Appendix A - Dissolved Oxygen Sensor - idronaut Ocean Seven 304Plus Operator's Manual

APPENDIX A – DISSOLVED OXYGEN SENSOR
A.1 INTRODUCTION
The OCEAN SEVEN 304Plus CTD interfaces an IDRONAUT Dissolved Oxygen sensor through
dedicated interface and software. This appendix contains all the information needed to operate the
CTD in the laboratory and in the field with the Dissolved Oxygen sensor. It should be mentioned that,
due to the lack of continuous polarization of the Dissolved Oxygen sensor, the CTD, depending on the
measurement and sensor conditions, needs a 30s warm-up before the oxygen measurements are stable
and accurate. Therefore, in case of profiling, we suggest waiting on surface for the stabilization time
before lowering the CTD into water. Instead, in case of acquisitions in function of time, a dedicated
configuration parameter allows the operator to set up a delay time "warm-up time-out" to be used at
the start-up before starting the measurements. In this case, it is necessary to consider the time spent
during the warm-up time-out in the computation of the CTD battery autonomy.
A.2 DISSOLVED OXYGEN SENSOR (standard version)
This section refers to the standard versions (150 and 700 bar) only.
The oxygen sensor is of the polarographic type and consists of two half cells, the anode and the
cathode. The anode is a silver tube inside the sensor which encircles a glass body where a platinum
wire, forming the cathode, is sealed. The platinum wire (cathode) ends at the tip of the sensor where
the glass body is rounded. A special membrane cap with a gas-permeable replaceable membrane
screws onto the sensor. The inside of the cap is filled with a special electrolyte which allows the
current (measuring) to flow between the anode and the cathode. The membrane is shielded from
accidental bumps by a protective ring. The anode acts as a reference cell, providing a constant
potential with respect to the cathode. The cathode, where oxygen is consumed or reduced, is
separated from the sample to be analyzed by a thin layer of electrolyte and a special composite
membrane. The electrolyte permits the chemical reaction to occur whereas the membrane constitutes a
barrier against ions and other substances. By applying a polarizing voltage to the half-cells, the sensor
develops a current proportional to the concentration of oxygen in the sample in front of the cathode.
Oxygen from the sample is drawn across the membrane, at the sensor tip, in the area of the cathode.
The applied polarization voltage is such that the sensor only responds to oxygen. The sensor is
insensitive to nitrogen, nitrous oxide, carbon dioxide and other gases. In order to avoid stray ground
current leaks, in case of membrane leaks, the anode is kept at ground potential while the cathode is
polarized at a fixed negative voltage. The oxygen sensor limits stirring effects on the measurement
and reads at least 97% of the true value, even with a stagnant aqueous sample. This is because the
very small cathode area and special cathode geometry, associated with a unique composite
membrane, minimize the consumption of the oxygen contained in the sample in contact with the
membrane. The function of this sensor depends on the reduction of oxygen at the cathode, as
expressed by the formula:
The developed electrons represent the measuring current and are supplied by
the silver/silver chloride anode.
Standard version 150 bar
Type:
Measurement range:
Accuracy:
Resolution:
Polarization voltage:
Response time:
Max Pressure:
IDRONAUT – Brugherio (MB)
O + 2 H O + 4e - >>> 4 OH
2 2
polarographic
Ag(99.99%) anode
0... 50 ppm 0... 500% sat.
0.1 ppm 1 % sat.
0.01 ppm 0.1% sat.
650 mV DC
3s @20°C
150 bar
OCEAN SEVEN 304Plus CTD
-
with Pt/Ir cathode and
12-2015
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