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Section 3. Operations

interval that will show the changes in turbidity and water depth that you wish

to investigate. Rat

e sets the number measurements per second, in Hz, taken

during a sample. The quicker turbidity and depth change, the higher the

sampling rate should be to get a stable average value for a sample. Finally,

Duratio

n sets the period of time for measurements and how long sensor

outputs will be averaged. For example, with an interval of 30 seconds and a

duration of five seconds, the OBS-3A will make measurements for five

seconds starting every 30 seconds. The number of measurements in a sample

(sample size) is the product of the duration and the rate. So if the rate was 25

Hz in the prior example, the sample size would be 5 X 25 = 125 measurements.

TABLE 3.9-1 provides some recommended ranges for these parameters in

various sampling environments. Always select duration and rate to give a

sample size of at least 30, and to reduce random sampling noise below 50% of

its maximum value, select them to give a size greater than 200.

TABLE 3.9-1. Sampling Schedules

Environment Rate (Hz) Duration (sec) Interval

River/Stream

2-5 30-100 300-900

Beach

5-25 30-200 60-900

Estuary

5-10 10-60 600-3600

3.10 Sampling Modes

Survey: Select the survey mode when operating the unit with a cable

connection to a PC and when high data rates are desired. Data can be logged

with a PC at rates up to 120 lines per minute (2 Hz).

Cyclic sampling: Use cyclic sampling to record data internally in the 8

Mb, non-volatile FLASH memory at regular intervals, e.g. every 1, 5, 15, or 30

minutes. Depending on the number of sensors measured and the statistics

selected, the OBS-3A can log as many as 200,000 lines of data (one per hour

for 23 years) including: time, date, depth, NTUs,

C, and salinity.

Scheduled sampling: The OBS-3A can be scheduled to sample at

specific times in hours and minutes on a 24-hour clock using this mode.

Setpoint sampling: Use this mode for fast sampling of events such as

storms, floods, dredging operations, and construction activities. The unit will

revert to slow recording between events. Sample events two to five times

faster than the rate chosen for the periods between events. For example,

program the OBS-3A to sample slowly for a duration of 30 seconds every 900

seconds (15 minutes), and to sample at a fast rate every 180 seconds (three

minutes), when the turbidity level exceeds a specified setpoint.

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