Notes:
1. There are several types of oxygen data that can be calculated, as desired, in all of these software versions:
• Oxygen, SBE (units of ml/l, mg/l, or micromoles/kg, as selected) – measured SBE 43 oxygen, based on the
equation shown above in Oxygen Algorithm.
• Oxygen saturation (units of ml/l or mg/l, as selected) – theoretical saturation limit of the water at the local
temperature and salinity value, but with local pressure reset to zero (1 atmosphere). This calculation
represents what the local parcel of water could have absorbed from the atmosphere when it was last at the
surface (p=0) but at the same (T,S) value. See Appendix A for computation of oxygen saturation.
• Oxygen, SBE, percent saturation – ratio of measured SBE 43 oxygen to oxygen saturation, in percent.
2. When entering calibration coefficients for the SBE 43 in the configuration file for the CTD, you can select the
older Owens-Millard equation or the recommended Sea-Bird equation (documented in this application note).
Data Conversions
Sea-Bird uses the following equations to convert oxygen to various engineering units:
[mg/L] = [ml/L] * 1.42903
[micromole/Kg] = [ml/L] * 44660 / (sigma_theta(P,T,S) + 1000)
For the micromole/Kg conversion, there is disagreement in the scientific community about the conversion constant
44660 should be used:
• The value 44660 is exact for oxygen gas.
• The value 44615 is the average value for atmospheric gas (N2,O2,Ar,H2O,CO2,...). It is not exact for any
individual gas, but has been used historically by oceanographers.
The argument distills to exact versus historic, with oceanographers split; Sea-Bird uses 44660 in all
software calculations.
Oxygen Sensor Cleaning and Storage
Sea-Bird has altered our recommendations with regard to cleaning and storage of the SBE 43. In the past, we
recommended using Triton X-100 detergent for the combined purpose of degreasing and discouraging biological
growth. We recently discovered that prolonged exposure of Triton X-100 to the sensor membrane is harmful and
causes the sensor’s calibration to drift. Our present recommendation, detailed below, is to continue to use
Triton X-100 for degreasing (with a short wash), then use a short wash with a dilute bleach solution to reduce
biological growth, and store the sensor in an anoxic (or near zero oxygen) condition. See Materials below for a
discussion of Triton X-100 detergent, bleach, and water.
Avoid fouling the oxygen membrane with oil or grease, as this causes a calibration shift toward erroneously
low readings. An oil-fouled membrane can be cleaned using the following procedures.
CAUTION: During service and storage, maintain temperature at or below 30 ºC (86 ºF). If temperatures are
raised above 40 ºC (104 ºF), sensors exhibit a temporary increase in sensitivity of a few percent. This relaxes back
to historical sensitivity after a few days when temperatures return below 30 ºC (86 ºF).
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