Evaluation of Ohmic’s Ultrasound Power Meters


 

Some biomedical technicians have asked how long Ohmic Instruments has been selling Ultrasound Power Meters. The first unit, the UPM-30 was made in 1976. It was evaluated by the FDA in 1977. The first digital power meters, models UPM-DT-1 and UPM-DT-10 were evaluated by the National Bureau Of Standards in 1988. See evaluation report.

 


 

DEPARTMENT OF HEALTH, EDUCATION. AND WELFARE
PUBLIC HEALTH SERViCE
FOOD AND DRUG ADMINISTRATION
ROCKVILLE, MARYLAND 20057

August 11, 1978

Mr. Denes Roveti
Technical Director
OHMIC Instruments Company
102 Chew Avenue
St. Michaels, Maryland 21663

Dear Mr. Roveti,

As you know, the Bureau of Radiological Health is in the process of evaluating the UPM-30 ultrasound power meter. As a result of the study of one instrument, a draft technical report for publication by the Bureau of Radiological Health has been prepared summarizing the equipment’s performance.

I am enclosing the first draft, of this report with the hope that you, or any of your fellow workers, will review it and send me any and all questions or comments that you may have. I would like to stress that this is only a draft and will be updated to incorporate various reviews, both internal and external.

Additionally, more testing is being done on the second instrument we have to corroborate or refute the information contained in the report. Your prompt, cooperation is appreciated so that this report can soon be published for public availability. ‘Thank you.

Sincerely yours,
Jesse Abzug
Staff Engineer
Acoustics Branch

Enclosure

 


EVALUATION BY THE NATIONAL BUREAU OF STANDARDS
OF MODELS UPM-DT-1 AND UPM-DT-10

In early 1988, Ohmic submitted two digital power meters to the National Bureau of Standards for verification of their accuracy. The N88 transducer calibration standards have a maximum uncertainty of ±3%. Both power meters performed within the ±3%, at their low/high power measurement ranges as outlined in the following excerpts from the two NBS reports:

   

March 30. 1988

US. Department of Commerce
National Bureau of Standards
Gaithersburg, MD 20899

EXCERPTS FROM THE TEST REPORT
OF
ULTRASOUND POWER METER
Model UPM-DT-1, S/N 207

Introduction
    The ultrasound power meter was tested pursuant to the manufacturer’s request for an evaluation ol the stability and accuracy of the instrument when used to measure ultrasonic power over a wide range at power levels. The Instrument was tested by using it to measure known levels of ultrasonic power ranging from 5 mW to 1 W; the test procedures were designed to generate data useful in determining the range or measured values over which reasonable accuracy might be expected. and the average accuracy obtained for this range
0f measured values.

Test Procedures
    The ultrasound used in these tests was generated by a half-wave resonant, air-backed lithium niobate transducer 1.6 cm in active diameter. Single-frequency continuous-wave excitation at 4.768 MHz was used. The magnitude and stability at the ultrasonic power thus generated was known from prior transducer calibration data obtained using the National Bureau of Standards (NBS) radiation force balance and procedures described in NBS Internal Report 78-1520. The known values of applied power cited herein are believed to differ from the values actually applied to the instrument under test by an amount no greater than three percent of the cited values. The instrument was used with the setup parameters programmed by its manufacturer.
    Nine levels of power, ranging from 50 mW to 1 W, were used in obtaining the data from which an estimate of overall instrumental uncertainty was developed. The choice of these levels was facilitated by use of the results ol earlier experiments intended to determine the magnitude of the smallest level at power that could be detected (but not necessarily accurately measured) and to determine the magnitude of the smallest change in power level that could be measured with reasonable accuracy. By switching small amounts of ultrasound on and off, and carefully watching the readout, it was found that 5 mW of power could be detected reliably, but lower values could not. This value is consistent with the 2-count truncation of displayed values established by setup parameters. In order to further check instrumental performance with small changes of input, the applied power was switched between 40 mW and 50 mW, and display readings were recorded as soon as they became stable, The process was repeated to allow computation of 13 successive differences, and 10 mW was subtracted from each successive difference. The average absolute value of these 13 values is 2.2 mW. This result suggests that, tor values of applied power large enough to allow the instrument to operate at full accuracy, the precision of the measured values is about 2 mW.
    Repeated measurements were made for each of 12 levels of applied power ranging from 25 mW to 1 W. For each measurement, the re-zero button was pushed and the transducer was energized just after the readout indicated zero. The display was read just after the stability lamp came on, except when the lamp had failed to extinguish while the displayed values increased. In this circumstance. it was necessary to continuously watch the display and take the reading the first time a stable indication was obtained. The transducer was de-energized iust after reading the display and another reading taken when the display had again settled. This procedure was repeated at least ten times for each level. Average measured values were computed for and compared to each known value of applied power. The fractional errors for three levels of power below 50 mW ranged from 7% to 13%, while the fractional errors for the nine levels between 50 mW and 1 W ranged from 0.1% to 3.67%, with an average absolute value at 1.7%. From this dichotomy it is inferred that the lower power limit for operation with tuii accuracy is 50 mW. The measurements just discussed were made without use of the Average Display feature; another set 01 measurements was made between 100 mW and 1 W in order to determine the benefits of using this feature. The average absolute fractional error for the measurements made with the Average Display feature was 3.0%. and the corresponding value for data taken without it was 1.4%. These results are taken to indicate that no significant advantage was obtained by use of the Average Display feature.

