An objective comparison of commercially-available cavitation meters

Accepted Manuscript An objective comparison of commercially-available cavitation meters Daniel Sarno, Mark Hodnett, Lian Wang, Bajram Zeqiri PII: DOI: Reference:

S1350-4177(16)30166-3 http://dx.doi.org/10.1016/j.ultsonch.2016.05.024 ULTSON 3234

To appear in:

Ultrasonics Sonochemistry

Received Date: Revised Date: Accepted Date:

24 November 2015 6 May 2016 13 May 2016

Please cite this article as: D. Sarno, M. Hodnett, L. Wang, B. Zeqiri, An objective comparison of commerciallyavailable cavitation meters, Ultrasonics Sonochemistry (2016), doi: http://dx.doi.org/10.1016/j.ultsonch. 2016.05.024

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An objective comparison of commercially-available cavitation meters Daniel Sarno, Mark Hodnett, Lian Wang and Bajram Zeqiri Acoustics and Ionising Radiation Division National Physical Laboratory Hampton Road Teddington Middlesex TW11 0LW [email protected]

Abstract With a number of cavitation meters on the market which claim to characterise fields in ultrasonic cleaning baths, this paper provides an objective comparison of a selection of these devices and establishes the extent to which their claims are met. The National Physical Laboratory’s multi-frequency ultrasonic reference vessel provided the stable 21.06 kHz field, above and below the inertial cavitation threshold, as a test bed for the sensor comparison. Measurements from these devices were evaluated in relation to the known acoustic pressure distribution in the cavitating vessel as a means of identifying the mode of operation of the sensors and to examine the particular indicator of cavitation activity which they deliver. Through the comparison with megahertz filtered acoustic signals generated by inertial cavitation, it was determined that the majority of the cavitation meters used in this study responded to acoustic pressure generated by the direct applied acoustic field and therefore tended to overestimate the occurrence of cavitation within the vessel, giving non-zero responses under conditions when there was known to be no inertial cavitation occurring with the reference vessel. This has implications for interpreting the data they provide in user applications.

1. Introduction The growth, oscillation and collapse of bubbles in a fluid caused by high power ultrasound, known as acoustic cavitation [1, 2], is a phenomenon utilised throughout manufacturing industry. Alongside applications of sonochemistry, sonocrystallisation and sonoprocessing, perhaps the most established practice is ultrasonic cleaning [3]. Fluid movements in the vicinity of cavitating bubbles – such as jetting, streaming and acoustic shockwaves – are considered to be the primary mechanisms for particle removal from surfaces [4]. Cleaning applications span a frequency range from 20 kHz to 3 MHz, dependent on the substrate and soil removal requirement [5, 6], and have been used for many decades in industries such as the medical and aerospace sectors. However, measurement of the cavitation generated in these vessels, long considered most relevant to the cleaning activity itself - has not yet reached consensus for standardisation [3, 7]. Proposed characterisation techniques for ultrasonic cleaners vary greatly in their approaches. Current standards in the medical industry [8] specify the aluminium foil test which, while possessing the ability to resolve a cavitation field spatially [9], can only provide qualitative and at best comparative measurements. Additionally, the procedure contaminates the cleaning bath it tests. Specifically designed soil test products like GKE Clean-Record® and Ultrawave STF Load Check Indicator Strips are also only semi-quantitative tests and therefore suffer from the same limitations. Other, more quantifiable, methodologies such as calorimetric measurements do not provide spatial resolution information, crucial in identifying the likely distribution of cleaning effectiveness within a vessel [9]. Characterisation of the acoustic and cavitation fields in ultrasonic cleaners is hence one of the most important stepping stones in the ultimate goal of a standardised, traceable cleaning effectiveness measurement, addressing the requirements of cleaning device manufacturers and users alike. Building on this paper, subsequent studies will correlate acoustical data with effectiveness measures such as particle removal efficiency. Acoustic measurements offer perhaps the most promising solution to cavitation field characterisation [10], with many commercial devices available to industry which claim to do just that (see Section 3 below). The phenomenon of acoustic emissions generated by cavitation has clear indicators such as its distinctive noise and frequency components [2, 11]. A significant contributor to cavitation noise comes from the impulses arising from individual and collective bubble collapses. Shockwaves from inertial cavitation generate broadband noise ranging from audible sound up to megahertz frequencies [12], whose level relates to the amount of cavitation being generated within the particular system, and in turn, to other observables such as material erosion and sonochemistry [13]. Additionally, acoustically driven nonlinear oscillations of bubbles, non-inertial cavitation, produce secondary waves of the vibrating wall of the bubble which appear as sub-harmonics and harmonics of the drive frequency in the spectrum [2]. Finally, acoustic pressure can itself be taken as an

