Campbell Scientific OBS-Mobile Software for Archer-OBS - Manual (Page 14)

Section 1. Operations
length or the 2048 / rate. Note: the product of the rate and the duration cannot
exceed 2048.
Rate: Rate is the frequency of sampling for the duration of measurements. All
sensors are sampled at the same rate, typically 2, 5, 10, or 25 times per second
(Hz). For example, a rate of 25 Hz for a 60-second duration will produce a
sample with 1500 measurements for each sensor. When wave statistics are
chosen, the rate must be selected in the Wave Setup box.
Power: This indicates the percentage of time over the duration of a sample that
sensors are ON. Higher power levels mean larger samples, better statistics, and
shorter battery life. Lower levels spare the batteries but result in more random
noise in sample statistics.
Record Length: When wave measurements are selected, this sets the time in
seconds for which depth measurements are made for the wave-spectral
computations. Use a record length of 512 seconds for inshore waters (lakes
and rivers), protected bays and estuaries. For coastal waters with intermediate
periods (6 to 9 seconds) use 1024 seconds. For the open ocean select a record
length of 2048 seconds to record long period waves (T
s
> 10 seconds).
Depth: This is the user’s best estimate of the water depth when the OBS-3A is
deployed. It is an initial value needed by the unit to compute wave heights and
correct for the attenuation of dynamic pressure with depth. When depth is
specified in the Wave Setup box, the OBS-3A automatically measures height
above bottom after reaching the deployment depth.
Height Above Bottom: This is distance above the bottom in meters or feet
where the OBS-3A will come to rest after it is deployed. It is an alternative
initial value used by the unit to correct for pressure attenuation. When height
above bottom is selected, depth is automatically computed once the unit has
come to rest.
1.9 Sampling Schedules
The main factors that need to be considered when setting up OBS-3A sampling
schedules include:
• Sampling interval needed to characterize the processes of interest (e.g.
water-level fluctuations, flood and transport duration, tidal and surf
conditions, etc.).
• Maximum sediment concentration.
• Statistical requirements, such as sample size and sampling rates.
• Battery capacity.
The goal is to pick a sampling scheme that gets essential information without
taking too many samples or sampling too often. Inefficient sampling produces
a data avalanche, unnecessary processing, and excessive battery consumption.
Sampling schedules are set with the interval, duration, and rate parameters.
Interval sets the time in seconds between the start of one sample and the
beginning of the next, e.g. how often data are recorded. Select the longest
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