Inexpensive Expendable Conductivity
Temperature and Depth (CTD) Sensor
R. Paradis, and S. L. Wood, Member, IEEE
Abstract— The Conductivity, Temperature, and Depth (CTD)
sensor is one of the most used instruments in the Oceanographic
field. These devices are the number one, most important sensor
for any research of the ocean depths. CTD measurements are
found in every marine related institute and navy throughout the
world because they are used to produce the salinity profile for the
area of the ocean under investigation. In order to deploy large
numbers of CTD sensors, an inexpensive version is needed,
especially if used as a one-time (throw-away) system.
Oceanographic institutes and navies require instruments such as
these that are reliable, accurate, precise and inexpensive.
The CTD developed in this paper is low cost and disposable
that can be easily and rapidly deployed to obtain measurements
throughout the earth’s oceans. Five (5) specific specifications for
the device (ECS Unit Sensor) were paramount: 1) Its weight had
to be under 5-lb (2.27-kg) maximum, but ideally as close to 1 to 2lb (0.45-kg to 0.9-kg) as possible; 2) Its dimensions must be no
bigger than 10-in (25-cm) in height and 3-in (7.6-cm) in diameter;
3) It must be reasonably priced to be expendable; 4) It must have
wireless communication; and 5) It only has to record the top 10m of the water column.
This paper describes the design, development, construction
and testing of the Expendable Conductivity Sensor Unit (ECS
Unit), which when deployed, takes multiple snapshots of the
water profile and sends its data out via a satellite
communications chip. The ECS Unit was tested against other
CTDs for accuracy. When compared to the other expendable
CTDs the ECS Unit, along with being much more economical, is
able to provide more snapshots than any other expendable CTD
instrument currently available on the market. The CTD
instrument (ECS Unit Sensor) developed in this project costs
around $982.00 per unit to make.
Index Terms—
temperature,
conductivity,
CTD,
depth,
expendable,
This work was submitted on July 25, 2013 and was supported in part by
the United States Navy (Naval Surface Warfare Center Panama City Division
(NAVSEA-PCD)).
S. L. Wood, is the Ocean Engineering program chair at Florida Institute of
Technology, Department of Marine and Environmental Systems, Melbourne,
FL 32901 USA (e-mail: [email protected]).
R. Paradis, graduated with his Masters from Florida Institute of
Technology and currently is an engineer at General Dynamics - Electric Boat,
75 Eastern Point Rd, Groton, CT 06340 USA (email [email protected]).
I. INTRODUCTION
The CTD obtains the in situ salinity and temperature at a
specific depth to provide parameters for various uses such as
determining the speed of sound at the depth. This information
is a key part in understanding deep ocean currents and the
ocean's acoustical properties. While research of the deep sea
currents is important to oceanographers; the Navy is more
interested in the acoustical properties of the ocean. These
properties play an integral role for SOund Navigation And
Ranging (SONAR) and are used to help determine the
vibration signatures for underwater vehicles. The cost of the
standard CTD instruments, whether expendable or not, is very
expensive. In order to deploy large numbers of CTD sensors,
an inexpensive version is needed, especially if used as a onetime (throw-away) system.
II. CTD'S
Scientists use CTDs to get the salinity profile of a water
column in the ocean. These sensors give scientists a precise
and comprehensive charting of the distribution and variation
of water temperature, salinity, and density that helps
understand how the oceans affect life [1]. These properties
help oceanographers study the ocean and understand how the
density variations affect the acoustical properties of the water.
Generally CTDs are large, expensive, and are tethered to a
support vehicle (i.e., the CTD is lowered down into the water
column). However, CTDs can be small, e.g., the units used in
an Autonomous Underwater Vehicle (AUV) Remotely
Operated Vehicle (ROV), or glider, but as their size decreases
cost increases. Consequently, due to their price CTDs are not
usually expendable.
The main sensors that make up a standard CTD package
include a temperature sensor, conductivity meter, and pressure
sensor. The temperature sensor provides measurements at the
instrument’s location in the water column and those readings
help determining the salinity. The conductivity meter
measures conductance, which is the amount of electrical
current that can pass through the water; and with a few
calculations and input from the temperature probe, salinity is
determined. The pressure sensor of course measures pressure,
which is a function of depth. So, ultimately as the instrument
is lowered into the water column it takes readings using the
different sensors and through on-board calculations and data
processing it provides temperature and salinity readings with
respect to depth. These measurements givee insight into the
properties of sea water [1].
III. SALINITY
Salinity provides the measurement of the aamount of salt by
mass, to a unit mass of seawater [2]. Thhe ocean surface
salinity is important in understanding the effeects of fresh water
input and output to ocean dynamics and phhysical properties
[3]. One of the most important physical pproperties for the
scientist is the ocean water’s density. This diirectly affects the
acoustical properties of the ocean with whhich the speed of
sound is calculated along with the sound wave reflection/
refraction/diffraction properties. The speed oof sound “c” is an
important property for the scientist, which iss calculated using
eqn. 1 provided by Hermin Medwin usingg the temperature
”T”, depth ”z”, and salinity of the water ”S” [[4].
2
3
c = 1449.2 +4.6T −0.055T +0.00029T +(1.344 −0.010T)
(1)
(S−35) +0.16z
Salinity plays a key role in the health of vaarious ecosystems
around the ocean (e.g., coral reefs), sliight changes in
temperature and salinity can wreak havoc on coral reefs [5].
