Sea-Bird Electronics SBE 19plusV2 User Manual page 225

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Temperature differences between the water and oxygen sensor can lead to errors in the oxygen measurement.
The SBE 43 minimizes this difference by using materials that equilibrate rapidly with the environment and
incorporating a thermistor placed under the membrane, at the cathode, for accurate temperature compensation.
As a result, the SBE 43 is less susceptible to error when profiling through areas of high temperature gradients than
previous oxygen sensors.
Use in Moored Applications
As discussed above, the oxygen sensor consumes the oxygen in the water near the sensor membrane. In moored
applications, this requires that water be pumped past the oxygen sensor. When used with a SEACAT (SBE 16, 16plus,
16plus-IM, 16plus V2, 16plus-IM V2, or 19plus or 19plus V2 in moored mode), the SBE 43 flow chamber
(plenum) is connected in-line between the pump and conductivity sensor. The pump does not run between samples,
trapping water in the plenum. Because the sensor is continuously polarized by an internal battery, oxygen continues
to be consumed between samples. The sensor depletes oxygen in the water close to the membrane. If you were to
observe the sensor output after the pump stopped, the oxygen concentration inside the plenum would approach a
steady state well below ambient oxygen levels. When the pump switches on at the beginning of the next sampling
interval, you would observe a curve similar to those shown below for a 0.5-mil membrane. The water flow
establishes a normal boundary layer at the membrane, and the sensor equilibrates to the ambient oxygen level. The
time required to reach 99% of the final equilibrium value depends on temperature (faster equilibration in warmer
water) and on the sensor membrane thickness (faster equilibration with a thinner membrane).
Vertical arrows on the plot show the point at which the sensor has achieved 99% of the final value at each
temperature.
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Prior to 2007, all SBE 43s were sold with a 0.5-mil thick membrane. Beginning in 2007, Sea-Bird began
offering two membrane thicknesses – 0.5 mil (faster response, typically for profiling applications) and
1.0 mil (slower response but more durable, typically for moored applications).
99% of final value
0 10
Time in Seconds
Response Time at Several Temperatures for 0.5-mil Membrane
7 seconds at 29.2 °C
20 30
2
28 seconds at 1.7 °C
1.7 °C
10.2 °C
19.7 °C
29.2 °C
T = 1.7
T = 10.2
T = temperature (°C)
T = 19.7
T = 29.2
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