Ocean Optics OOISensors - Manual (Page 50)

3: Oxygen Sensors
The Stern-Volmer constant (k) is primarily dependent on the chemical composition of the ruthenium
complex. Ocean Optics probes have shown excellent stability over time, and this value should be largely
independent of the other parts of the measurement system. However, the Stern-Volmer constant (k) does
vary among probes, and it is temperature dependent. All measurements should be made at the same
temperature as the calibration experiments. If this is not possible, temperature-monitoring devices should
be used.
Compensating for Temperature with the Linear (Stern-Volmer) Algorithm
If you decide to compensate for temperature, the relationship between the Stern-Volmer values and
temperature are defined as:
I
0
= a
0
+ b
0
* T + c
0
* T 2
k = a + b * T + c * T 2
The intensity of fluorescence at zero pressure of oxygen (I
0
) depends on various factors in the optical
setup: the power of the LED, the quality and type of optical fibers, light loss at the probe due to fiber
coupling, and backscattering from the sample. To account for these inconsistencies, you must measure the
intensity of fluorescence at zero pressure of oxygen (I
0
) for each experimental setup.
It is evident from the equation that the sensor will be most sensitive to low levels of oxygen. Deviations
from the Stern-Volmer relationship occur primarily at higher oxygen concentration levels. To correct
these deviations, use the Second Order Polynomial algorithm.
Second Order Polynomial Algorithm
The Second Order Polynomial algorithm requires at least three standards of known oxygen concentration.
The first standard must have 0% oxygen concentration and the last standard must have a concentration in
the high end of the concentration range in which you will be working. The Second Order Polynomial
algorithm is considered to provide more accurate data because it requires at least three known
concentration standards while the Linear (Stern-Volmer) algorithm requires a minimum of two known
concentration standards. The Second Order Polynomial algorithm is defined as:
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