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This calibrator is capable of calibrating eight hydrophones at a time at depths up 3500 m. The unknown and reference hydrophones are placed in a hydraulic pressure vessel that contains electrically-shielded sound projectors in the bottom. After the vessel has been sealed and bled of air, the vessel is pressurized to the chosen depth, and the calibration software activated.

This equipment comes in two versions, both of which produce calibrations of the same accuracy. In the simpler version, the operator manually changes the pressure with a hand-operated hydraulic pump and manually activates the calibration software. In the automatic version, the operator enters a pressure-versus-time profile and the times at which calibrations are to be performed. The software controls a hydraulic pump and electric valves to produce the specified pressure profile and calibrates the hydrophones at the specified times. The calibration results are written to a file in both versions.

Screen shot of the automatic pressure-control/calibration software. The blue line in the upper left graph is the user-specified pressure profile; the red line is the pressure achieved by the system. The red dots are the times at which calibrations are performed. The graph on the upper right shows a time trace of the reference and unknown hydrophone signals. The graph immediately below shows the FFT of the hydrophone signals.

Calibration System
Parameters measured Voltage sensitivity
Capacitance
Absolute accuracy of voltage sensitivity ┬▒ 2.0 dBV
Relative accuracy of voltage sensitivity
(one hydrophone to another)
┬▒ 0.1 dBV
Measurement frequency 15 to 200 Hz
Cycle time for calibration 30 seconds
Number of unknown hydrophones 8
Number of calibration points Unlimited
Data recorded Voltage sensitivity
Capacitance
Charge sensitivity (calculated)
Depth
Temperature
Time of calibration
Pressure Control Systems
Depth range 30 to 3500 m
Measurement medium Hydraulic fluid
Number of pressure steps in the pressure profile Unlimited
Maximum time of the pressure profile Unlimited
Maximum rate ofchange of pressure Equivalent to 10 m/s
Usable calibration chamber dimensions 30 cm high, 11 cm diameter
(other sizes available)