Oceanographers are also interested in the salinity of the ocean,
because along with water temperature, itt determines the
density of the ocean water. The salinity profiles give a
snapshot of the density variations in the oceann at that location,
and those profiles are important since as the oocean water cools
or salinity increases, the ocean water becom
mes either more or
less dense. The change in water density ddrives the ocean
circulation at greater depths more than wind ddriven circulation
near the surface [6]. These salinity profilees are extremely
important in understanding the ocean. By keeeping track of the
ocean’s surface salinity/density, scientists can monitor the
fluctuations in the water cycle and get a bettter understanding
of how the ocean interacts with various ennvironments on a
global scale [3].
To measure salinity, the following equatiions provided by
Bibby Scientific [7] are used. A probe with a set cell constant
“K” will have to be known. “K” is calculated by dividing the
distance between the probes (D) by the surrface area of the
probes (A):
K = D /A
(2)
By passing a known current and voltage aacross the probe
while submerged in the salt water a conducctance reading is
measured in units of Seimens (S’). To get conductance (G),
the electrical current I is divided by the knownn voltage (V) :
G = I/V
(3)
With the conductance known and the proobe cell constant
known, conductivity (C) can be calculated by dividing the
conductance by the probe cell constant “K”:
C = G(K)
(4)
Once conductivity is calculated it is converteed into Resistivity
(R), which is the reciprocal of conductivity.
(5)
R = 1/C
With resistivity and temperatture the salinity (S) is
calculated using the Practical Salin
nity Scale of 1978 (PSS)
equation by Edward Lewis [2] (E
Eqn. 6). This equation has
other additional variables: a temperrature reading and unity k
of the CTD instrument. The unity iss determined by taking the
conductivity value of a known solutiion and comparing it to the
conductivity reading of the solution. This ratio is the last
variable used in the equation off the PSS [7]. With the
combination of those variables and a few given constants, the
PSS equation is calculated.
(6)
where:
5.3851, a3 = 14.0941
a0 = 0.0080, a1 = -0.1692, a2 = 25
a4 = -7.0261, a5 = 2.7081
k is the sensors measure of unity
RT is the resistivity of the water sample at temperature T is
the temperature of the water samp
ple
b0 = 0.0005, b1 = -0.0056, b2 = -0.0066, b3 = -0.0375
b4 = 0.0636, b5 = -0.0144
Equation 6 is valid for temperatu
ures between -2 C and 35
C and salinity between 2 and 42 [2].
[ Although this equation
is good for a wide salinity range, the probe measuring the
conductivity of the sea water mustt have the appropriate cell
constant if the instrument cannott regulate the amount of
current it can pass across the pro
obes. The cell constant of
saltwater should be 10, for brackish water 1, and for fresh
water 0.1 cell constant [7]. Since th
he ECS Unit was designed
for saltwater, the cell constant thatt was used was the k =10
constant.
IV. CTD TYPES
CTDs come in all shapes and sizzes, ranging from a casting
CTD (the most basic and popular ty
ype), the autonomous Argo
Floats, and the expendable eXpend
dable CTD (XCTD)s. CTD
casting methods include ship assistted casting/manual casting
and remote sensing. Ship assisted casts use manual input to
complete a CTD cast (i.e., the low
wering and raising of the
CTD through the water column). For remote sensing, the
OV, or glider.
operation of the CTD is by AUV, RO
Most ship assisted CTDs are larrge and measurements are
recorded via communications cablee or logged on a memory
chip. These systems require a cable to be plugged into the unit
ments for analysis. When a
in order to download the measurem
communications cable is used, scien
ntists can observe the water
properties in real time. A standard
d CTD cast, depending on
water depth, usually requires a cou
uple of hours to collect a
complete set of data. Small, low-p
powered CTD sensors are
used on expendable CTDs or auton
nomous platforms such as
profiling floats, AUV’s, Expendablee or moored profilers [1].
A. AXCTD/XCTD
The XCTD by Lockheed Martin - produced by Sippican,
Inc. (Fig. 1) is launched from a specially designed apparatus
(hand held or hull mounted) over the side of a vessel; the
sensor itself contains a reel of a micro wire that is connected
to the launching apparatus. This micro wire is spooled out of
the probe and the launching apparatus simultaneously to
prevent the wire from binding or kinking while it descends
through the water column. The launching apparatus is
connected to a specialized data acquisition system (the MK21
DAQ System) designed for the XCTD via a data cable. The
data acquisition system records the data at a rate of 800-Hz
from the probe while the micro wire continues to unspool, and
upon reaching the extent of micro-wire length the wire breaks
and data logging ceases. The unit is only used once.
Remote sensing CTDs are used on unmanned / autonomous
underwater vehicles with onboard CTD packages incorporated
into the vehicle’s system. They are also used on expendable
units that require a support vehicle nearby to receive the data.
The autonomous vehicles send out data packets to a central
computer when it surfaces or record measurements onto a
memory chip and downloaded when the vehicle is retrieved.
The expendable CTDs used are usually the AXCTD, which is
very similar to the XCTD (made by Sippican, Inc., Fig. 2).
The Aerial expendable CTD (AXCTD) is launched from a
plane or AUV. The unit floats on the surface using a wireless
transmitter to send the data back to the aircraft. The floating
unit serves a similar role of the XCTDs launching apparatus.
