APPLICATION NOTE NO. 64 Revised November 2008
SBE 43 Dissolved Oxygen Sensor – Background Information,
Deployment Recommendations, and Cleaning and Storage
General Description
The SBE 43 is a polarographic membrane oxygen sensor having a single output signal of 0 to +5 volts, which is
proportional to the temperature-compensated current flow occurring when oxygen is reacted inside the membrane.
A Sea-Bird CTD that is equipped with an SBE 43 oxygen sensor records this voltage for later conversion to oxygen
concentration using a modified version of the algorithm by Owens and Millard (1985).
The SBE 43 determines dissolved oxygen concentration by counting the number of oxygen molecules per second
(flux) that diffuse through the membrane from the ocean environment to the working electrode. At the working
electrode (cathode), oxygen gas molecules are converted to hydroxyl ions (OH-) in a series of reaction steps where
the electrode supplies four electrons per molecule to complete the reaction. The sensor counts oxygen molecules by
measuring the electrons per second (amperes) delivered to the reaction. At the other electrode (anode), silver
chloride is formed and silver ions (Ag+) are dissolved into solution. Consequently, the chemistry of the sensor
electrolyte changes continuously as oxygen is measured, resulting in a slow but continuous loss of sensitivity that
produces a continual, predictable drift in the sensor calibration with time. This electro-chemical drift is accelerated
at high oxygen concentrations and falls to zero when no oxygen is being consumed. Accordingly, sensor storage
and deployment strategies that produce zero- or near zero-oxygen environments when the sensor is not being
sampled can be used to substantially reduce electro-chemical drift, improving long-term data quality.
Membrane fouling also contributes to drift by altering the oxygen diffusion rate through the membrane, thus
reducing sensitivity. Non-biological fouling, occurring for example if the SBE 43 was profiled through an oil slick,
typically produces an immediate jump toward low oxygen. Biological fouling, particularly on moorings, can be
troublesome, because the living organisms either consume or create oxygen. Without protection and/or routine
cleaning, a micro-environment around the sensor can produce oxygen levels that are different from the true ambient
conditions. By recognizing fouling, both episodic and gradual in nature, and promptly cleaning the sensor using the
procedures in this application note, accuracy can be restored.
In 2007, Sea-Bird began shipping SBE 43s with a black plastic plenum in place of the original white plastic
plenum, for SBE 43s intended for mooring applications. When our inventory of white plastic plenums is used up,
we will begin building all SBE 43s with black plastic plenums. In 2007, Sea-Bird also began shipping SBE 43s
intended for mooring applications with black Tygon tubing in place of clear Tygon tubing (SBE 43s intended for
profiling applications will continue to be plumbed with clear tubing). The black plenum and tubing minimize light
entering the system, and reduce biological fouling.
The concentration of oxygen in the environment can be computed given the flux of oxygen and the geometry of the
diffusion path. The permeability of the membrane to oxygen is a function of temperature and ambient pressure and
is taken into account in the calibration equation. The algorithm to compute oxygen concentration requires
measurements of water temperature, salinity, pressure, and oxygen sensor output voltage. When the oxygen
sensor is interfaced with a Sea-Bird CTD, all of these parameters are measured by the CTD system.
The oxygen sensor consumes the oxygen in the water very near the surface of the sensor membrane. If there is not an
adequate flow of new water past the membrane, the sensor will give a reading that is lower than the true oxygen
concentration. Additionally, if the flow rate is not constant, the sensor response time will vary, causing dynamic error,
particularly when profiling. Maximum accuracy requires that water be pumped (across the membrane) at rates from
20 to 40 ml/second, as provided on Sea-Bird CTDs with SBE 5T or 5P pumps.
Sea-Bird Electronics, Inc.
1808 136th Place NE
Bellevue, WA 98005
US
Phone: (425) 643-9866
Fax: (425) 643-9954
E-mail: seabird@seabird.com
Web: www.seabird.com
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