Alec Electronics Co., Ltd. has developed a new optical DO sensor with fast response and
high accuracy. Here, we introduce the instrument in an overview.
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Development In the field measurements of DO, the galvanic electrode sensors have usually been used for several decades. Recently, various in-situ optical DO sensors were developed and have been commercialized. But both type sensors have late response time, such as 20~50 seconds. Moreover the sensors are sometimes unstable in the field measurements. Therefore, many users waited for someone to develop a new-technology sensor with fast response time, high stability and good accuracy. We have collaborated with Nara Women's University since March 2007 to develop the reactive chemical material with suitable characteristics for DO sensor, based on existing PSP (Pressure Sensitive Paint) technology. Although the development is extremely difficult because of our ultimate target to make sensor to be fast response (1sec), seawater resistance, high stability, good accuracy, low price and so on, we have reached our final goal and now are making our effort toward the commercial products. The prototype sensor was named "*RINKO". We already did some tests in deep-ocean with depth of 2000m to 6000m and obtained many valuable data. The tests show RINKO's great potential application on relative research. (*Rinko means phosphorescence in Japanese.) |
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Principle
and Futures We applied phosphorescent material to the sensing foil of RINKO. When a pulse-excitation light is emitted from the inside to its material, it radiates the red colored phosphorescence. Its intensity varies in a negative correlation with oxygen partial pressure [Fig.3]. Thus, the phosphorescent intensity is the highest at anoxic environment. Moreover, the lifetime of the phosphorescence is varied, as well as its intensity. During the reaction, the oxygen molecules are not consumed. RINKO measures the phosphorescent lifetime by a phase differential detection to estimate DO [Fig.4]. This optical method provides two advantages; one is very stable without the influence of bio-fouling, the other, the sensor does not need water mixing like the galvanic electrode sensors. The optical DO sensor presently being used over the world has a similar principle as RINKO. But their sensing foils are too weak to the ambient light from the outside, so they must need a layer, like black silicones, to shade it. It is known well that the sensor's response time gets extremely delayed by this layer. According to our new technology, RINKO's sensing foil does not need a layer to shade it. This technology is greatly improved by our technology, which makes us successful to develop a new type DO sensor with high response and long-term stability [Fig.5]. We are planning to provide 4 models for RINKO instrument based on its special characteristic. To coincide with the release of RINKO, we will discontinue manufacturing COMPACT-OPTODE and COMPACT-DOW. |
Phosphorescence is a photo-luminescence including fluorescence. Fig.6 shows the energy
diagram and the interaction between PSP and the oxygen molecules. The luminescence molecules
absorb lights and are excited from its ground state S0 to excited state S1. According to the spin
angular momentum (spin) of their excited molecules, their excited states are called as singlet or
triplet state. Both excited states are relaxed to the ground state via non-radiative transition and
radiative transition, where some of energies are lost as heats and lights. If the radiative transition
occurs between the singlet excited state and the ground state (S1 to S0), the emission light (photon)
is called as fluorescenscence. If the radiative transition occurs between the triplet state and the
ground state (From T1 to S0), this process is called as phosphorescence. Phosphorescence has a
feature where its lifetime is longer than that of fluorescence.
RINKO's PSP emits the luminescence. But PSP is categorized as a phosphorescence material,
because the contribution of the fluorescence to PSP's luminescence is very little. The intensity and
the lifetime of its phosphorescence vary in a negative correlation with oxygen partial pressure,
because the PSP's molecules, at the triplet state, lose some of the excitation energies by the
transmitted oxygen molecules into the PSP layer. A relationship between the phosphorescence and
the partial pressure of oxygen defines as Stern-Volmer equation. The measurement of DO by
RINKO applies this equation.
RINKO is equipped
with Alec's data logger which is used in current COMPACT series or
next-generation INFINITY series. The sonde is made of Titanium alloy which
is completely free from biofouling. We also have a plan to adopt plastic
material only for the shallow water application in the near future.
-Specifications
Model : Infinity-RINKO (ARO-USB)
provisional values
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Parameters |
Temperature |
Thermistor |
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Saturation |
Rinko
(fluorescence quenching) |
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Measuring
Range |
Dissolved
Oxygen |
0~20[mg/l]
*1 |
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Saturation |
0~200[%] *2 |
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Temperature |
-5~45[°C] |
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2.5~3.6[V] |
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Resolution
*3 |
Dissolved
Oxygen |
0.002~0.008[mg/l] |
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Saturation |
0.01~0.04[%] |
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Temperature |
0.001[°C] |
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Accuracy |
Saturation |
±1%[FS] (1 atmosphere, 25°C) |
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Temperature |
±0.02[°C] |
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reproducibility |
Saturation |
Drift |
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Response speed *5 |
Saturation |
within
0.4[sec] (63%) |
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Communication
format |
USB
(Ver2.0-compliant, Ver1.1 compatible) |
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Data
transfer method/ |
Trough
PC : abt 50KByte/s |
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Memory
type |
Mini-SD
card (16 MB ~ 2 GB) |
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Creatable
file numbers |
Format
FTA16 : abt 100 ( depending length, max :128) |
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A/D
Converter |
16
bits digital conversion |
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Recording
Mode |
Continuous,
Burst |
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Measuring
Interval |
0.1~600[s] |
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Burst
time |
1~1440[min]
(to be set per minute) |
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Sampling
Numbers |
1~6000 |
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Warming-up
Time |
5
[s} |
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Power
Source/ Capacity *6 |
CR-V3
lithium battery (3V) 1~2 pieces / 3.3Ah, 6.6Ah |
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Current
Consumption |
At measuring :110[mA] |
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Main
Material |
Housing
: Titanium (Ti-6AI-4V) |
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Dimensions
*7 |
Outer
Length : 235.5 mm Mooring
hole diameter: 8.5 mm |
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Weight
(incl. 2 batteries) |
In
water 570g In air 900
g |
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Pressure
Resistance |
7000
m depth equivalent |
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Others |
With
buzzer function (1~1440[min] adjustable) |
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is affected by temperature and water pressure, so these parameters should
be measured at the same
time for converting into saturation value. Temperature is measured by
the instrument but water
pressure is required to measure and input by user. The calculation is
made automatically by the software.
*3 Resolution goes down as oxygen pressure grows up.
*4 Accurate temperature limits in the region 3-31°C.
The accuracy of saturation shows linearity error.
*5 Standard responding speed in gaseous atmosphere.
*6 Never use used battery together with brand new one.
*7 Outer length includes sensor guard and mooring hook.
*8 In comparison with Galvanic electrochemical DO, there is extremely
smaller drift but calibration
function is equipped for users' calibration.
RINKO-II is a real-time
model with digital sensor (DO, Temperature, Depth), 50m cable and a
portable display.
- Application : Water quality measurements in surface water, automatic
observation systems.
This is a model to
be integrated in CTD-RMS and operated by DC12V. Both signals of DO and
temperature are output by analog voltages (0~5V). There is no difference
in DO profiles of upward
cast and downward cast in the normal winch speed, because RINKO has fast
response time within 1
sec.
We plan to supply
the OEM model of only the optical sensor part (without the sonde). We
will
design the sensor part depending on the requirements from various customers,
including the
manufactures of industry, medicine and institutions.
