What is the minimum dissolved oxygen concentration that can be accurately measured by the SBE 43 DO sensor?
The measurement range and accuracy of the SBE43 are not defined in discrete units (mL/L, mg/L, µMol/L, etc.) Rather, the range and accuracy specification are expressed as a ratio of the calculated oxygen saturation point. This saturation value depends on the temperature and salinity of the water, decreasing in higher temperatures and higher salinity.
The lower limit of detection of dissolved oxygen concentration is going to be constrained by the accuracy specification, which is a ratio of the calculated oxygen saturation (initial accuracy: +/- 2% of saturation). This saturation value varies depending on the temperature and salinity of the water. Common oxygen saturation values in seawater are in the range of 4-7 mL/L. Using this value as an estimate, you can extrapolate that the accuracy of the sensor in many conditions can be as low as +/- 0.1 mL/L. Therefore, the close you get to zero oxygen concentration, the closer the resolution of your measurement will be to the accuracy spec, until you get below 0.1 – 0.2 mL/L and your measured value is smaller than the nominal error margin.
While the theoretical lower limit of the instrument’s measurement range is zero, the measurement becomes less meaningful the closer your dissolved oxygen measurement gets to 2% of the calculated saturation. For example, if you were in an environment where the oxygen saturation was 7 mL/L but the oxygen concentration measured by the SBE43 was 0.28 mL/L, then your accuracy would effectively be +/- 50% of the measurement, making it relatively useless.