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OBS-5+ System
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from the descending limb of the near detector curve. A calibration consists of
a set of 15 to 30 calibration points that each includes an SSC value, a near
detector count, and a far detector count.
FIGURE 7-2. Lookup tables and table limits (a, b, and c)
Six to ten calibration points are needed to define each table. For the lookup
tables to function properly, the peaks in the ND and FD curves must be within
± 2,500 counts of one another, and to maximize resolution, the peak heights
should be between 62,000 and 64,500 counts. Start with the schedule shown in
TABLE 7-1 and adapt it to your sediment as required during the calibration
procedure.
Sediment preparation is a critical factor in calibration quality. Use dry material
whenever possible because it can be accurately weighed. Keep in mind that
mixing, grinding, and sieving can produce smaller sediment than you will
measure in the field and that the OBS-5+ is very sensitive to particle size (see
Section 10, Interfering Factors). This means that disaggregation can produce
measurement errors. Vigorous disaggregation with a sonic probe, for example,
can produce smaller particles that result in more ND and FD counts per unit of
SSC than less aggressive methods. An OBS-5+ calibrated with the former
material will underestimate SSC in the field.
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