Q&A: Negative Fluorescence Data
December 2019 Newsletter
Fluorescence is the light emitted by a particle after excitation from another light source. Sea-Bird Scientific fluorometers are widely used to measure fluorescence from chlorophyll as a method of measuring primary productivity.
The figure above shows a 10 meter profile of an ECO-Fl fluorometer. The shape of the profile and most of the data are reasonable, but fluorescence in the upper 5 meters (marked in red) appears to be negative. Since the signal should never be below zero (there cannot be negative amounts of plant life), something is obviously wrong.
Can you identify the problem?
Answer: Uncalibrated “Dark Offset”
Sea-Bird Scientific’s ECO fluorometers have a calibration value known as the “Dark Offset”. This value represents the zero-output of the sensor: when the fluorometer’s detector is not picking up any fluorescence (e.g. when it is completely dark), the sensor will output a small nonzero signal (in counts if outputting serial data, or volts if outputting analog data). This signal may vary slightly when moving the ECO to a different platform, especially when outputting analog data from the ECO.
Sea-Bird Scientific calibrates the dark offset, but it is best to recalibrate this offset with the ECO’s exact deployment setup (same cable, same CTD or logger, and same power supply). The process is quick and easy:
- Cover the LED and detector with black electrical tape. Remove any bubbles and ensure the tape is smooth.
- Turn the system on and gather ECO data (volts if capturing the analog output, counts if capturing digital output).
- Record the ECO’s output for 60 seconds.
- Identify the minimum value. This value is the new “Dark Offset”.
- Replace the existing “Dark Offset” value in the ECO’s calibration data. If using the ECO with a Sea-Bird Scientific CTD, enter this into the “Dark Offset” field in the .xmlcon file.
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