indicator of where cavitation is likely to form as areas of elevated peak-negative (rarefactional) acoustic pressure, beyond a threshold [14], will instigate inertial cavitation. The link between the applied acoustic field and cavitation field is not a direct one however, as the presence of bubbles in the system perturb the fields through mechanisms such as the absorption and scattering, in addition to altering the water’s properties such as the speed of sound. In particular, for systems operating at or near the threshold pressures of acoustic cavitation, only selected fluid regions where peak negative pressures exceed this threshold may exhibit cavitation behaviour. These acoustical indicators of cavitation are utilised in various ways as detection methods by the devices studied in this paper [15, 14].

2. Concept of an ideal sensor Prior to describing the measurement results for the various devices studied using the NPL reference cavitation vessel described in Section 4, it is useful to identify the performance characteristics that an ideal measurement device should possess. Whilst there have been a number of reviews of measurements in this area [2, 11, 16, 17] that have highlighted potential methods based on the detection of the secondary effects of cavitation such as sonoluminescence or chemiluminescence [18], free radical production [19] or erosion [20], significant research activity has been applied to study methods based on acoustic emission [12, 21, 15]. Indeed, the measurement devices described in Section 3 that have been the focus of this study, generally work in this way, through employing some form of acoustic detection and signal conditioning. It is well established [14] that the spectral content of the acoustic waveform at any point contains information about the driving acoustic field, which we will call the “Direct Field”, in conjunction with acoustic radiation arising from cavitation, generated by the acoustically-induced oscillation and collapse of cavitating bubbles within the liquid medium. We will call this second component the “Cavitation Field”, and in itself it contains further information about the energetics of the cavitation process generated through the sub- and ultra-harmonics, harmonics and broadband cavitation noise continuum [22]. These two distinct parameters, the “Cavitation” and “Direct” components of the acoustic field, constitute the measurement quantities which will be considered here, or the measurands. Table 1 describes these two quantities in relation to idealised properties of the measurement device; specifically, the spatial resolution, its bandwidth and calibration. Other characteristics of the device affecting its suitability for practical application are clearly also important, such as its robustness to withstand potentially damaging effects of cavitation, its maintenance of a stable calibration under the demanding conditions encountered, and a suitable geometry to be useful in confined spaces and in loaded vessel configurations.

These two acoustic measurands describing the “Direct” and “Cavitation” fields provide distinctly different and potentially complementary information about what is happening within a cavitating environment such as a cleaning vessel. This can be understood through the following analysis. The acoustic pressure generated within the vessel at any point is clearly key in terms of controlling the nature and degree of cavitation which occurs at that point. Additionally, it is important to characterise fluid content, as dissolved gases provide nucleation sites for cavitation. At 25 kHz for example, the threshold for acoustic cavitation in water at ambient conditions is close to 100 kPa [14]. Cavitation is a highly nonlinear process and so at higher applied acoustic pressure levels energy is progressively removed from the “Direct” acoustic field into the “Cavitation” field, with former typically saturating whilst the latter continues to increase [23]. An instance where this might occur is one in which the cleaning vessel is driven sufficiently hard that we achieve cavitation shielding close to the transducer, preventing transmission of acoustic pressure to the point of interest within the vessel. In this scenario, a device with relatively poor spatial resolution of the Cavitation field might measure an increase in Cavitation signal as it is likely to measure an integration of the signals from an unspecific volume of the vessel. Whereas the “Direct” acoustic field signal might actually reduce at these elevated transducer drive levels due to cavitation shielding. Table 1 - Summary of the desirable characteristics of an idealised measurement device able to identify the spatial distribution of acoustic and cavitation-related acoustic signals generated within an ultrasonic cleaning device. Here, spatial resolution quantifies how close regions of high cavitation activity can be to each other but still be resolved Property

Spatial resolution

Measurement bandwidth

Measurand:

Measurand:

Direct Field

Cavitation Field

Defined by the geometry of the active surface of the hydrophone, or similar device, positioned at a location in the environment under test. For systems with strong acoustic fine structure, for example generated by the existence of acoustic standing-waves, spatialaveraging over the active surface of the device may occur leading to an underestimate in the acoustic pressure being applied.

