO
2
is the quantity of CO
2
produced by
the body and exhaled through the lungs, and V
O
2
is
the quantity of O
2
acquired by the body and inhaled
through the lungs.
In consequence, the respiratory rate or breathing
frequency of the diver remains the same at depth
(Bennet, 2003). A normal respiratory rate for an
adult is known to be 12 to 20 cycles per minute
(M. A. Cretikos and Flabouris, 2008).
2 DETECTION OF BREATHING
2.1 First Observations
An analog gauge was placed on the IP and it was
observed visually that the IP would slightly oscillate
while the diver inhales gas from the second stage of
the regulator. The amplitude of the oscillation would
not go over 0.5 bar. There was no such phenomenon
observed at the diver’s exhalation due to the one-way
valve of the second stage of regulator -item 4 dis-
played in Figure 1- letting the diver exhale through
the second stage regulator directly to the water. This
effect can be explained by the mechanism of the reg-
ulator, triggering gas input to the IP only when the IP
drops.
2.2 Digitization of IP
A digital sensor MS5541C was connected to the IP
and plugged to a development board. The pressure
sensor used has a resolution of 1.2 mbar and a max-
imum pressure of 14 bar. The maximum sampling
frequency obtained with this sensor was 2 to 4 Hz.
Although the sampling frequency enabled by the
pressure sensor was low -a normal breathing fre-
quency for an adult is between 12 and 20 breaths per
minute, so a breathing cycle every 3 seconds- it was
expected to be sufficient to observe the phenomenon
on the IP signal.
The microcontroller MSP430F5529 was pro-
grammed to acquire the IP sensor measures as fast
the sensor allowed, to acquire the ambient pressure
measures at a rate of 1 Hz, and to transmit the live
measures to the USB interface.
A User Interface (UI) was developed on PC, us-
ing the .NET Framework 4 and Visual Studio as de-
velopment environment. The PC was connected to
the development board by USB and the user interface
displayed the live measures received from the micro-
controller.
Figure 2 shows the UI during the acquisition of the
live measures from the development board, for a total
recording time of 107 seconds and 272 IP samples.
The normal IP, when the diver is not inhaling, is about
148 psi (10.2 bar). The ambient pressure at time 107
second is 14.6 psi (1.007 bar).
Each drop in the IP corresponds to an inhalation
through the second stage of the regulator. In Figure 2,
a total 10 inhalations are observed, with different du-
ration and intensities. At time 85 sec, a very shallow
IP drop is observed, corresponding to a very short,
low volume of inhalation by the diver.
BIODEVICES 2016 - 9th International Conference on Biomedical Electronics and Devices
120
Figure 2: First UI and digitization of IP at an average sam-
pling rate of 2.54 Hz.
At each inhalation, the IP starts dropping until a
certain trigger level where the first stage of the regu-
lator rapidly balances the IP by injecting higher pres-
sure gas from the tank.
In the observation of Figure 2, excluding the drop
at 85 sec, each inhalation results in a drop in IP of
6 to 13 psi (400 to 900 mbar). The slopes could not
be measured accurately due to the low sampling fre-
quency.
2.3 Automatic Breathing Detection
In order to detect the breathing events automati-
cally, algorithms are being studied and developed
off-line, using data collected from tests carried out
with Nerites. Eventually, Nerites system will imple-
ment an algorithm for real-time detection of breathing
events, enabling automatic breathing monitoring for
the diver’s safety.
3 REGULATOR PERFORMANCE
MONITORING
As recalled in Section 1.3, a first stage regulator is
designed to drop the gas pressure from the tank to an
Intermediate Pressure of 9 to 11 bar above the ambi-
ent pressure. The exact value of the IP differs from
manufacturer. However an accurate IP is important
for the effective operation of the second stage of the
regulator, the mouthpiece used by the diver.
If the IP is not properly regulated by the first stage
regulator and is higher than its design range, there is
a risk for the second stage regulator to let the gas free
flow, uncontrollably letting the gas from the tank es-
cape to the water during a dive. The diver may end up
without sufficient breathing gas to surface and drown.
In a similar manner, if the IP is lower than its
defined range, the diver may experience difficulty in
breathing due to excessive Work of Breathing (WoB).
Whereas a mild WoB may only cause discomfort, an
excessive WoB will cause the diver to physical ex-
haust, hypoxia, or drowning.
By monitoring the ambient pressure with a digi-
tal pressure sensor, it is possible to compare the IP
and the ambient pressure (also called Low Pressure or
LP). Monitoring the IP LP value enables monitor-
ing the proper functioning of the first stage regulator.
