Steven E. Fick

ULTRASONIC POWER OUTPUT MEASUREMENT BY PULSED RADIATION PRESSURE

Direct measurements of time-averaged spatially integrated output power radiated into reflectionless water loads can be made with high accuracy using techniques which exploit the radiation pressure exerted by sound on all objects in its path. With an absorptive target arranged to intercept the entirety of an ultrasound beam, total beam power can be determined as accurately as the radiation force induced on the target can be measured in isolation from confounding forces due to buoyancy, streaming, surface tension, and vibration. Pulse modulation of the incident ultrasound at a frequency well above those characteristics of confounding phenomena provides the desired isolation and other significant advantages in the operation of the radiation force balance (RFB) constructed in 1974. Equipped with purpose-built transducers and electronics, the RFB is adjusted to equate the radiation force and a counterforce generated by an actuator calibrated against reference masses using direct current as the transfer variable. Improvements made during its one overhaul in 1988 have nearly halved its overall measurement uncertainty and extended the capabilities of the RFB to include measuring the output of ultrasonic systems with arbitrary pulse waveforms.

Ultrasound power measurement by pulsed radiation pressure

In a specially designed radiation force balance (RFB), low-frequency pulse modulation of the incident ultrasound allows high-accuracy measurement of time-averaged spatially-integrated ultrasound power radiated into a reflectionless water load. Errors characteristic of force sensors are precluded by operating the RFB as a force comparator, without directly measuring force. Equipped with purpose-built transducers and electronics, the RFB is adjusted to equate the radiation force and a counterforce generated by an actuator calibrated against reference masses using direct current as the transfer variable. Special techniques enable RFB measurements of the output of continuous-wave and pulsed diagnostic medical systems. Nearly twenty years ago, customer feedback indicated that the accuracy of power levels replicated using radiation conductance was seriously compromised by limitations of equipment typically available. Accordingly, an alternative scheme, based on a stable radio frequency (rf) voltage sensor treated as part of the transfer standard, was developed which allows ultrasound power-transfer measurements to be achieved with relative standard uncertainty components as low as 2 × 10-3 (1 σ), using equipment already available to most customers. A detailed description is given of the dc-level method (DCLM), which since 1985 has been, by a wide margin, the method most frequently requested.

An IEEE 1451.5–802.11 standard-based wireless sensor network with embedded WTIM

This paper introduces a reference implementation of the Institute of Electrical and Electronics Engineers (IEEE) 1451.5–802.11 standard-based wireless sensor network (WSN) developed at the National Institute of Standards and Technology (NIST). The WSN consists of a Network Capable Application Processor (NCAP) and two Wireless Transducer Interface Modules (WTIM). The NCAP, a gateway node of the WSN, was developed on a laptop in Java language according to the IEEE 1451.5–802.11 standard. The embedded WTIM, a wireless sensor node, was developed based on the IEEE 1451.5–802.11 standard on a single board computer in Dynamic C language. The wireless communications between the NCAP and WTIMs are based on IEEE 1451.0 messages using Transmission Control Protocol/Internet Protocol (TCP/IP) and User Datagram Protocol/Internet Protocol (UDP/IP) sockets. A few examples are provided to illustrate the functionalities of the WSN.

New Measurement Service for Determining Pressure Sensitivity of Type LS2aP Microphones by the Reciprocity Method

A new National Institute of Standards and Technology (NIST) measurement service has been developed for determining the pressure sensitivities of American National Standards Institute and International Electrotechnical Commission type LS2aP laboratory standard microphones over the frequency range 31.5 Hz to 20 000 Hz. At most frequencies common to the new service and the old service, the values of the expanded uncertainties of the new service are one-half the corresponding values of the old service, or better. The new service uses an improved version of the system employed by NIST in the Consultative Committee for Acoustics, Ultrasound, and Vibration (CCAUV) key comparison CCAUV.A-K3. Measurements are performed using a long and a short air-filled plane-wave coupler. For each frequency in the range 31.5 Hz to 2000 Hz, the reported sensitivity level is the average of data from both couplers. For each frequency above 2000 Hz, the reported sensitivity level is determined with data from the short coupler only. For proof test data in the frequency range 31.5 Hz to 2000 Hz, the average absolute differences between data from the long and the short couplers are much smaller than the expanded uncertainties.

Long-Term Stability of the NIST Standard Ultrasonic Source.