The actual sensor gets dropped from the floating unit and a
small micro wire spool on the sensor is connected to the
floating unit relaying the data out to the aerial support vehicle.
The data is sent out as a Frequency Modulated Signal (FM
Signal). When the AXCTD sensor reaches its maximum depth
the micro wire breaks. The unit is only used once.
Fig. 1: XCTD: [8]
Fig. 2: AXCTD: [8]
B. Argos Floats
Argos Floats are a type of profiling float that has a
complementary relationship with the Jason satellite altimeter
mission. This relationship gave birth to its name of Argo, the
ship in Greek mythology from the tale of Jason and the
Argonauts. The large global float array with the Jason satellite
give real time data to computer models to help forecast the
ocean climate. There are over 3000 floats located throughout
the oceans providing a 100,000 temperature/salinity (T/S)
profiles (Fig. 3) and velocity measurements per year [10].
The Floats cycle to a 2000-m depth every 10 days. Once the
maximum depth is attained the onboard pump pushes fluid
into an external bladder making it positively buoyant.
Approximately 6 hours is required for the instrument to
surface during which measurements of temperature and
salinity are continuously taken. Once the float reaches the
surface, the float’s position is determined via GPS signal and
data is transmitted back. After the data is sent the bladder is
deflated and the float sinks to its predetermined depth and the
cycle is repeated. Each Argos Float is designed to travel up
and down in the water column 150 times (i.e., giving each
float a 4 to 5 year lifespan) [10].
Fig. 3: Argos Float [9]
C. AUV or Glider CTDS
AUV’s and gliders are used in every ocean around the
world. AUV’s, such as the Remote Environmental Monitoring
Units (REMUS), actively roam the oceans days or weeks at a
time working on missions that scientists have programmed.
Autonomous underwater gliders are similar to the standard
AUV, but generally are much smaller and use considerably
less power. Rather than using thrusters, they dynamically
adjust their buoyancy and convert the upwards or downwards
motion, from the change in buoyancy, to forward motion with
the pitch of their wings as an airplane does. The only
difference is that the plane usually stays level while the
underwater glider’s center of gravity changes thus changing
the glider’s angle, e.g., the Slocum glider (Fig. 4).
AUV’s typically have a higher payload capacity and
therefore can carry higher grade CTDs than gliders. For
example, the SBE 49 Fast-CAT CTD from Sea-Bird
Electronics, generally used in AUVs like the Bluefin Robotics
AUV’s, has a data logging rate of 16-Hz. This CTD,
depending on the model, has depth ratings up to 7000-m; and
a resolution of 0.00005 Seimens per Meter (S’/m).
This CTD is directly powered from the AUV’s battery
bank. Gliders like the Slocum Glider, mindful of their payload
weight, cannot carry such high end CTDs. Generally gliders
tend to use CTDs like the Glider Payload CTD from SEA-Bird
Electronics, which has a 1-Hz data logging rate and a depth
rating to 1500-m; and a resolution of 0.0003-(S’/m). The lack
of power available to the CTD on gliders requires the CTD to
have its own battery pack to power it for up to 45 days. [12]
VI. DEVELOPMENT
A. Design Considerations
The five (5) requirements for the ECS-Unit are: 1) Its
weight must be under 5-lb (2.27-kg) maximum, but ideally as
close to 1 to 2-lbs (0.45-kg to 0.9-kg) as possible; 2) Its
dimensions must be no bigger than 10-in (25-cm) in height
and 3-in (7.6-cm) in diameter; 3) It must be reasonably priced
to be expendable; 4) It must have wireless communication;
and 5) It must record the top 10-m of the water column.
Secondary requirements are: 6) it should have a 50-m depth
rating; and 7) it could be reused if retrieved.
The physical design of the ECS Unit’s housing was
determined by the shape of an AUV’s payload bay that would
hold the ECS Unit (i.e., a bay of 3-in (7.6-cm) diameter by 10in (25-cm) long) gave the ECS Unit its size and shape (Fig. 6).
Due to the size restraint, the ECS Unit telescopes to allow the
bottom of the unit to slide up into the buoyancy chamber.
When deployed, the ECS Unit expands when the buoyancy
chamber is inflated.
The operation of the ECS Unit is designed to be deployed
via an aerial vehicle or AUV. To prevent early activation the
unit is powered on only when a pair of water-contacts
becomes submerged for any period longer than a few seconds.
When the unit is deployed and becomes activated, it begins its
D. Moored Profilers
A Moored Profiler is a CTD that travels up and down the
water column attached to a subsurface mooring, using a
traction motor to travel up and down the mooring line. The
sub-surface mooring stretches to the seafloor (Fig. 5) and is
used in all regions of the ocean (i.e., from shallow coastal
waters to deep open ocean waters). This type of CTD profiler
stores its data on-board for the duration of its deployment and
is offloaded when the instrument is recovered. [14]
V. BUOYANCY CONTROL
Buoyancy control is the ability to maintain negative,
neutral, or positive buoyancy while in the water column.
Buoyancy control is the adjustment of the water displacement/
density of the platform on which the CTD unit is mounted. For
AUVs, buoyancy control comes from a neutral buoyancy
design and through the use of thrusters to determine water
column position. Gliders control their buoyancy by either
adjusting an inflatable bladder or by manipulating materials
that can adjust their density such as thermal waxes and then
use the pitch of their wings to control its ascent/descent in the
water column [11]. The Argos Float changes its density by
pumping fluid out of its body into an external air bladder. As
long as the water displaced/density can be manipulated,
buoyancy control has been achieved.
