Principles of Operation
Section 5
YSI Incorporated Drinking Water Monitoring Systems Operation Manual
5-17
Most commercial fluorometers fit into two categories. In the first category are benchtop instruments that
generally have superior optical flexibility and capability but are relatively expensive and are often difficult
to use in the field. In the second category are sonde-type fluorometers that have a fixed optical
configuration, but are less expensive, can be more easily used in the field, and are usually compatible with
data collection platforms. The use of a pump is recommended for some sonde fluorometers and this can
result in the need for large capacity batteries for field use.
The unique YSI chlorophyll system available as an option for use with YSI sondes consists of a probe
which is similar in concept to the sonde-type fluorometers, but is much smaller, making it compatible with
the probe ports of the YSI 6920DW sonde. The output of the sensor is automatically processed via the
sonde software to provide readings in either generic fluorescence units (percent full scale; % FS) or µg/L of
chlorophyll. No pump is required for the YSI system allowing the sensor to operate off of either the sonde
internal batteries or the batteries in the YSI 650 MDS display/logger. Like the YSI 6026 and 6136 turbidity
probes, the YSI 6025 chlorophyll probe is equipped with a mechanical wiper to periodically clean the
optical face either by manual or automatic activation. With these features, the YSI chlorophyll sensor
provides the same level of performance as the sonde fluorometers, but is much easier to use and can be
deployed in environmental water for several weeks without the need for service. In addition, the probe will
be a component in sondes that can acquire up to ten other parameters simultaneously with chlorophyll,
rather just providing the single parameter.
CALIBRATION METHODS -- GENERAL
As described in
Section
2 of this manual, the sonde software offers the option of either zeroing the generic
fluorescence parameter (% FS) or of performing 1-point, 2-point, or 3-point calibration procedures in µg/L
of chlorophyll. For most applications, a 2-point calibration at 0 and middle range (e.g., ca. 100 µg/L) is
sufficient. Slight non-linearities of the system can be overcome with a 3-point calibration across the
normally encountered environmental chlorophyll range (0-400 µg/L).
If the range of chlorophyll content in
the environmental sample is well known, standards of any chlorophyll values can be utilized in either 3-
point or 2-point routines. However, one of the standards must be 0 µg/L and this standard must be first in
the calibration sequence.
For calibration of the YSI chlorophyll system,
only one type of standard will assure accurate field
readings: a suspension of phytoplankton of known chlorophyll content
. The chlorophyll content of this
Optical Fiber
Photodetector
Light Source
470 nm
Optical Filter