The National Institute of Standards and Technology (NIST) Standard Ultrasonic Source (SUS) is a system comprising a transducer capable of output power levels up to 1 W at multiple frequencies between 1 MHz and 30 MHz, and an electrical impedance-matching network that allows the system to be driven by a conventional 50 Ω rf (radio-frequency) source. It is designed to allow interlaboratory replication of ultrasonic power levels with high accuracy using inexpensive readily available ancillary equipment. The SUS was offered for sale for 14 years (1985 to 1999). Each system was furnished with data for the set of calibration points (combinations of power level and frequency) specified by the customer. Of the systems that had been ordered with some calibration points in common, three were returned more than once to NIST for recalibration. Another system retained at NIST has been recalibrated periodically since 1984. The collective data for these systems comprise 9 calibration points and 102 measurements spanning a 17 year interval ending in 2001, the last year NIST ultrasonic power measurement services were available to the public. These data have been analyzed to compare variations in output power with frequency, power level, and time elapsed since the first calibration. The results verify the claim, made in the instruction sheet furnished with every SUS, that “long-term drift, if any, in the calibration of NIST Standard Sources is insignificant compared to the uncertainties associated with a single measurement of ultrasonic power by any method available at NIST.”

In-situ Attenuation Corrections for Radiation Force Measurements of High Frequency Ultrasound With a Conical Target

Radiation force balance (RFB) measurements of time-averaged, spatially-integrated ultrasound power transmitted into a reflectionless water load are based on measurements of the power received by the RFB target. When conical targets are used to intercept the output of collimated, circularly symmetric ultrasound sources operating at frequencies above a few megahertz, the correction for in-situ attenuation is significant, and differs significantly from predictions for idealized circumstances. Empirical attenuation correction factors for a 45° (half-angle) absorptive conical RFB target have been determined for 24 frequencies covering the 5 MHz to 30 MHz range. They agree well with previously unpublished attenuation calibration factors determined in 1994 for a similar target.

Precision Ultrasonic Wave Measurements With Simple Equipment.

We describe the design and construction of a relatively simple, inexpensive laser interferometer system for accurate measurements of ultrasonic surface displacement waveforms in reasonably friendly environments. We show how analysis of a single waveform can provide both the calibration constant required for absolute measurements and an estimate of the uncertainty of these measurements. We demonstrate the performance of this interferometer by measuring ultrasonic waveforms generated by a novel conical-element ultrasonic transducer.

Pressure reciprocity calibration of a MEMS microphone

This article reports the first use of the pressure reciprocity technique to calibrate a micro-electromechanical system (MEMS) microphone. This standardized primary calibration method is conventionally used to calibrate laboratory standard microphones. Results for the pressure reciprocity calibration of a MEMS microphone and two laboratory standard microphones are presented for the frequency range 100-10 000 Hz. Because the amplifier in the MEMS microphone package prevents reciprocal operation, this microphone was used only as a receiver of sound. A description of the procedure is presented along with checks of the measurement results and data regarding the uncertainties of these results.

Effect of Acoustic Excitation on R134a/Al2O3 Nanolubricant Mixture Boiling on a Reentrant Cavity Surface

This paper quantifies the influence of acoustic excitation of Al2O3 nanoparticles on the pool-boiling performance of R134a/polyolester mixtures on a commercial (Turbo-BII-HP) boiling surface. A nanolubricant with 10 nm diameter Al2O3 nanoparticles at a 5.1% volume fraction in the base polyolester lubricant was mixed with R134a at a 1% mass fraction. The study showed that high-frequency ultrasound at 1 MHz can improve R134a/nanolubricant boiling on a reentrant cavity surface by as much as 44%. This maximum enhancement occurred for an applied power level to the fluid of approximately 6 W and a heat flux of approximately 6.9 kW/m2. Applied power levels larger and smaller than 6 W resulted in smaller boiling heat transfer enhancements. In total, five different applied power levels were studied: 0 W, 4 W, 6 W, 8 W, and 12 W. The largest and smallest enhancement averaged over the tested heat flux range were approximately 12% and 2% for the applied power levels of 6 W and 4 W, respectively. In situ insonation at 1 MHz resulted in an improved dispersion of the nanolubricant on the test surface. An existing pool-boiling model for refrigerant/nanolubricant mixtures was modified to include the effect of acoustic excitation. For heat fluxes greater than 25 kW m−2, the model was within 4.5% of the measured heat flux ratios for mixtures, and the average agreement between measurements and predictions was approximately 1% for all power levels.

Long-Term Stability of the NIST Conical Reference Transducer

The National Institute of Standards and Technology (NIST) Conical Reference Transducer (CRT) is designed for purposes requiring frequency response characteristics much more uniform than those attainable with ultrasonic transducers conventionally used for acoustic emission (AE) nondestructive testing. The high performance of the CRT results from the use of design elements radically different from those of conventional transducers. The CRT was offered for sale for 15 years (1985 to 2000). Each CRT was furnished with data which expressed, as a function of frequency, the transducer sensitivity in volts per micrometer of normal displacement on the test block. Of the 22 transducers constructed, eight were reserved for long term research and were stored undisturbed in a laboratory with well controlled temperature and humidity. In 2009, the sensitivities of these eight units were redetermined. The 2009 data have been compared with data from similar tests conducted in 1985. The results of this comparison verify the claim “Results of tests of the long term stability of CRT characteristics indicate that, if proper care is taken, tens of years of service can reasonably be expected.” made in the CRT specifications document furnished to prospective customers.