Fig. 4: Slocum: [11]
Fig. 5: Sub Surface Moored Profiler: [13]
mission as depicted in Fig. 7. After the ECS Unit is deployed
it releases enough compressed air to create poositive buoyancy.
When the unit reaches the ocean surface it eestablishes a GPS
location and makes contact with a T
Tactical Satellite
(TACSAT) (one of 4 in a series of U.S. milittary experimental
reconnaissance and communication satellites)).
The device starts its first dive by openinng the buoyancy
chamber’s venting solenoid valve allowing thhe air chamber to
flood, creating negative buoyancy. While thee unit descends to
a specified depth, readings are obtained and uupon reaching the
specified depth releases compressed CO2 gass into a chamber.
Water is expelled creating positive buoyanncy allowing the
device to ascend to the surface where the nexxt GPS position is
obtained and data is transmitted to the TACSAT. After
mmed sleep period
transmission the unit goes into a preprogram
until it is time for the next dive. Upon w
waking, the unit
establishes another GPS location and repeats all the steps. The
ECS Unit developed in this research is ablee to complete 12
dive cycles to a 10-m depth or 3 dive cycless to its maximum
50-m depth rating.
Fig. 6: ECS unit compressed an
nd expanded for storage
B. Buoyancy System
As stated before, the buoyancy system ffor the ECS Unit
uses compressed CO2 gas to displace waterr in its buoyancy
chamber and opens a solenoid valve to vent thhe CO2 gas in the
buoyancy chamber to flood the chamber w
with water again.
This complete system uses one 16-g CO2 ccanister, one CO2
puncture device, one pressure regulator, twoo solenoid valves,
and a flood-able chamber. The 16-g CO2 caanister’s pressure
when full is nominally around 900 to 10000-psi (63 to 70kg/cm). The 16-g CO2 canister’s pressure possed a problem for
the 5-V miniature solenoid valves with their 1100-psi (7-kg/cm)
rating, which was remedied by a pressure reegulator. Pressure
regulators are used to reduce the presssure acting on
downstream components of a pneumatic circuuit [11].
The pressure regulator was made out of 1-iin (2.5-cm) round
brass stock with a male thread to attach tthe CO2 canister,
shortening the device. From the input side oof the regulator a
0.040-in (1-mm) hole was made to connect tthe reduction side
with an interior diameter of 0.625-in ((16-mm), which
accommodated a 0.5-in (13-mm) seat. Ussing a spring to
supply an additional 8-lbs (3.6-kg) of fforce a pressure
reduction of 80-psi (5.6-kg/cm) was achieeved. While this
regulator worked, it will be replaced by a llow cost, off-theshelf version for higher quality when the EC
CS Unit goes into
production. The designed pressure regulator is a basic single
stage regulator reducing the 1000-psi to 800-psi (70 to 5.6kg/cm) (size limitations prevented a further pressure
reduction). This single stage pressure regulaator design leaves
about a 100-psi (7-kg/cm) remaining in the CO2 canister. A
two stage pressure regulator is recommendedd to optimally use
the compressed air in the ECS Unit.
The buoyancy system allows the ECS U
Unit to be able to
complete 12 dive cycles for a 10-m depth, orr 3 dive cycles for
50-m. The following equations and data connstants from [15]
expand on how these numbers were achieved..
First, take the 16-g of compressed CO2 annd convert it to a
Fig. 7: ECS Sequencce of Events
g) = 0.3636-mole.
molecular weight: 16gx(1-mole/44-g
Next, take that weight and calcu
ulate the volume (L) at the
exhaust ambient pressure and div
vide it by the amount of
volume needed in the buoyancy chaamber to make it positively
buoyant and subtract 1 cycle for a faactor of safety: For a 10-m
dive depth exhaust ambient pressuree will be 2-atm:
V = [(0.3636mole)(0.0822)(293)]/2
2atm = 4.38L.
= 13 − 1 = 12 Dive Capability
C
(7)
For a 50-m dive depth exhaust ambient pressure will be 63)]/6atm = 1.46L.
atm: V = [(0.3636mole)(0.0822)(293
pability
= 4 − 1 = 3 Dive Cap
(8)
E
VII. CTD SENSOR
An inexpensive CTD sensor is required for the ECS Unit to
be a success. Multiple designs were tried before a proof-ofconcept CTD design was obtained. First, a linear 4-pin probe
design was tried where the probes were placed in a line and
spaced equally apart. This type of probe is optimum to get a
stable conductivity reading from the water while avoiding
electroplating the probes. In the four-pin probe design, the
outermost probes alternate in pushing and pulling current
through the water while the two middle probes read the
voltage. The two probes reading the voltage at separate
locations, gives a change in voltage over the distance between
the probes. Combining the change in voltage with the known
current being sent across the outermost probes produces the
resistivity of the water and along with a temperature reading,
salinity can be calculated by using the PSS equation. This
design was abandoned since the readings were not stable.
Next, a linear four-pin probe design with a four concentric
circular pin design was tried (Fig. 8).
This four pin circular pin design proved to be more stable
but the readings still fluctuated and required more calculations
than what is normally used to get the resistivity of the water.
