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Section 3. Operations

3.6 Water-Density and Barometric Corrections

Since depths are estimated from pressure measurements, it is important to set

the water temperature and salinity so the OBS-3A can correct for water density

and calculate depth in meters or feet (this will not affect temperature or salinity

measurements). Also, the sensor measures absolute pressure so another

correction must be made for barometric pressure. Be sure to do this while the

OBS-3A is at the surface. Doing so when the instrument is submerged will

result in large errors in the depth measurement. The error will be

approximately equal to the instrument depth when the correction is made.

Depending on the magnitude of barometric pressure fluctuations at the

sampling site and the desired accuracy, you may want to correct data for

atmospheric effects using barometric pressure simultaneously recorded at a

nearby site.

3.7 Sample Statistics

Three types of statistics can be selected for the OBS-3A measurements.

1. Measures of central tendency, the mean and median.

2. Measures of variation or spread within a sample, the standard deviation (V)

and cumulative percentages, such as X

and X

(where X is the measured

depth or NTU)

3. Wave statistics, significant height and dominant period.

Statistics are computed for each sample and logged in the FLASH. The raw

data are not saved. The mean is the arithmetic average of the values (¦ x / n),

where ¦ x is the sum of the sample values (x) and n is the number of values

(sample size). The median (X

) is the value that exceeds 50% of the sample

values and is the best measure of central tendency when a sample has outliers.

The percentages, X

, X

, etc. exceed 25, 50, and 75% of the sample

values. The OBS-3A uses a spectral method developed by the U.S. Army

Corps of Engineers to calculate wave heights in depth units and periods (H

and

). H

is the average height of the one-third largest waves, and reports it in the

selected depth units (meters or feet). T

, is the time in seconds associated with

the peak spectral-density in the wave spectrum.

3.8 Definitions

The following definitions are useful when programming the OBS-3A.

Interval

: The time in seconds between the start of one sample and the

beginning of the next. In cyclic mode, this is the time between samples, and in

setpoint mode, there are two intervals, one slow and the other fast. The

interval must be longer than the duration plus some time for statistical

computations. OFW will prompt you if too short an interval is selected.

Duration

: This is the length of time in seconds that the OBS-3A is measuring

its sensors. The duration must always be less than the interval. The minimum

duration is five seconds and the maximum is the longer of the wave record

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