The ECO sensors primarily image a volume that is approximately 1 cm3, centered 1 cm off the face of the instrument.
NOTE: This does not preclude return of photons from outside of this volume and in particular it is best that no fixed objects are in the field of view of the instrument, such as cables or cage hardware.
We recommend that the instruments be mounted in such a way that they are seeing only the free field. Often when mounted on a CTD, mounting the ECO face-down or away from the CTD can improve reading accuracy.
That said, the ECO’s have been used in fairly tight quarters with excellent results, including in flow-through housings and dense instrument cages.
When the ECO is mounted in such a way that there is an object in front of the Instrument, the ‘wall effect’ should be established. We are assuming that the material does not fluoresce.
To establish the “wall effect”:
1) Turn the instrument on in air, or better, in a clear water bath, and collect an ‘offset’ reading.
2) Compare this to the factory calibration offset. If the difference is small (e.g. a few counts or mV), then no further action is necessary.
3) Rotate the instrument to find the minimum offset. The backscattering or turbidity channel is the best for this.
4) Mark this position and record the output values of all channels.
5) To minimize the wall effect any object in front of the face of the instrument should be dull black or grey. Tape is usually the easiest solution for this on frames. Grey or black matte plastic is the solution for underway systems.
6) Use these values as the offset values in generating engineering unit output, as specified on the characterization sheets:
Output = ScaleFactor x [InstrumentOutput – Offset]
7) After collecting data from the field, check to confirm that the offset you are using is appropriate. You may find that your minimum values are lower than the offset you have established.