Since there were multiple probe pins, the current flowing
around each pin converted each pin to act like a magnet,
which also changed the cell constant. The cell constant would
have to be normalized for the current flow of an electrical
dipole field [16]. This version was also abandoned and the
next version was based on the usual two pin design.
The two pin design once again improved the readings but
the still proved insufficient. The two pin CTD design was
replaced by completely new CTD design that incorporated an
Atlas conductivity chip. The Atlas is a small embedded CTD
chip that any probe could be hooked up to it as long as it had
the same cell constant it was designed for (i.e., a cell constant
of 10 for solutions that have a high concentration of salt like
oceanic waters). This new probe was designed to have the cell
constant K = 10.
This new design for the CTD probe still proved difficult
since the cell constant K = 10 was not easy to set up by hand.
It needed to be very precise and accurate. First designed with
two cylindrically shaped stainless steel pins set at a distance of
one centimeter apart, the stainless steel quickly corroded as
soon at a current was passed between them. These pins were
upgraded to gold plated nickel pins that are slightly larger, and
placed about two and a half centimeters apart. The gold plated
nickel pins of this CTD probe also corroded, albeit at a much
slower rate. The probe pins were then upgraded to platinum.
As seen in Fig. 9, the pins were changed to the miniature flat
bar shape in the existing probe. Each pin is 2-mm wide and
6.35-mm high, with a spacing of 12.7-mm in between. This
was determined by using the cell constant value of K = 10 and
working the equation backwards, using the known surface area
of the pins, to get the distance between them.
VIII. SYSTEM ELECTRONICS INTEGRATION
The ECS Unit consists of five primary electronic systems:
GPS, CTD, Communications, Data Logger, and Controller.
Each serves a crucial role in the operation of the unit. The
GPS system shows how the unit is drifting with the currents of
the ocean and the location where each dive was performed.
Fig. 8: 4 Pin Concentric Circular Probe
Fig. 9: New Probe for K=10
The CTD system evaluates the water samples, and is the key
system component of device. The communications system
allows the data to be offloaded; but as a backup, a micro SD
card is onboard to store all data in case of communication
failure. Lastly, the controller of the device is a PIC18LF4520
microchip.
A. Global Positioning System
The GPS was first established in 1978 by the United States
Department of Defense. By sending a signal to a minimum of
four satellites, a position can be triangulated on the Earth’s
surface down to about a 10-m radius [17]. GPS operations
depend on a very accurate time reference, given by atomic
clocks from the U.S. Naval Observatory, and each GPS
satellite has atomic clock on board [18]. The ECS Unit uses
the Venus GPS board from Sparkfun Electronics.
This GPS outputs the standard National Marine Electronics
Association (NMEA) sentences with update rates up to 20-Hz.
The GPS requires a regulated 3.3-V supply to operate, and
when powered up uses up to 90-mA. The main NMEA
identifying string of information sent by the Venus GPS is the
GPRMC string. This string of information provides a valid
signal indicator, date, time, latitude, longitude, speed, bearing,
and magnetic variation. The valid signal indicator, date, time,
latitude, and longitude used by the ECS Unit are obtained
from this string using a GPS parse loop. The parse loop looks
for the start of the GPRMC string and saves each character
until it sees the end of the string. With the GPRMC string in
memory, the parsing loop looks for commas that are used to
separate the values of each data point. Using the commas the
parse loop separates the data points that is wanted and discards
those that are not.
B. Conductivity Temperature Depth Sensor
The CTD is the system that handles the water sampling that
the unit was made for and uses the E.C. Circuit from Atlas
Scientific (Fig. 10). Atlas Scientific claims that the chip is
accurate enough for lab work yet rugged enough for a longterm field deployment. Almost a plug-and-play device, it
requires minimal calibration. With included calibration
solutions, the sensor reads out the conductivity, total derived
solids (referenced to KCL), and the salinity.
The Atlas chip outputs these values in a string that is parsed
via a carriage return character at the end of the string. Unlike
most chips outputting strings that end with the standard null
character, the Atlas chip ends its strings with a carriage return.
So a parse loop is used to save the string and look for when
the data ends. The salinity values are derived from the
Practical Salinity Scale. The ECS Unit uses one of the Atlas
chip’s commands to take a temperature calibrated reading.
After sending the ambient temperature value to the Atlas chip
it responds by sending back the temperature calibrated
reading. This process takes just under a second to accomplish
and regulates the rate of sampling for the ECS Unit to 1-Hz as
it descends through the water column.
Fig. 10: Atlas E.C. [19]
Fig. 11: XBee WIFI: [20]
Fig. 12: Iridium [21]
Fig. 13: OpenLog [22]
C. Communications
The communication system of the ECS Unit allows data to
be offloaded. The XBee chip from Digi (Fig. 11) uses a form
of Wireless Fidelity (WIFI) called ZigBee Protocol and has a
data transfer range of one hundred and twenty meters line-ofsight. To retrieve data via the XBee, the receiver must be
located fairly close as the data transfer range is optimistic.
Like the Venus GPS it is powered with 3.3-V and uses 200mA when transmitting data.
The XBee chip when the ECS Unit is deployed will be
replaced by an Iridium Satellite Communications Chip (Fig.
12). This chip is able to communicate with the NAVYs
TACSAT network providing data access from the ECS Unit
anywhere in the world.
D. Data Logging
The data logging system is run by an open source data
logger “OpenLog” (Fig. 13). This logger takes any serial
stream sent to it and logs it.
