1. What should I consider when selecting a measurement instrument?

There are two main components: (1) the hydrophone to detect the acoustic pressure and (2) the meter to process the acoustic signal.

When selecting the hydrophone, factors such as the frequency range, chemistry compatibility, robustness, size, and cost must all be taken into account. The HCT hydrophone has a wide bandwidth and can support frequencies up to 1.2 MHz. The shaft material is Teflon and is compatible with a broad range of chemistries. Finally, the HCT has an outer diameter of 3 mm and the sensing element is mechanically isolated from the shaft to localize measurements to a point.

To select the right meter depends on the importance of absolute versus relative measurements. Process control use-cases may consist of multiple production lines across different facilities and require absolute measurements to directly compare one tank to another. In this case, a calibrated MCT-2000 may be more suitable. Relative measurements with the MCT-1200 may be adequate for quick spot checks or studies to characterize process trends (e.g., time).

Another factor is the need to quantify the cavitation performance. For R&D and process control, it may be useful to acquire lower-level transient and stable cavitation data to develop, tune, and monitor a process.

Both hydrophones are compatible with either meter. The cavitation meter does require a hydrophone calibration.

Please review a product comparison chart HERE.

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2. What units of measure are relevant?

To evaluate the performance of an ultrasonic system (e.g., cleaning tank), one must take into account the relationships between voltage, pressure, intensity, power, and frequency.

Hydrophones are designed to measure the mechanical sound pressure in water. Most hydrophone sensors are piezoelectric materials which convert mechanical energy to electrical energy. The acoustic pressure (Pa) detected is converted to a voltage output (V) which can be acquired and analyzed with an instrument such as the MCT-2000 cavitation meter, MCT-1200 pressure meter, or even an oscilloscope.

Because each hydrophone has an inherent frequency response, an acoustic calibration of the hydrophone determines the sensitivity as a function of frequency. This is measured in units of electrical output per unit of physical pressure (V/Pa) over a frequency range (Hz) and is traceable to a primary calibration laboratory. Only with a calibration is it possible to make absolute measurements of physical pressure.

The acoustic waveform is the acoustic pressure as a function of time. By post-processing the waveform, the total pressure can be separated into the direct field and cavitation pressure. Click HERE for further information about how the pressure components are determined.

The acoustic intensity or acoustic power density (W/cm2) is the product of pressure and velocity at any location. Since particle velocity can be measured only under very specific conditions (typically not an ultrasonic tank), the approximation that velocity is the pressure divided by the impedance of the medium is commonly used. However, this approximation is only valid when away from the source – and when the wave propagates “cleanly” without the presence of reflections.

Ultrasonic systems consist of a transducer driven by electronics, such as a generator. The drive electronics delivers electrical power (W) to the transducers to generate the acoustic field. The efficiency () of an ultrasonic system can be estimated as the acoustic power inside the vessel divided by the electrical power delivered to the transducer. The efficiency for each ultrasound system will vary since it depends on many design factors.
For additional information about measurement units, please refer to HERE.

What should I beware of?

Relative measurements are common and have some merit. They describe the difference between ‘condition one’ and ‘condition two’, and are useful for comparative studies with a single variable (e.g., time). However, ultrasonic cleaning is based on a complex multi-variable system, and it is important to measure in standard units that accurately represent the ultrasound physics that do the cleaning.

Some instruments measure on a unitless scale of, say, one to ten. Some simply output the measured voltage. Others output slick units such as ‘Cavins’ or Cavitation Intensity. Unfortunately, the measured values represent the total acoustic signal, not just the cavitation.

Another unit of measure that has gained some traction is ‘Watts per Gallon’. It perhaps offers some usefulness as a relative measure. However, it should NOT be recognized as a standard unit of measure to characterize acoustic fields.

To prove this as an invalid unit of measure, let us consider an example by comparing two cleaning tanks of equal volume. Assume both tanks are 20 gallons; only each tank has different physical dimensions, say 12 x 16 x 24 in and 16 x 24 x 12 in. Also, assume identical generators are delivering 100 Watts to each tank. In both cases, 5 Watts per Gallon are evidently delivered to each tank. However, it would be inaccurate to claim this “power per volume” value as a useful measure of the cleaning performance at any given location. This is because ultrasound is directional and in any cleaning tank, there will be substantial acoustic reflections from many directions. Ultimately, the uniformity of the acoustic field is dependent on several system factors such as the tank geometry and construction, transducer configuration, and loading conditions. Not to mention, the electrical power being delivered to each transducer will result in some loss and not completely convert to acoustic power.

For reference, there is an analogous debate about the usefulness of ‘Watts per Gallon’ applied to fish aquariums. Here, ‘Watts per Gallon’ refers to the amount of light being delivered to a given size fish tank to indicate how well plants will grow. Here is one discussion thread similarly debunking this as a valid unit of measure.

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3. How often should I calibrate my hydrophone? How are they calibrated?

Typically, hydrophones are calibrated annually.

Each Onda hydrophone includes an acoustic calibration certificate which follows IEC 62127-2, by performing the calibration by comparison to a reference hydrophone whose calibration is traceable to NPL. The detailed calibration method is described HERE.

A copy of a traceability certificate for calibrations of hydrophones and preamplifiers at Onda is available upon request.

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4. How do I correlate the acoustics in my tank to the cleaning of my parts?

It may be useful to work backwards.

Two parameters commonly used to control a cleaning process include the particle removal efficiency (PRE) and the localized damage. These parameters are often determined by visual, optical, or chemical methods, counting and binning particle or defect information. To translate these parameters to the acoustic performance, a correlation study to determine how process variables such as frequency, generator power, chemistry, temperature etc. influence the acoustic parameters including the direct field, stable cavitation, and transient cavitation pressure.

For reference, there are published studies available HERE.

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5. What are the best practices to make consistent measurements?

1. Position the hydrophone in a consistent manner. Use a fixture to maintain the location in XYZ as well as the angular orientation, which all can contribute to the measurement repeatability. To view available fixtures from Onda, please click HERE.
2. Monitor and control the gas concentration of the solution which directly affects the cavitation level. A dissolved oxygen meter may be used.
3. Monitor and control the temperature of the solution which directly affects the cavitation level. A thermometer or thermal couple may be used.
4. Keep the surface level of the solution consistent. Changes in the surface level because of factors such as evaporation will affect the acoustic reflection behavior.
5. Ensure the output from the ultrasonic transducers is stable when measuring. Some tanks require some time to stabilize.
6. Check your measurement instrument by routinely testing it in a stable reference tank under controlled conditions. Acoustically calibrate the instrument on a periodic basis (e.g., annually). For more information about Onda’s calibration services, please CONTACT US.

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