As a regulator will generally not start delivering an
IP out of its design range suddenly, but rather slowly
on the course of several weeks or several consecutive
dives, often due to a lack of maintenance, monitoring
the value IP LP continuously allows detecting the
improper functioning of the regulator before it gener-
ates serious consequences for the diver’s safety.
While detecting the breathing requires a sampling
rate of 20 Hz from the IP sensor, the ambient pres-
sure cannot change quickly due to restrictions on div-
ing practise. Recreational diving limits ascending and
descending speeds to 10 m.min
1
. Technical diving
generally limits descending speed to 30 m.min
1
and
ascending speed to 15 m.min
1
. Therefore, the maxi-
mum absolute value of ambient pressure derivative in
open-circuit scuba diving equals:
30
60
.(100.518)[m][s
1
][mbar.m
1
]
= 50.259[mbar][s
1
]
(2)
where 100.518 mbar.m
1
represents the standard
pressure variation with depth variation in salty water,
often rounded to 100 mbar.m
1
.
The typical acceptable range for a first stage reg-
ulator being 2 bar (from 9 to 11 bar, typically), a
sampling rate of 1 IP measure per second is far suf-
ficient. For comparison, dive computers generally
record depth measures once every 2 to 10 seconds,
depending on the model.
A resolution of 1 cm of salty water, equivalent to
10 mbar, is required in order to monitor the proper
functioning of the regulator and to record the depth of
the diver in a dive log.
4 SYSTEM COMPOSITION
Following the initial test on automatic breathing de-
tection, a system was designed for underwater appli-
cation.
The main difference in its composition is that the
sensor used for the IP was of better performance. It
was selected to reach a sampling frequency of 20 Hz,
with a 24-bit resolution and withholding a pressure
Nerites - Underwater Monitoring System of Diver’s Respiration and Regulator Performance using Intermediate Pressure Signal
121
up to 30 bar, corresponding to the IP at a depth of
approximately 100 meters in salty water.
4.1 Brief Specifications
The system has the following characteristics:
Single piezo-switch button and four LEDs for
minimal user interface
USB connector for connection to PC and ad-
vanced UI
Two buzzers to be heard easily underwater
One bicolor LED: green to indicate the charge of
the battery, red to indicate the low level of the bat-
tery
One yellow LED: blinking when a the diver’s
breathing is detected to indicate its proper func-
tioning
One red LED: blinking when an alarm is triggered
(non breathing or regulator performance alarm)
Pressure sensor 89BSD: 0 to 30 bar, 24-bit reso-
lution and sampling frequency used at 20 Hz
Depth detection using a second 89BSD pressure
sensor, with a sampling rate of 1 Hz
Operation depth: 0 to 100 meters (0 to 11 bar am-
bient pressure, 12 to 29 bar IP)
A microcontroller Texas Instruments
MSP430F5529, selected for its ultra-low power
characteristics and its USB implementation, acquires
the signal from the IP sensor, stores the data in an
internal flash memory and controls the buzzers and
LEDs.
4.2 PCB Design
The system was designed for an easy integration into
a waterproof casing. It aimed at being easy to use for
the diver, therefore staying as small as possible.
The components used for the electronics were se-
lected for their small footprint, their availability and
their price. The PCB was designed to remain as small
as possible while allowing soldering its components
with limited equipment for a limited series. As a re-
sult, all of the selected electronics components are
Surface-Mount Devices (SMD), with a small foot-
print but large enough to solder using a hand iron.
None of the parts is of Ball Grid Array (BGA) foot-
print, which can only be soldered using a oven (not
available).
Figure 3: Nerites 3D-printed prototype.
Figure 4: Nerites final design.
4.3 Casing
The casing was redesigned several times to fit best the
diver’s equipment and minimize its size.
It was designed to be plugged to the diver’s equip-
ment, between the buoyancy compensator (BC) and
the IP hose. Its single button allows the diver to start
the device, turn it off, trigger manually an alarm while
underwater and turn off any alarm. The case was de-
signed to minimize the interaction with the diver. It
should be used only in case of emergency, and in such
case, be used very quickly and easily.
Figure 3 shows the 3D-printed prototype of Ner-
ites, with it PCB mounted. Figure 4 shows Nerites
system mounted on a BC. Figure 5 shows Nerites
mounted on a diver’s equipment for a first test.
4.4 Product Use Cases
As described in Figure 6, Nerites can run in three dif-
ferent modes-
Sleep mode: the user doesn’t use Nerites. The
battery can be charged while in sleep mode.
