YSI 6 series User Manual page 332

Chlorophyll Measurements
What Data Filter settings should I use to obtain the best possible field chlorophyll readings?
As described in Section 5, Principles of Operation, some processing of raw chlorophyll data is usually
beneficial in terms of outputting values that reflect the "average" chlorophyll at the site. Filter options
designed to optimize this data processing are located in the sonde menu structure under both the 3-Sensor
and the 4-Data Filter selections in the Advanced submenu.
For most applications involving both spot sampling and monitoring, the following settings are
recommended with regard to data processing:
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In 3-Sensor, Disable the "Chl Spike Filter" if present
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In 4-Data Filter, "Enable" the filter.
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In 4-Data Filter, "Disable" the "Wait for Filter" selection.
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In 4-Data Filter for chlorophyll, set the Time Constant to 12
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In 4-Data Filter for chlorophyll, set the Threshold to 1
These default settings will normally produce data that is reflective of the "average" chlorophyll without
significantly slowing the response time of the sensor for either sampling or monitoring applications.
Increasing the values of the time constant, threshold, or both can further smooth the chlorophyll values.
Variation in the default settings should be based on empirical data taken at your particular site.
How often should I calibrate my YSI 6025 sensor?
You should always perform a zero point calibration in deionized water before each use of the YSI 6025
sensor.
Testing at YSI has indicated that the overall sensitivity of the optical system of the YSI 6025 is very stable
and is unlikely to show significant drift over time. Thus, if you are using the dye "calibration" method
(with its limitations), you may only have to carry out the routine very infrequently (Remember that it is
basically only useful as a drift check anyway). In general, you should perform periodic 2-point calibrations
using a dye sample more frequently during your initial studies with the YSI 6025 to empirically determine
its drift rate and use these results to set up your calibration frequency.
If you are post-calibrating the sensor with phytoplankton suspensions to obtain accurate readings relative to
chlorophyll as determined by laboratory extractive analysis, then you will effectively be performing a 2-
point calibration for every sampling or monitoring study.
Are there precautions that I should take when acquiring samples for laboratory analysis?
Yes. The key is to obtain a sample which is representative of the water being measured by the YSI 6025 in
vivo. The mistake most often made is to simply immerse an open bottle in the environmental water. Under
these conditions, any macroscopic surface plant or algal matter (which also contains chlorophyll) will also
be introduced into the sample even though it is not present in the subsurface zone where the probe is
measuring fluorescence. If this happens, your laboratory analysis will always be erroneously high relative
to the in vivo chlorophyll data from the YSI 6025.
For surface sampling you can minimize this effect greatly by placing the sealed bottle a foot or more below
the water surface and only then removing the cap and allowing the container to fill. The cap should then be
reinstalled before removing the bottle from the water. A better technique is to acquire commercially
YSI Incorporated Environmental Monitoring Systems Operations Manual
Appendix I
I-5