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Measurement effects

Some typical measurement effects are described below

Drift

Drift
Drift occurs mainly through the gradual evaporation of water and through stabilisation of the target being used with the temperature of the water. For a stable system, the effect is prominent at diagnostic levels (output power < 100mW) where the rate of drift is significant compared to the output power being measured. For an absorbing target drift can also occur from heating and cooling within the material.

Provided all other environmental effects are reduced, drift should not be an issue with measurement. It is advisable to use the extrapolation method for calculation as this will reduce an underestimation in the calculated power.

Target heating

Target heating
Heating is most significant when using an absorbing target. The transition shown to the right shows the initial insonation period is relatively flat with a consistant drift downward typical of the bouyancy change in the target. For subsequent measurements variation during the insonation period can be seen and is a results of accumulated heating within the material.

One solution would be to allow sufficient time for the heat to dissipate from the target. As a guide, for measurements above 1W, a 'cooling' period of up to 10 times the insonation period would be appropriate.

Cavitation
The effects of caviation can be seen both on a time history and visually through bubbles forming in the path of the ultrasound beam. The transition shows variation during the insonation period.