In-Situ TROLL 9500 Operators Manual - Manual (Page 133)

128
TROLL 9500 Operator’s Manual 0095110 rev. 007 01/09
RESETTING DEFAULT COEFFICIENTS
The sensor’s calibration may be reset back to factory defaults at any
time. As long as there is no contamination on the optical windows, this
will restore the factory accuracy (± 5% or 2 NTU).
1. Establish a connection to the instrument in Win-Situ 4 or Pocket-
Situ 4.
2. Select Turbidity in the Parameters list and click Calibrate.
3. In the first screen, select Use Default Coefficients, then Next.
4. In the final screen, click Finish to restore the sensor’s factory
calibration coefficients.
SENSOR SLOPE AND OFFSET
The offset is factory-set at 0 NTU. The zero offset may be recalculated
for any appropriate value by performing a single-point calibration
using a calibration standard of the desired NTU value. The sensor
response is very linear up to 200 NTU.
UNITS AND CALCULATED MEASUREMENTS
Two units are available for readings from the turbidity channel:
s
sor has been calibrated with polymer suspensions.
s
been calibrated with Formazin.
USAGE RECOMMENDATIONS AND CAUTIONS
The operational pressure rating of the turbidit y sensor is
150 psi. Do not submerge it deeper than 346 ft (105 m).
Avoid use of the stirrer accessory (recommended for monitoring dis-
solved oxygen in stagnant water) when measuring turbidity.
When used without a wiper, dirty sensor optics can be compensated
for to some extent by changing the offset.
Optical absorbancy (“color”) will lessen the turbidity signal.
Turbidity readings are temperature-commpensated.
The optics need 5 seconds warm-up time to take the first reading
later. Subsequent readings can be returned instantaneously.
COMMON INTERFERENCES
Light scattering depends upon the size, shape, refractive index, and
other characteristics of the particles and the wavelength of the light.
Optically black particles, such as those of activated carbon, may
absorb light and effectively decrease turbidity measurements.
Nephelometers are relatively unaffected by small changes in design
parameters and therefore are specified as the standard instrument for
measurement of low turbidities. Nonstandard turbidimeters, such as
forward-scattering devices, are more sensitive than nephelometers to
the presence of larger particles and are useful for process monitoring.
Reported turbidities are heavily dependent on the particulate matter
contained in the suspensions that are used to prepare instrument
calibration curves.
Due to current technological limitations, field turbidity measurement
is “a snapshot of averages,” Field measurements can be an excellent
indicator of in-situ turbidity; final determination for reporting purposes
should be conducted in a laboratory.
PROFILING TURBIDITY
The turbidity sensor’s 5-second warmup will result in a slight delay be-
fore the first Profiler reading for all parameters. Subsequent readings
can be taken within the Profiler’s 2-second cycling.
If a turbidity wiper accessory is installed, it performs an initial wipe of
the sensor optics—this takes about 15 seconds—then displays the
first turbidity reading. If the profiling rate is longer than 15 seconds,
this 15 second wipe will happen before each reading. To avoid this
delay, set the profiling rate to less than 15 seconds. See Customizing
the Profiler in Section 5 for details.
LOGGING TURBIDITY DATA
The wiper is activated automatically before turbidity readings during
tests, so long as the readings are 15 seconds or more apart. To
prolong battery life when running a wiper, we recommend the use
of external power or two internal lithium D-cells installed in the MP
TROLL 9500.
SENSOR CARE
INSPECTION/MAINTENANCE/CLEANING
The optical windows of the sensor are made of scratch-resistant
sapphire. The optical components are not user-serviceable. Serious
mechanical and temperature shock are about the only things that can
damage the LED. If you feel the instrument has suffered such dam-
age, contact In-Situ Technical Support.
However, the windows may need frequent cleaning, especially if used
in a biologically active environment. A wiper accessory can help to
prevent the accumulation of foreign material.
SECTION 18: TURBIDITY