Introduction | Radiation Force Balances | Measurement | Additional Information | Further Reading

Making a measurement

It is necessary to measure the change in the force on the target when the ultrasound is turned ON or OFF. The Balance takes a few seconds to stabilise after a sudden change in the radiation force caused by turning the ultrasound ON or OFF. During this stabilisation time water continues to evaporate and so the weight of the target decreases (by perhaps 0.1 mg to 1 mg, equivalent to approximately 1.5 mW to 15 mW). This means that accurate results cannot be obtained by simply observing the change in the displayed value as this contains contributions from the radiation force and weight loss. Two methods can be recommended. A timer will be needed for both. Worked examples have been provided.

Approximation Method

This method is suitable for a 'quick check' to see if the transducer is working or to gain an approximate value for the output power. It is most suitable for outputs above 100 mW.

• Measure the temperature of the water
• Note the balance reading (T = 0s). Switch on the drive signal to the transducer.
• Wait for 10 s. Note the balance reading (T = 10s). Turn off the drive signal to the transducer.
• Wait for 10 s. Note the balance reading (T = 20s)
• Take the difference between the OFF-ON transition (T(10) - T(0)) and ON-OFF transition (T(10)-T(20)).
• The method can be repeated continuously by taking the reading for T(20) as the initial value T(0)

An equivalent acoustic power can be obtained from the standard equation, P = c.F = c.m.g. Using a nominal water temperature of 20°C (c = 1482.36 m/s), the results from the approximation method yield an equivalent acoustic power of 97.5mW.

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