YSI 600DW-B Sonde - Manual (Page 235)

Principles of Operation Section 5
EFFECT OF CELL STRUCTURE, PARTICLE SIZE, AND ORGANISM TYPE ON IN VIVO
READINGS: Even if the only fluorescent species present for in vivo measurements were chlorophyll, and
reliable calibration standards were available, its absolute quantification would probably still be difficult
because samples are not homogeneous. Differing species of algae with differing shape and size will likely
fluoresce differently even if the type and concentration of chlorophyll are identical and this significantly
limits the accuracy of in vivo measurements.
EFFECT OF TEMPERATURE ON PHYTOPLANKTON FLUORESCENCE: As noted above, YSI
experiments indicate that phytoplankton fluorescence increases as temperature decreases. Thus, readings
taken on a phytoplankton suspension at cold temperature would erroneously indicate the presence of more
phytoplankton than when the suspension is read at room temperature. Unless this effect is taken into
account, most field readings will be somewhat in error, since the field temperature will differ from the
temperature of calibration. The use of the “Chl Tempco” factor found in the Advanced|Sensor menu will
help to reduce this error, but must be used with caution since each species of phytoplankton is likely to
have a slightly different temperature dependence.
EFFECT OF LIGHT ON PHYTOPLANKTON FLUORESCENCE: It is well documented in the
literature that the fluorescence of chlorophyll resident in phytoplankton can be inhibited by light. This
“photoinhibition” is confirmed by empirical data that indicate that, at constant phytoplankton level, the
fluorescent signal can change significantly on a diurnal schedule, showing less fluorescence during the day
and more fluorescence at night. It is clear that this effect would produce errors in the absolute values of
chlorophyll unless it were accounted for by the user.
The chlorophyll section of Standard Methods substantiates these limitations along with application notes
that are offered by current fluorometer manufacturers. The limitations result in the realization that any in
vivo “chlorophyll” sensor will be much less quantitative than any of the other sensors offered for use with
our sondes.
MEASUREMENT AND CALIBRATION TIPS
1. For best results, analyze field samples to be used for “calibration” of the sensor as soon as possible
after collection.
2. If unusually high or jumpy readings are observed during calibration, it is likely that there are bubbles
on the optics. The surface should be cleaned by manually activating the wiper before confirming the
calibration.
3. The output of the YSI fluorescence sensor is susceptible not only to the overall phytoplankton
concentration in the environmental medium, but also to the size and rate of movement of the
suspended particles that pass across the optics on the probe face. Thus, although the phytoplankton
content of an environmental sample may appear to the eye to be relatively stable, the displayed
chlorophyll reading can vary significantly depending on the nature of the particles in the optical path at
the instant of measurement. In a discrete sample study of environmental water, for example, the
variability of the output can be significant. This apparent jumpiness is not observed in dye standards,
since these are homogeneous solutions containing no suspended matter.
4. The sonde chlorophyll system allows the user to apply a mathematical filter to the raw data so that the
sensor output may be more representative of the average phytoplankton content of the environmental
sample. From the 8-Advanced menu of the sonde software, the user can activate the data filter and
adjust its performance. For typical sampling and monitoring applications, YSI recommends that the
Data Filter settings for chlorophyll be selected as follows: Enabled -- On; Wait for Filter -- Off;
Chlorophyll Time Constant = 12; Chlorophyll Threshold = 1. The advantage of the filter is a more
stable display of chlorophyll readings.
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-22