ALEC TECHNICAL EXPRESS
Sept. 2004, No. 3
CT logger with new wiper function and new type sensor
Alec Electronics is pleased to introduce great new model to the COMPACT series of
instruments. The new, COMPACT-CTW is an innovative instrument that allows stable
measurements of water temperature and salinity without maintenance for long-term
deployments.
COMPACT-CTW
CT Sensor with
anti-fouling wiper
Development of the new CTW sensor
The original Alec Electronics COMPACT-CT sensor combines an inductive conductivity cell
and a temperature sensor in a small, autonomous package. This rugged and compact design
has proven to be an excellent solution for deployments up to one week. Various attempts were
made to increase the deployment time of the COMPACT-CT which included covering the
sensor with an anti-fouling net; housing the sensor in an anti-fouling chamber that was
continually flushed with a pump; using a costly mooring system which employed a winch,
where the sensor was kept in air during stand-by and lowered into the water only for the
duration of the measurement. None of those attempts achieved reliable long-term fouling
protection. Eventually, Alec Electronics developed the seven-electrode conductivity sensor
with a piston type wiper.
Alec Technical Express 1
Design of new CTW Sensor
The conductivity cell of the new Compact
CTW is a seven-electrode cell with a
plunger that periodically sweeps the
measurement cavity, as shown in the
schematic illustration on the right. A pair of
electrodes of equal polarity (–) are located
at the ends of the cavity and they share a
common electrode (+) in the centre of the
cell. The opposing electrodes drive a
current that is completely contained in the
inner cavity of the sensor. In this
arrangement the measurements are not
affected by any growth on the outside
walls of the sensor. A pair of pick-up
electrodes measures the potentials
created by the drive electrodes (which are
in the fluid contained in the cavity) and this
potential is a function of the fluid’s
conductivity.
Compact-CTW Sensor Schematic
Piston type wiper
A piston wiper with a circular silicone blade
gently brushes the inside of the cell at
regular intervals (see pictures at right) to
keep the electrodes and the inside cavity
free of growth. In addition, the motion of
the wiper helps to flush the cell and suck
surrounding water into the cell, so that the
measurement accurately reflects the
conditions of the water surrounding the
sensor.
Wiper retracted
Data logger
The COMPACT-CTW has been developed
based upon the fundamental design
criteria from our compact and lightweight
COMPACT series of instruments. The
unit’s dimensions are 60 mm in diameter
and 416.4 mm in length. It weighs 1440 g
in air and 510 g in water.
The unit is fully autonomous and a cabled
version is also available for integration into
telemetry observation systems.
Wiper fully extended
Alec Technical Express 2
Field Tests and Results
The Compact CTW was tested in Nomi Bay off Japan’s southern island of Shikoku between
February 6 and March 7, 2004. The total deployment time was 728 hours (30 days). The
Compact CTW was moored on a raft next to one original Compact CT sensor at a depth of one
meter. Each instrument recorded a data sample of conductivity and temperature every 10
minutes. The wiper mechanism on the CTW was activated once before each sample.
Intense bio-fouling on both instruments was evident after their retrieval. The growth covered all
instrument parts and sensing surfaces, except the cavity of the CTW instrument. Over the
entire period of the deployment, the temperature data of both systems matched each other
perfectly (within the instruments’ accuracy). The salinity data of the CTW instrument (red trace)
shows a nearly constant value of 34.9 PSU, which is typical for the Nomi Bay site. The
occasional excursions towards lower salinities are due to rain events. The salinity data of the
CT instrument (blue trace) departs significantly from the CTW measurement after 170 hours (7
days). After 240 hours of deployment, the CT’s salinity values show erratic excursions.
These excursions cannot be correlated with any observed or known phenomena at the site, so
the excursions must be an artifact of the bio-fouling. After 700 hours of deployment, the
average difference between the salinities reported by the CTW and CT instruments exceeded
4 PSU, which is approximately 12% of the expected salinity value.
Before deployment
Temperature and salinity data from the deployment in Nomi
Bay, February 6 to March 7, 2004.
CTW after retrieval
Summary
Bio-fouling is the bane of long-term measurements in the ocean. The new Compact
CTW conductivity and temperature data logger provides an effective way of measuring
conductivity and temperature during extended deployments.
Alec Technical Express 3
General Specifications
Sensor Specifications
Parameter
Sensor Type
Measuring
Range
Resolution
Accuracy
Time Constant
（Average）
Temperature
Thermister
-5～+45℃
0.001℃
±0.05℃
1 sec.
Conductivity
Sevenelectrode
0～60mS/cm
0.003mS/cm 0.05mS/cm
1 sec.
Data Logger Specifications
Memory Capacity
178439 data
Measuring Mode
Continuous mode, Burst mode
Measuring Interval
Burst Interval
Sampling Number
Battery/Capacity
Current Consumption
Material
Dimension
2, 5, 10, 15, 20, 30 sec.
１～255 Min.
1, 10, 15, 20, 30, 60, 120, 180, 240, 300, 600, 1200
D cell type Lithium Battery, 14 AH
52 mA during measurement, 110 mA during wiper in operation
Titanium
60 mm dia. (Flange ø69 mm), length 416.4 mm
See drawing below
Weight
Depth Rating
1440 g in air, 510 g in water
100 m
Drawing
For more information, please contact:
Rockland Oceanographic Services Inc.
Oceanographic Instruments and Consulting
ALEC ELECTRONICS CO.,LTD.
7-2-3 Ibukidai-Higashimachi, Nishi-ku, Kobe,
Japan 651-2242
TEL +81 78-997-8686 FAX +81 78-997-8609
Web site www.alec-electronics.co.jp
E-mail [email protected]
Toll Free: (877) 370-1688 Fax: (250) 370-0234
Email: [email protected]
Web: www.rocklandocean.com
www.alecvictoria.com
Alec Technical Express 4