Conclusions
    An experimental investigation of the operating characteristics of the ultrasound power meter designated UPM-DT-1 S/N 207 has allowed estimation of its accuracy while undergoing testing, Because NBS has no knowledge or control of the degree to which the performance of other equipment might approach that of the instrument tested, nor of the degree to which performance might vary over time, the uncertainty estimate cited herein should be construed to apply only to the designated device while tested and no other time, and should not be considered ‘relevant to the performance of any other equipment during any period of time. From the test results for the instrument designed UPM-DT-1 S/N 207, it is estimated that measured values differed from the true values of applied ultrasonic power by an amount not greater than the sum of 0.002 W and three percent of the true values, for true values between 0.05 W and 1.0 W. The present Investigators know of no way to predict accuracy or stability for operation under conditions different from the ones described.

Signed by:
Donald G. Eitzen, Group Leader
Steven E. Fick. Electrical Engineer
Ultrasonic Standards
Automated Production Technology Division
Center for Manufacturing Engineering


   

February 24, 1988

US. Department of Commerce
National Bureau of Standards
Gaithersburg, MD 20899

EXCERPTS FROM THE TEST REPORT
OF
ULTRASOUND POWER METER
Model UPM-DT-10, S/N 188

Introduction
    The instrument designated UPM-DT-10, S/N 188. which will hereinafter be called DT-10 for the sake of brevity, was found to behave as might be expected of a device of Its type. That is, the discrepancies between indicated and actually applied levels of power were consistent with the number of displayed digits. and no unusual behavior was observed during the course of testing.

Test Procedures
    For all test reported here, the magnitude and stability 01 the applied ultrasonic power WOIB independently known from data generated in transfer calibrations from the National Bureau of Standards (NBS) radiation force balance described in NBS Internal Report 78-1520. Levels of power from 0.05 W to 1.0 W were generated using a standard source transducer operated at 4.768 MHz, while levels between 2 W and 10 W, inclusive, were obtained using a commercial transducer operating at 0.926 MHz. Single-frequency continuous wave rf excitation was used in all cases. The known values of applied power cited herein are believed to differ from the values actually generated by an amount no greater than three percent of the cited values. The power meters were set for the highest possible display resolution in milliwatts and were used with the other operational settings (e.g.. averaging mode) unchanged from those established by the manufacturer. Distilled, recently-degassed water was the test medium. The instruments were allowed to warm up overnight before each test.
    Furnished with the instruments were conical targets of two designs. The target hereinafter called the “metal” one exposed bare sheet metal to the incident ultrasound, while the “rubber” target was covered with a material similar in appearance to sheet neoprene. The response of the two Instruments to sudden, large changes in Incident power was tested with both types of target installed in each instrument. Ten levels of power ranging from 0.1 W to 10.0 W were used for these tests. For each measurement, the re-zero button was pushed and the transducer was energized just after the readout indicated zero. The display was read after the stability lamp came on, except when the lamp had failed to extinguish while the displayed values increased. in this circumstance, it was necessary to continuously watch the display and take the reading the first time a stable indication obtained. The transducer was de-energized lust after reading the display and another reading taken when the display had again settled. This procedure was repeated at least five times for each of the ten levels of power used in testing each instrument with both targets.

Conclusions
    An experimental investigation of the operating characteristics of the ultrasound power meter designated UPM-DT-10 S/N 188 has allowed formulation of uncertainty estimate and of the operating conditions under which the estimates are valid, Because N88 has no knowledge or control of the degree to which the performance of other equipment might approach that ot the instruments tested, nor of the degree to which performance might vary over time, the uncertainty estimate cited herein should be construed to apply only to the designated devices while tested and at no other times. and should not be considered relevant to the performance of any other equipment during any period of time. From the test results for the instrument designed SIN 188, it is estimated that measured values diftered from the true values of applied ultrasonic power by an amount not greater than the sum of 0.01 W and three percent of the true values, tor true values between 0.16 W and 10 W. The present Investigators know of no way to predict accuracy or stability for operation under conditions different from the ones described for each instrument.

Signed by:
Donald G. Eitzen, Group Leader
Steven E. Fick. Electrical Engineer
Ultrasonic Standards Group
Automated Production Technology Division
Center for Manufacturing Engineering

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