Nerites

Underwater Monitoring System of Diver’s Respiration and Regulator Performance

using Intermediate Pressure Signal

Corentin Altepe

, S. Murat Egi

2,3

, Tamer Ozyigit

and Paola Pierleoni

Bogazici Underwater Research Center, Yavuzturk Sk. 32/1 Altiyol 34716, Istanbul, Turkey

Galatasaray University, Comp. Engineering Dpt., Ciragan Cad. 36 Ortakoy 34349, Istanbul, Turkey

DAN Europe Research Division, 12 Via Basilicata, Roseto degli Abruzzi TE, Italy

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

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

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

and its CO

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

(1)

Where V

is the quantity of CO

produced by

the body and exhaled through the lungs, and V

the quantity of O

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.

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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:

.(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

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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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