Hach-Lange FILTERTRAK FT 660 sc User Manual - Manual (Page 24)

Table 4 DIAG/TEST menu (continued)
Option Description
SERVICE MODE Enable or disable service mode—On or Off (default).
SERVICE DIAGS Passcode protected. Menu options are only available to
service personnel.
Theory of operation
The FilterTrak 660 sc nephelometer uses nephelometric detection of
light scatter from particles in a sample. The optical design is shown in
Figure 9 . The geometry of the incident light source and the centerline
into the detector receive fiber is fixed at 90 degrees, which is consistent
with the detection angle associated with current nephelometry
technology.
The light source is a laser diode with a wavelength of 660-nm. The
response system is optimized to detect this specific wavelength. In
addition, the incident light is collimated into a narrow beam, resulting in a
relatively high beam power density. This high power density results in
higher scattered intensity from smaller particles. In addition, the portion
of the narrow beam that passes through the sample without being
scattered is absorbed by a light trap at the bottom of the nephelometer
body. These features combined to create a measurement system with
dramatically lower stray light.
The FilterTrak 660 sc nephelometer detection apparatus is coupled with
fiber optic technology to convey the scattered light signal to the detector.
The high power source and very sensitive detector combine to produce a
strong signal even at the lowest of turbidity levels. The optimization of
the optical components results in an increase in sensitivity of more than
two orders of magnitude over the sensitivity that is achieved with the
best turbidimeters in use today. This allows for a very stable baseline
capable of detecting a change in turbidity down to 0.05 mNTU
(1.000 NTU = 1000 mNTU). Since sensitivity is defined as the detection
of a change in turbidity, the lowest numerical value that an instrument
can read is not as significant as the smallest change that it can detect.
Figure 9 Laser nephelometer optical configuration
1 Laser module 5 Light receiver
2 Light detector 6 Water level
3 Optical fiber 7 Aperture
4 Scattered light 8 Incident light beam
RSD parameter
The FilterTrak 660 sc offers a complementary parameter to the laser
nephelometry measurement. This parameter is known as the relative
standard deviation (RSD). RSD is a dimensionless parameter that
provides a quantative assessment of the variability (fluctuation) of the
laser turbidity measurement. Studies have shown that the baseline of a
turbidity measurement will often increase in fluctuation before the actual
22 English