YSI 600DW-B Sonde - Manual (Page 230)

Principles of Operation Section 5
is the basis for all commercial fluorometers capable of measuring the analyte in vivo. Fluorometers of this
type have been in use for some time. These instruments induce chlorophyll to fluoresce by shining a beam
of light of the proper wavelength into the sample, and then measuring the higher wavelength light which is
emitted as a result of the fluorescence process. Most chlorophyll systems use a light emitting diode (LED)
as the source of the irradiating light that has a peak wavelength of approximately 470 nm. LEDs with this
specification produce radiation in the visible region of the spectrum with the light appearing blue to the
eye. On irradiation with this blue light, chlorophyll resident in whole cells emits light in the 650-700 nm
region of the spectrum. To quantify the fluorescence the system detector is usually a photodiode of high
sensitivity that is screened by an optical filter that restricts the detected light. The filter prevents the 470 nm
exciting light from being detected when it is backscattered off of particles in the water. Without the filter,
turbid (cloudy) water would appear to contain fluorescent phytoplankton, even though none were present.
The following diagram can be used to better understand the principles of the YSI system.
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.
Optical Fiber
Photodetector
Light Source
470 nm
Optical Filter
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-17