OpenLog logs continuously, but commands can be sent to it
to initiate other operations, for example: sending the command
”new File” will create a new file named File and then sending
the command, ”write” puts it back into logging mode again, in
doing so, saves data to the new file. Cycling the power creates
a new file named with incrementing numbers. The ease of use
from OpenLog made it an easy choice to use for data logging
operations.
E. Controller
The controller for the system is a PIC18LF4520 chip from
Microchip (Fig. 14) (i.e., an 8-bit microcontroller that uses a
16-bit architecture [23]). The PIC18 family is one of the most
popular chip families for embedded systems that support both
3-V and 5-V applications like the ECS Unit. The LF version
of the chip has a power supply ranging from 2-V to 5.5-V and
draws minimum current when compared to other chip
families. This chip controls when the GPS, WIFI, CTD, and
data logger are switched on and off, parses the GPS and Atlas
Figure 14: Pic chip [23]
Chip output strings, writes/reads data to and from the data
logger, and sends data out through the WIFI chip. This chip
also is able to puts itself into sleep mode between dives and
wakes up when it is time to start a new dive.
Two PIC18LF4520 chip problems arose during testing: 1) a
memory problem (i.e., too much data to store on the onboard
flash memory of each dive before transferring it to external
memory); and 2) it had only one hardware data transfer port
(i.e., the GPS, WIFI, CTD, and Data Logging systems all use
this port and sometimes at the same time). The first issue was
solved by saving the data to the data logger as it was sampled
and the second issue was solved by setting up a secondary
software data transfer port.
F. Software and Application
When the ECS Unit is activated, a start-up sequence runs
followed by the main operation’s loop. The program exits only
when the maximum number of pre-programmed dives is
reached. This sequence of events is depicted visually in the
flow diagram in Fig. 15. Upon activation the ECS Unit
determines whether it is floating at the surface of the water by
sampling the pressure sensor and if so obtains a GPS position.
If the device is below the surface it inflates the buoyancy
chamber and rises to the surface to make the satellite link
where it makes contact with the satellite and obtains a GPS
position, parses the GPS signal and records the date, time, and
location. Once the information is recorded, the ECS Unit starts
the dive sequence. The dive sequence consists of shutting
down the communications chip and GPS chip, starting the
continual background sampling of the pressure sensor, as well
as venting the buoyancy chamber to start the dive. While the
ECS Unit sinks, a time based sampling loop runs until the unit
reaches the pre-programmed depth for its dive.
The time based sampling loop reads the water temperature
each second, which is sent to the Atlas conductivity chip
(returning a temperature compensated reading in the form of
conductivity and salinity), and with the pressure reading the
depth is calculated. With the temperature, depth, conductivity,
and salinity readings obtained, the combined data is sent to the
data logger. Upon reaching the preprogrammed depth the
sampling loop is exited and the end dive sequence begins,
turning off all but the pressure sensor. A set of rapid bursts of
the compressed CO2 is sent into the buoyancy chamber for
positive buoyancy to raise the device to the surface. The ECS
Unit continues to sample the pressure sensor to make sure it is
not sinking, and if it is more CO2 is released into the buoyancy
chamber. When the ECS Unit reaches the surface, the GPS is
turned on. After receiving a valid signal the GPS is turned off
and the communications chip is activated. The ECS Unit then
sends out a data packet consisting of its initial location, dive
data, and resurfaced location, and then the dive-counter is
increased by one. The dive count is checked against the max
number and until that number is reached the unit powers down
into a low-power state and sleeps for a preprogrammed time
until the next dive. If the max number of dives is reached, the
unit opens the buoyancy chamber’s venting valve, powers off,
and sinks beneath the surface of the ocean for the last time. If
the unit is to be retrieved the device remains on the surface
and broadcasts its location every few hours so it can be
located.
Figure 15: ECS unit Program Flow Chart
IX. TESTING AND ANALYSIS
Each sensor subsystem was tested individually during the
design and construction phase, and when integrated into the
ECS Unit each sensor was sequentially bench tested for
functionality. The GPS, Communications, and Buoyancy
systems all tested satisfactorily. The final ECS Unit
prototype’s CTD system was tested against YSI’s YSI-85 (a
hand-held instrument that measures oxygen, conductivity,
salinity, and temperature), and IDRONAUT’s Ocean Seven
320 Plus (a standard CTD used in ship casting operations).
The IDRONAUT Ocean Seven 320 Plus is a full scale CTD
that can be mounted in a rosette wheel.
The ECS Unit’s CTD testing consisted of three phases: 1) a
basic bench test of all the subsystems; 2) a characterization
bench test for the CTD system; and 3) a controlled field test of
the CTD system. Upon completion the readings of the ECS
Unit were compared to the readings of the YSI-85 and
IDRONAUT-CTD, and then the functionality of the ECS Unit
to the XCTD was compared.
The bench test of the ECS Unit consisted of initial testing of
the CTD system along with characterization of the CTD
system readings. The CTD system was calibrated and verified
with a saltwater solution against the YSI-85, using the Atlas
Scientifics Conductivity Circuit Data sheet [19] and the
calibration solution provided by Atlas Scientific.
A final check was done to test of the calibration of the
Fig. 16: Bench Test Set Up
salinity reading of the ECS Unit by verifying it against the
YSI-85. It was seen that for a salinity solution of 26 Parts Per
Thousand (ppt), the ECS Unit consistently read 3-ppt higher
than that of the YSI-85, which is a key note for the testing and
analysis section.