In Use: either on the surface or while diving
BIODEVICES 2016 - 9th International Conference on Biomedical Electronics and Devices
122
Figure 5: Nerites equipping diver for a first test.
Figure 6: Modes of operation.
USB Interface: Nerites is connected to a PC to ei-
ther download dive log, run a regulator diagnostic
or configure the internal parameters
While in Use mode, the breathing monitor is ac-
tive. The breathing monitor is a part of the program
which was prepared to implement the breathing detec-
tion events (currently in development). At each new
IP measure (20 Hz) or LP measure (1 Hz), the breath-
ing monitor verifies the alarms conditions for breath-
ing detection and regulator performance (see section
2.2). It triggers the alarms if their respective condi-
tions are met.
Figure 7: Breathing pattern recorded with the system for
two minutes on one diver.
5 CONCLUSIONS
The system has allowed collecting IP signal data
while on surface. Breathing signals for a length of
one minute were recorded for six divers on the surface
using a total of two different regulators. These pro-
files were later used in the CADDY project, aiming at
identifying the diver health status (normal, in panic, or
in danger). CADDY is a collaborative project funded
by the European Community’s Seventh Framework
Programme FP7 which aims to establish an innovative
set-up between a diver and companion autonomous
robots (underwater and surface) that exhibit cognitive
behaviour through learning, interpreting, and adapt-
ing to the divers behaviour, physical state, and ac-
tions(S. M. Egi, 2015). Nerites system is also ex-
pected to help regulator manufacturers to measure the
performance of their products while underwater.
The developed User Interface allows easy cus-
tomization of the parameters to fit the equipment
specifications and the habits of the diver. The UI also
allows running a full, live diagnostic of the regulator
while on surface, helping the maintenance of equip-
ment.
5.1 Regulator Mechanism
Figure 7 displays the IP signal of a diver using Nerites
for about 2 minutes, with a total of 18 inhalations.
With a sampling frequency of 20 Hz, it allows a more
detailed observation of the regulator mechanism but
only confirms the preliminary observations of Section
2.2.
Each drop of amplitude 350 to 500 mbar in the IP
corresponds to an inhalation through the second stage
of the regulator. At each inhalation, the IP starts drop-
ping until a certain trigger level where the first stage
of the regulator rapidly balances the IP by injecting
higher pressure gas from the tank.
Nerites - Underwater Monitoring System of Diver’s Respiration and Regulator Performance using Intermediate Pressure Signal
123
6 FUTURE WORK
The algorithm for breathing event detection will be
developed in order to implement a real-time, efficient
and reliable breathing event detection on Nerites. Al-
though the system Nerites was used to record six
divers on surface, with a total of two different regu-
lator models, the system will be used to gather further
data.
The factors preliminarily identified which are be-
lieved to influence a breathing event IP signal pattern
are:
Model / Brand of the regulator
Tank pressure
Depth
Therefore, several additional tests will be car-
ried out in order to collect data at different depths,
tank pressures and using different models of regula-
tor. These collected data will later be used off-line
to develop and compare on a bench test the reliability
and efficiency of each event detection algorithm.
The system Nerites has yet to be used underwater
and in pressure chambers, as the manufactured pro-
totype casings have not allowed testing under higher
than surface pressure.
As the model and brand of the regulator is be-
lieved to greatly influence the IP pattern of breath-
ing event, a calibration will have to be run in order to
optimize and adapt the breathing detection algorithm
parameters to the diver’s equipment. Such a calibra-
tion process will be implemented in the User Interface
such that it will be guided and automatic. It will guide
the diver step by step into the calibration process such
as opening the first stage regulator and inhaling from
the second stage when required.
REFERENCES
Bennet, Elliott, A. O. B. T. S. N. (2003). Physiology and
medicine of diving, pp.77-114. Saunders, 5th edition.
M. A. Cretikos, R. Bellomo, K. H. J. C. S. F. and Flabouris,
A. (2008). Respiratory rate: the neglected vital sign.
In MJA, vol. 188 Num. 11.
P. J. Denoble, A. M. and Vann, R. D. (2010). Annual fa-
tality rates and associated risk factors for recreational
scuba diving. In Recreational diving fatalities work-
shop proceedings. Divers Alert Network.
S. M. Egi, C. Balestra, M. P. D. C. G. T. A. M. (2015). Cog-
nitive autonomous diving buddy (caddy): operational
safety and preliminary results. In Caisson, N.1.
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