Best Practice Guide to Measurement of Acoustic Output Power (Measurement)
Introduction | Radiation Force Balances | Measurement | Additional Information | Further Reading
Uncertainties
Random (Type A evaluation)
The random component of measurement is a good indicator of the stability of a system. Unlike the systematic component, which generally remains unchanged for a standard measurement protocol, the random component can vary depending on the stability of the system and the conditions under which the unit is measured. Generally, a unit which is less stable will show higher random uncertainty.
The method incorporated in an NPL calibration can be found in the UKAS document 'The expression of Uncertainty and confidence in Measurement' and two worked examples of the method have been provided. The first set of data is from a Reference source used as a transfer standard for calibrations at NPL and has itself been calibrated prevously against the NPL Primary Standard Radiation Force Balance. The second set of data is from a second standard off the shelf device.
| Data set 1 | Data set 2 | |
| 99.48mW 99.15mW 99.20mW 99.07mW 99.20mW 98.74mW 99.12mW 98.83mW 99.06mW 98.81mW 98.64mW 98.70mW |
94.22 mW 96.91mW 95.64mW 97.99mW 96.38mW 98.16mW 96.89mW 98.49mW 97.21mW 99.17mW 97.76mW 99.84mW |
|
Mean value |
x = 99.00 mW | x = 97.39 mW |
Standard deviation |
s(xj) = 0.254 mW | s(xj) =1.541 mW |
Standard uncertainty |
U(x) = 0.073 mW (0.07%) | U(x) = 0.445 mW (0.46%) |
Data set two shows an increase in the measured force with an increase in the number of insonation periods. A systematic difference can be seen between the two transitions (OFF-ON and ON-OFF). The variation for the results is reflected in the overall calculated value for the random uncertainty. Calculating each transition can be a good indicator of any performance characteristics of a device and can contribute to an appropriate procedure for measurement. An example for data set 2 would be to use only one set of transitions, say ON to OFF, and a specified insonation period as the basis for deriving a reference value. The method used should be replicated for most accurate transfer to a device under test.
References
- United Kingdom Accreditation Service document M3003 Edition 1 ' The expression of Uncertainty and Confidence in Measurement' (1997)
- NPL Measurement Good Practise Guide No 11 'A beginner's guide to Uncertainty of Measurement' Stephanie Bell (1999)
- BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, OIML, 1993, Guide to the expression of the uncertainty in measurment (GUM)