Best Practice Guide to Measurement of Acoustic Output Power (Measurement)
Introduction | Radiation Force Balances | Measurement | Additional Information | Further Reading
Uncertainties (2)
Systematic (Type B evaluation)
The systematic uncertainty (Ub in this case) is a measure of the uncertainty that is inherent in the system being used. As a result, it will always need to be factored into the uncertainty applied to any measurements completed using the system. For commercial balances, the manufacturer may supply a figure in the technical manual that can then be used in the overall uncertainty. For a balance that is being built it is advisable that an uncertainty budget is evaluated through testing. In the table below is an example of an uncertainty budget defined for the NPL Reference Therapy Level Force Balance. The balance has been assessed over the frequency range 1MHz to 3MHz, typical for therapy level, and over the acoustic power range 1W to 20W. The uncertainties are appropriate for an absorbing target.
| Source of measurement uncertainty | 1MHz |
3MHz |
| Imperfections in target and geometry Imperfections in the absorber |
0.21% | 0.46% |
| Effects of changes in target buoyancy | 0.23% | |
| Tank wall lining imperfections Effects from reflections (standing waves) |
1.2% | 1.8% |
| Sartorius Balance accuracy Deviations from linearity Temperature dependance of balance Balance Resolution |
0.5% 0.12% 0.09% |
|
| Extrapolation of power back to reference distance Effects of Streaming Absorption on water path |
0.1% neg |
0.6% 0.35% |
| Derivation of power from OFF-ON and ON-OFF | 0.29% | |
| Type B (systematic) uncertainty UB-NPL | 1.38% | 2.08% |
- IMPERFECTIONS IN THE TARGET PROPERTIES AND GEOMETRY Measurements of the acoustic properties of the NPL absorber were carried out using a membrane hydrophone to record the transmission and reflection coefficients of the material. The transmission loss leads to underestimates in the measured radiation force. The effect of reflections indicates an over estimate in the measured radiation force.
- EFFECT OF CHANGES IN TARGET BUOYANCY The coefficient of weight change for the absorbing target was calculated from known linear expansivity values of the polycarbonate base and polyurethane materials used in the absorber is -13 mg °C-1.
- TANK WALL LINING IMPERFECTIONS The significant effect is of reflections back to the transducer face which produces standing waves and can affect the output.
- SARTORIUS BALANCE ACCURACY The accuracy has been assessed through measurement of its linearity and from manufacturers specifications relating to the temperature dependence. The linearity was established through applying a series of calibrated weights and comparing against traceable results.
- EXTRAPOLATION OF POWER BACK TO A REFERENCE DISTANCE Power measurements made at a fixed distance have to be corrected for absorption in the water within the intervening path. Measurement of power can be affected by acoustic streaming currents set up within the water path separating the transducer from the target. This bulk movement of water arises from absorption of ultrasound within the water path and is more apparent at frequencies above 5MHz.
- DERIVATION OF POWER FROM OFF-ON AND ON-OFF This component relates to the sensitivity of the derived power to the method used to analyse the data produced by the balance. For typical traces produced the effect is small.
References
- EC project report. Therapy Level Ultrasonic Power measurement - SMT4-CT96-2139
- The measurements of the radiation conductance of quartz crystal ultrasonic transducers between 1 and 10MHz - A Shaw, NPL report RSA (RES) 14 (1992)