A. Characterization Bench Test
After calibration the ECS Unit readings were characterized
(i.e., the readings were tested in varying salinity and
temperatures to see how or if their values were affected by
changing those variables). This test consisted of testing water
with three different salinity solutions at the same temperature,
then testing one of the salt water solutions at different
temperatures. These solutions were set in separate baths that
could fit all three instruments at once. The setup for this test is
shown in Fig. 16.
Ten gallons of saltwater and five gallons of reversed
osmosis filtered water were obtained from the local aquarium
store and tested by a salinometer to have a salinity of 26-ppt.
This was then divided into three separate solutions one of
which the salinity was changed by diluting the solution with
the reversed osmosis water to a salinity of 18-ppt. Then the
reversed osmosis filtered water and the three saltwater
solutions were stored in an air conditioned room to bring their
temperature down to simulate normal ocean temperatures.
The first test to characterize the device was to measure the
salinity of different saltwater solutions at the same
temperature (22 oC). Freshwater was the first solution
measured by all three instruments at the same time. After
waiting a few minutes to allow the readings to stabilize, their
readings were recorded. Each had an average reading of 0-ppt
as would be expected for fresh water. This procedure was
repeated for the 18-ppt and the 26-ppt solutions (Table 1).
Table 1: Characterization Test 1
Steady Temperature (20oC) − Variable Salinity
Solution
IDRONAUT CTD
YSI-85
RO (0-ppt)
0.042
0.1
18-ppt
18.128
18.5
26-ppt
26.174
26.7
ECS Unit
3
21
29
For the reverse osmosis water the instrument readings were
IDRONAUT-CTD: 0.042-ppt, YSI-85: 0.1-ppt, and the ECS
Unit: 3.0-ppt; For the 18-ppt saltwater solution the
IDRONAUT-CTD: 18.128-ppt, YSI-85: 18.5-ppt, and the
ECS Unit: 21-ppt; For the 26-ppt saltwater solution the
IDRONAUT-CTD: 26.174-ppt, YSI-85: 26.7-ppt, and the
ECS Unit: 29-ppt. (Note: the ECS Unit consistently read 3-ppt
higher than the other CTDs and is used to calculate the unity
of the CTD sensor.)
The second part of the characterization test was to measure
the salinity of the saltwater solution (26-ppt) at different
temperatures. Simultaneously, all three instruments measured
the solution at 19.5 oC. After waiting for a few minutes to
allow readings to stabilize, the following were obtained: the
YSI-85 and the IDRONAUT-CTD had an average reading of
26-ppt and the ECS Unit read 29-ppt. This procedure was
repeated for 22.1 oC, 28.2 oC and again for or the 26-ppt
solution. The salinity readings changed minimally, only the
temperature changed (Table 2).
Table 2: Characterization Test 2A
Steady Salinity (26-ppt) − Variable Temperature
Solution
IDRONAUT-CTD
YSI-85
19.5 oC
26.173
26.6
22.1 oC
26.175
26.8
28.2 oC
26.176
26.7
ECS Unit
29
29
29
The remaining 26-ppt saltwater solution that was kept in an
air conditioned room was used to show the dynamics of the
water while raising the temperature from 19 oC to 28 oC (e.g., to
see how the readings would change when passing through a
thermocline). In this test the ECS Unit was compared to the
IDRONAUT-CTD only, since the YSI-85 could not record
data automatically. The salinity readings changed minimally,
only the temperature changed significantly (Fig. 17).
B. Controlled Field Test
After successfully passing the characterization testing, the
ECS Unit underwent a controlled field test in the brackish
water of the Indian River Lagoon, at the mouth of Crane
Creek in Melbourne Florida.
The ECS Unit was tested against the IDRONAUT-CTD
where three separate casts were performed off the dock. Each
cast was compared individually and as an average between the
two instruments. The two CTDs were lowered at the same
time and rate to the bottom of the marina where they remained
for enough time to obtain accurate readings (i.e., few seconds).
These casts were performed to verify how the ECS Unit
operates (Fig. 18-20 field casts). After each cast the logged
data was saved to separate files for each instrument and the
process repeated until all three casts were completed and their
information gathered and logged.
X. ANALYSIS OF TEST DATA
The test data gathered from all four tests performed on the
ECS Unit was compared to the data collected from the
IDRONAUT as well as the YSI-85. It should be noted that the
initial calibration of the commercial CTDs might have drifted
over time and induced an error in the readings, but the focus of
these tests was to look at the trend of the data. Any percent
error in readings is easily fixable, but an error in the data trend
is a fundamental issue.
During the initial bench test it was noted that the ECS Unit
had a consistent reading of 3-ppt higher than the YSI-85, this
was true of all the tests. The initial bench test and the first
characterization tests were not graphed due to the data being
recorded by hand and therefore not being continuously logged.
This was also the case with the first half of the second
characterization tests as well. However, the second half of the
last characterization testing was recorded continuously;
therefore this is where the graphical comparison begins for the
ECS Unit. The data comparison was made between the
instruments by observing and comparing their outputs through
the graphical representation of their data.
