Page 21 - HydrophoneHandbook_eReader_Ed2_20151208
P. 21

Choosing a Hydrophone

environment. For instance, it is particularly important to
minimize the dissolved oxygen content in water to limit bubble
formation which increases the likelihood of cavitation damage.
Other factors that contribute to the likelihood of damage
include drive frequency, duty cycle, and temperature.
The following plot estimates the pressure range that can be
measured with each hydrophone model. However, it should be
noted these are only guidelines. It is important that the user
considers the details of other factors that contribute to the
actual pressure thresholds (e.g., aperture size, preamplifier
noise, water quality, drive conditions, etc.).
As can be seen from the plot, the only pressures that are clearly
acceptable for all circumstances are below approximately 100
kPa. This is particularly true for continuous or quasi-continuous
waves, which at pressures higher than 100 kPa make the
hydrophone more susceptible to cavitation damage. So from a
conservative point of view, the only clearly safe conditions are
below 100 kPa peak negative pressure.
Nevertheless, user-history has indicated that certain
hydrophone models have been used successfully under the
following conditions:
• HGL and HM: intended to be used to measure pressure

       amplitudes for diagnostic B- and A-mode ultrasound of
       one to three cycles of excitation with low duty factor (~
       1%). Stress tests on these devices also indicate that they
       should withstand short bursts (1-2 cycles) of pressure less
       than 3 MPa x [fc/1 MHz ]1/2 where fc is the center
       frequency in MHz. For example, a 1 MHz field indicates a
       negative pressure of 3 MPa, whereas a 4 MHz field
       corresponds to a maximum negative pressure of 6 MPa.


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