a,b,k,h, and nSee footnotes to tables 2,3,4, and 5. For 3-16, 18, 20, and 22 respectively. This average is badly biased by the poor accuracy of the low-power values. If these are disregarded as outliers, the value becomes -0.1%. mSame as d but value becomes 10%. "Same as d but value is 17%. (although a 3- rather than a 2-MHz unit) exhibited a large shift in the measured G,. Upon autopsy it was found to have suffered a small crack in the epoxy seal and therefore a small leak. Upon being dried out and recemented the transducer yielded its original G, One could speculate that a small leak occurred while transducer 3-18 was at the participant's laboratory and that on the way back to the pilot laboratory the transducer dried out. Sixteen months later, when the last measurements were made, the transducer was still behaving properly, at least under the pilot laboratory conditions. While we were at it, we took additional measurements on the other transducers as well. All of the measurements made by the modulation radiationpressure method at the pilot laboratory are summarized in table 8. From the material in table 8, we could conclude that whatever drifts occur in either the transducers or the ap Frequency, MHz 2 No. Fraction % 3 11 10 2 6 22 17 3 9 33 4 14 52 5 [1] Zapf, Thomas L. Calibration of quartz transducers as ultrasonic power standards by an electrical method. IEEE Ultrasonics Symposium Proceedings: Cat. #74 CHO ISU; 1974; 45-49. [2] Greenspan, M., Breckenridge, F.R., Tschiegg, C.E. Ultrasonic transducer power output by modulated radiation pressure. J. Acoust. Soc. Am. 63(4): 1031-1038; 1978. [3] Zapf, T.L., Harvey, M.E., Larsen, N.T., Stoltenberg, R.T. Ultrasonic calorimeter for beam power measurements. Natl. Bur. Stand. (U.S.) Tech. Note 686; 1976 September. 36 p. [4] Snedecor, George W. Statistical methods, 6 ed. Ames: Iowa State University Press; 1967. 593 p. 6. Appendix International Intercomparison of cw Ultrasonic Beam Power Measurements Procedures and Instructions National Bureau of Standards Washington, DC 20234 This document provides instructions and a typical procedure for the intercomparison of ultrasonic beam power measurements. The intercomparison is being conducted by circulating to participating laboratories four transfer standards, namely, half-wave resonant, airbacked, quartz transducers. 6.1 Equipment Supplied by NBS 1. Transfer standards. The resonance frequencies of the transducers have been measured at NBS. 2. Matching circuit. A matching circuit may be needed to provide adequate voltage to the quartz transducers, which have a high electrical input impedance. It also helps to minimize distortion in the voltage applied to the transducers. 3. Connectors. Adapters are supplied for use with the transducers. 6.2 Care of the Standard Transducers 1. Avoid any severe mechanical shock. 2. Voltage limitations on the transducers are 3 V unloaded, and 350 V water loaded. Transducer voltage should be monitored at all times when connected to a power amplifier. Experimental procedures should be used that will ensure that the test voltage is not applied when the transducer is out of water and that the voltage is removed before removing the transducer from the water. 3. The transducer connectors are not waterproof, and should not be submerged. 4. The front faces of the transducers are gold plated. They should be carefully wiped with lens tissue to remove water. Avoid excessive force. Measurements To Be Made The quantity to be measured is the total cw beam power radiated by a transducer (the supplied transfer standard) into a reflectionless water load at a specified temperature, frequency, and sinusoidal input voltage to the transducer. At the power levels to be used in the intercomparison, the beam power is proportional to the square of the applied voltage. Each participant is responsible for obtaining a suitable rf voltmeter and having it calibrated at the voltages and frequencies required. The following table indicates the specified frequency, f,, and the specified voltage, V,, at which each measurement is to be made. The actual frequency, fm of the measurement should be within ±0.02% of f,. The measured voltage, V, should be within ±50% of V (subject to the 350 V maximum limitation). The voltages are arbitrary, but have been chosen to cover the range of interest. 