Looking at the graph in Fig 21, the trend between the ECS
Unit and the IDRONAUT-CTD is extremely similar (when
not normalized the consistent offset can be seen and easily
corrected). When the trends in Fig. 21 are overlaid onto each
other, there is no discernible difference between the two
outputs. The field test data shows that, between the
IDRONAUT-CTD and the ECS Unit, the output readings still
have a similar trend with the 3-ppt offset. The three separate
casts performed during the controlled field test were averaged
(to normalize the readings) and graphed. As with the
characterization test, the normalized data from the field test
showed that the trend between the two instruments was
practically identical, with the only difference being the 3-ppt
offset (Fig. 22).
When looking at the graph of the normalized field test data,
the temperature and conductivity appear to have a stair like
trend, this is due to the field test data being split into three
sections. The first section covered the lowering of the
Fig. 17: Characterization Test 2B
Fig. 18: Field Test Cast 1
Fig. 19: Field Test Cast 2
Fig. 20: Field Test Cast 3
instruments through the water column, second section
was the Characterization Test 2B
Fig. 21: Normalized
instruments through the water column, second section was the
bottom time of the instruments, and the third section was the
retrieval of the instruments. The data files were separated into
those three sections to keep the difference in run times, for
each field cast, from skewing the normalized data.
XI. INSTRUMENT COMPARISON
Comparing the ECS Unit sensor to the XCTD/AXCTD
sensors for cost, life expectancy, depth rating, and utilization
the following was obtained. Keeping the cost as low as
possible for the ECS Unit, and the total price of parts
(including the iridium satellite chip) was $982. While the price
per unit for the XCTD/AXCTD respectively is $550 and
$1,680.
The life expectancy of the ECS Unit is determined by the
CO2 volume and battery life. These two factors are affected by
the dive frequency, dive depth, and the duration of hibernation
between cycles. For 10-m dive depths, with no hibernation
periods, using the maximum number of dives places its life
expectancy within 36 hours. However if hibernation is
included in the operation of the ECS Unit, with the maximum
number of dives for a 10-m depth the operational range
increases to 30 days. When the ECS Unit is programmed for
50-m dive depths with no hibernation periods, its life
expectancy (for the maximum number of 3 dives) is
approximately 18 hours. However, if hibernation is included,
life expectancy is increased to 2 weeks. These numbers are for
maximum and minimum operating parameters, variations can
be made to alter its life span by mixing dive depths and
hibernation periods. While the life expectancy for the ECS
Unit is variable, the XCTD/AXCTD is only used for one dive.
To record the data from the ECS Unit a WIFI chip and
inexpensive dongle (a $45 USB plug-in antenna to receive the
signal) can be used to record the data on any laptop or
desktop; using an Iridium satellite communications chip, the
ECS Unit can send data to any server in the world for the price
of Iridium’s standard messaging rates. The XCTD instrument
system requires the data to be transferred through its launching
apparatus into its specialized data acquisition system. The cost
of the launching apparatus is $1,256 and the cost for its
specialized data acquisition system is $8,638, for a total of
$9,894 to retrieve the data. The AXCTD requires its data to be
sent wirelessly to a receiver/data acquisition system which
costs $37,131. The ECS Unit, even with the shortest
deployment configuration, is still more economical than using
multiple units of the XCTD/AXCTD along with the man hours
and ship time to perform the same mission (Table 3).
Table 3: Instrument Comparison
ECS Unit
Cost per Unit
$982
Life
3-12 Dives (within a period
Expectancy
of 30 days)
Depth Rating
50m
Deployment
Single or Multi-use
DAQ Cost
XCTD
$550
1 Dive
AXCTD
$1,680
1 Dive
1000m
Single
Use
$9,894
1000m
Single
Use
$37,131
$45 - WIFI (or) Std Msg
Rates - Iridium
Currently there are only two downsides of ECS Unit when
compared to the XCTD/AXCTD: 1) the ECS Unit has a max
depth of only 50-m, whereas the XCTD/AXCTD can go down
to 1,000-m, and 2) the ECS Unit only has an accuracy of 1-ppt
for its salinity readings, whereas the XCTD/AXCTD is 0.01ppt. With some additional modifications the ECS Unit can be
updated to an accuracy of 0.1-ppt.
XII. CONCLUSION
The ECS Unit was constructed to be a multi-use,
expendable instrument for less than $982. The device was
tested against IDRONAUT’s Ocean Seven 320 Plus CTD and
YSI’s YSI-85 hand-held conductivity meter to prove its
functionality. This research covered the design, development,
and testing of the ECS Unit but more specifically its
conductivity sensor. Between all the system prototypes, the
ECS Unit meets all 5 specifications for this instrument: 1) Its
weight had to be under 5-lbs (2.27-kg) maximum - the ECS
Unit currently weighs 3.8-lbs (1.7-kg); 2) Its dimensions must
be no bigger than 10-in (25-cm) in height and 3-in (7.6-cm) in
diameter - Its dimensions are 3-in (7.6-cm) in diameter and
10-in (25-cm) in height; 3) It must be reasonably priced to be
expendable - It cost $982 to build; 4) It must have wireless
communication - It currently uses WIFI and can easily be
switched to satellite communications; and 5) It only has to
record the top 10-m of the water column - It can record up to
the top 50-m of the water column. Plus the ECS Unit is
designed not only to have a maximum dive depth of 50-m, but
has the option to be expendable or be retrieved and deployed
again if needed. The ECS Unit with its multi-dive capability,
low cost, and multi-use option makes it one of the most viable
expendable CTD instruments of today.
Fig. 22: Normalized Field Test Casting
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