Specifications

5.4.6 Calibration of the vertical accelerometer

A calibrated vertical accelerometer and stabilized platform should perform within limits over 3

to 6 years, depending on operating conditions. Consequently Datawell recommends

recalibration of your buoy every 3 to 6 years. To eliminate any doubt about the calibration, the

following tests may be carried out.

To test the vertical accelerometer, the buoy must be set in motion first. With a weight of

over 200 Kg, a buoy in motion must be approached with great caution! There are two easy ways

to obtain a vertical motion:

(1) set a suspended buoy swinging

(2) use the rubber cords to set the buoy oscillating vertically (not WR-SG 0.7 m)

The measured vertical motion can be monitored using the buoy receiver and wave software.

For the swing use a rope of 10 m and set the buoy swinging with 1 m horizontal

amplitude. Maintain the amplitude by gently pushing the buoy each cycle. A heave oscillation

with roughly 6 s period and 2.5 cm amplitude should result.

When testing with the rubber cords, the procedure is as follows. Fold the 30 m rubber

cord twice or (each of) the 15 m rubber cords once, to construct a 7.5 m pendulum, excluding 4-

6 m. elongation of the rubber cord under de weight of the buoy. The expected elongation of

10 m. explains why this test is not suitable for WR-SG 0.7 m. Attach the lower end of the

complete string of rubber cords to a long rope tied to both buoy handles. The upper end of the

complete string must be attached overhead. An elastic system with a natural period of vertical

oscillation of around 4 seconds will result. Amplitudes of about 2 m are obtainable. Hoist the

buoy to a height that a standing man can just reach the mooring eye. The oscillation amplitude

will be approximately equal to the height of the mooring eye over the ground. Draw the buoy

down until the mooring eye touches the ground and let go. The buoy should be kept oscillating

in a way so that the eye just touches the ground by pushing down gently each time the buoy

comes down. On hard ground, place a soft pad to avoid bumping. For exact calibration of the

heave the buoy must be sent back to Datawell Service.

5.4.7 Platform offset and stability

Here two tests are described that focus on the stabilized platform, more precisely its offset and

stability. Let us start with testing the offset. Place your buoy upright on a revolving frame or

trolley. Take measures to log both pitch and roll signals; consult subsection 5.9.9 on the test

box. Rotate the frame or trolley 1.5 times 360º around the vertical axis and start logging for 0.5-

1 hour. Now plot the logged pitch and roll against each other and skip the first 1.5 minute or so.

A circle should be written over a time lapse of approximately 30 minutes. The radius of this

circle represents the platform offset. It should stay below 1º.

To test the stability, leave the buoy at rest on a trolley for a while. Then start logging the

buoy motion. Push and stop the buoy fiercely thus moving it a few metres. In particular, the

horizontal displacements will show disturbances at the natural swinging period of 40 s of the

stabilized platform. If large disturbances occur at all kinds of frequencies, the platform has

become unstable. For example, this could be due to a separation of the sensor fluid after cooling

down to below −5 ºC. Compare doing the same experiment with a half-filled fish-globe. Contact

Datawell Service for repair.