6.4 Reporting Please use the reporting form included herewith. Report the test data in the sequence in which the data were taken. The pilot laboratory will accumulate and summarize the results from all participating laboratories. As discussed in the following paragraphs, numerical results from each participant should be reported in a manner to allow comparison on a common basis. An error can result from attenuation in the water coupling medium between the transducer and the receiver target. The radiated beam power, P,, equals the received power measured by the participant's equipment, Pm, plus the power loss in the coupling medium. This loss should be determined by measurement or calculation, and applied as a correction as indicated below. To correct for the attenuation, the following formula may be used: So that the fractional uncertainty, Up, associated with the common-basis power is properly determined at each test point, the participant should estimate the fractional uncertainty, U,, in the radiated power, taking into account the uncertainty in measuring or calculating the loss in the coupling medium. For example, if P, is 0.137 watts and the uncertainty in P, is ±0.004 watts, then U, = ±0.004/0.137 = ±0.03, or ±3%. The commonbasis power uncertainty, Up, is then calculated as U1 = U1+ 2U2 Ub where U, is the fractional uncertainty in the voltmeter calibration and the factor 2 is a result of the square-law relationship between voltage and power. Uncertainties associated with the application of the temperature correction will be negligible if temperatures are kept within the specified range. The estimate of U, should be based on the participant's experience with his equipment; it should not be influenced by the variations observed in the present intercomparison. Provide a brief description of each set of equipment and method of measurement used in the intercomparison. If more than one method of measurement is used, please report each on a separate data sheet. If the quantities Vm and Pm are inappropriate to the method of measurement used, then please supply at least the following information for each measurement: temperature of the measurement, Tm; frequency of the measurement, fm; the radiation conductance, G,, measured for the transducer; and the estimated uncertainty, U, of the measured G. 6.5 Precautions The transducer must be oriented so that the entire ultrasonic beam will be received by the participant's measuring equipment. Reflections that would cause ultrasonic energy to return to the transducer must be reduced to a negligible level to prevent interference at the face of the transducer that may change the characteristics of the transfer standard. Care should be exercised to avoid the presence or formation of bubbles on the transducer face during test. The use of degassed water may eliminate this problem. 6.6 Typical Procedure The following information is included but may be disregarded if not applicable: Typical equipment provided by participants: 1. Voltmeter. A radio-frequency voltmeter is needed to measure the voltage applied to the transducers. The voltmeter must be calibrated at the frequencies and voltages listed under Measurements to be made. The voltmeter should be calibrated with a short cable. This will minimize loading on the matching network. This cable then becomes part of the calibrated voltmeter. A calibration uncertainty of 0.25% or better is desirable. 2. Power amplifier. A power amplifier capable of supplying 5 watts into a 50-ohm load should be adequate. 3. Signal generator. A stable generator must drive the power amplifier with a cw sinusoidal waveform of low distortion (preferably less than 0.5%). 4. Frequency counter. This is needed to set the frequency of the signal applied to the transducer. sinusoidal waveform. Set the frequency controls of the signal generator approximately to the specified resonance frequency (loaded) of the NBS transducer. Adjust the measurement frequency, fm to within ±0.02% of the specified resonance frequency f,, of the transducer using the frequency counter as an indicator. Frequency should be monitored continuously during the test and adjusted, whenever necessary. 5. Set the voltmeter range switch to a suitable low voltage range. Increase the signal generator output to obtain a reading of perhaps 3 to 10 volts. Adjust the controls in the matching-circuit to obtain maximum voltage, being careful not to exceed 3 volts unloaded to 350 volts water loaded (if necessary reduce the signal from the generator). 6. The signal can now be increased to a desired test level. Table A-1 gives the test points (in terms of frequency and voltage) at which measurements are to be made by all participants. The number of measurements to be made at each point is not specified, although a number from 3 to 10 would seem practical. |