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OBS-5+ System
9.
Maintenance
9.1
OBS-5+ Sensor
9.2
Pressure Sensor
32
Bright sun near the surface (< 2 m) or black-colored sediments cause
erroneous OBS readings.
Do not survey in shallow water between 10:00 and 14:00 local time and avoid
areas with suspended black mud; see Section 10.2, NIR Reflectivity.
OBS-5+ indicates different NTU values in the field than other
turbidimeters.
Not all turbidity meters read the same! OBS-5+ sensors are checked with U.S.
EPA-approved AMCO Clear turbidity standards before leaving our factory (see
Appendix A). Other turbidimeters will read different NTU values on natural
water samples.
OBS-5+ indicates different suspended sediment levels in the field than in
the laboratory.
This results from a change in sediment size or color (see Section 10, Interfering
Factors). You may have to perform a field calibration with water samples.
The sapphire windows over the laser diode and the detectors must be kept
clean to make accurate SSC measurements (FIGURE 4-1 and FIGURE 4-2). A
gradual signal decline over a period of days to weeks indicates fouling with
mud, oil, or biological material. Regular cleaning with a water jet, mild
detergent and warm water, or a scouring pad will remove most contaminants
encountered in the field. A cloth with solvent or mineral spirits can be used to
remove oil and grease. However, do not use MEK, benzene, toluene, acetone,
TCE, or electronic cleaners as they could damage the epoxy bond between the
sapphire and the optic bushings. At the conclusion of each survey or
deployment, clean the OBS-5+ sensor. If thick bio-fouling has developed,
scrape the material off the window with a flexible knife then swipe it with a
scouring pad.
The silicon strain-gauge pressure sensor is located under a perforated disk and
spring-clip that protects the Hastelloy diaphragm isolating it from water
(FIGURE 4-1). Do not touch the diaphragm with tools or pointed objects, as
the instrument will leak if it is pierced. Clean the sensor with a water jet
directed at the disk after each survey or deployment to flush sediment from
between the disk and the sensor. Do not allow sediment to dry on the sensor
diaphragm because dry sediment will reduce accuracy and is difficult to wash
off. To clean the diaphragm, remove the spring clip with tru-arc pliers and the
disk with plastic tweezers, then gently wipe sediment off the diaphragm with a
wet cotton-tipped swab. Replace the disk and spring clip and flush with a water
jet.
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