For acoustic methods, this is generally unknown as the cavitation bubble events contributing to the output signal of the device may be of uncertain location within the vessel and the sensor output will be a spatial integration of events. Sensor geometry can be used to endow the device with spatial resolution [21]. Neppiras [24] indicated that some spatial resolution could be induced within a device by monitoring at very high frequencies (> 15 MHz) where absorption of acoustic waves within the fluid medium restricts the sensitive spatial volume of the device.

The requirement here is for the device to have a suitable sensitivity at the frequency or frequencies of interest.

Must be broad enough to reliably capture the cavitation-induced acoustic radiation: both the discrete signals related to the

Calibration

For the frequency range of interest here (close to 21 kHz), this normally demands an underwater acoustics hydrophone, such as those produced by Reson or Brüel and Kjær.

driving or fundamental frequency and the broadband continuum radiation which has been linked to inertial cavitation [2, 12]. There is evidence that for an ultrasonic cleaning environment, the bandwidth must be in excess of 5 MHz to reliably capture energy contained in the broadband continuum [9]. For sonochemistry devices, acoustic signals up to 20 MHz can be generated [23].

Typically given in terms of µV Pa-1, calibration is provided by manufacturers or specialist measurement laboratories. Calibration involves positioning the device in a known acoustic pressure field and recording the device output. It should be noted that this calibration can be strongly dependent on temperature.

This cannot currently be achieved for devices which respond directly to cavitation, although attempts are being made to establish an environment where known and reproducible cavitation can be applied at specific points with a “cavitation reference” vessel [14, 25, 26].

3. Devices In total five commercially-available cavitation meters were compared within the NPL cavitation reference field along with two underwater acoustics hydrophones employed to measure the acoustic pressure distribution. The hydrophones used in this study were the Reson TC4038 and the Reson TC4013. As supplied, both of these devices are prone to damage by cavitation and were therefore not useable in strongly cavitating fields. Reson’s TC4013, the larger probe diameter of the two, is more sensitive and has a frequency range of 1 Hz to 170 kHz while the TC4038 is specifically designed as a standard reference hydrophone for a broader frequency range of 10 kHz to 800 kHz [27, 28]. The commercial cavitation meters are: CM-3100 from Alexy Associates, HCT-0310 from Onda, pb502 from PPB Megasonics, Hygea Ultrasonic Activity Meter from Ultrawave and NPL’s CaviSensorTM device, and are shown in Figure 1. Figure 1 – Photograph of the hydrophones and commercial cavitation meters used in this study. From left to right: TC4038, TC4013, CM-3-100, HCT-0310, pb502, Hygea Ultrasonic Activity Meter and CaviSensorTM CM-3-100 Alexy Associates Corp. (Swan Lake, NY, USA) are the manufacturers of the CM-3100, of which there are a number of similar designs on the market. The meter consists of a 450 mm long stainless steel cylindrical probe which is permanently fixed to an analogue meter at the top of the handheld system. This is also where the sensing element is found, with the cylindrical tube acting as a waveguide. The materials used, the sensing element location and overall design of this meter aim to make it cavitation-damage resistant. The device manual states it measures “an integrated signal based on the total intensity in the cleaning tank in the immediate vicinity of the

probe” [29], implying it has spatial resolution. The CM-3-100 measures cavitation activity in units of “Cavins” (cavitation intensity), where 1 W/gal is approximately equivalent to 5.2 Cavins and 1000 Cavins is deemed the “highest intensity generally encountered in present typical high-intensity aqueous cleaning systems” [29]. The analogue meter includes two scales, 0-100 Cavins and 0-1000 Cavins, with a x10 switch as a selector. The recommended mode of operation states that in order to monitor cavitation in a system the user must first take readings in various locations in a system in which standards have been previously established under optimum conditions. The performance of the system over time is then compared to these initial readings. HCT-0310 The Cleaning Tank Hydrophone, HCT-0310, manufactured by Onda (Sunnyvale, CA) is a 300 mm long probe with a small 3 mm diameter. The probe tip is a “single-point” design with a sensing element that is mechanically isolated from the chemically-inert (Teflon) shaft to enable the acoustic field of ultrasonic cleaning tanks to be spatially mapped in detail [30]. As such, it is designed to be more robust than the Reson devices described above. The hydrophone includes an acoustic calibration from 30 kHz to 300 kHz, with an option to extend up to 1.2 MHz, covering typical ultrasonic cleaning ranges. All calibration measurements are traceable to national standards, in terms of acoustic pressure. This device can be used in conjunction with Onda’s companion handheld electronics, the MCT-1010, to provide measurement readouts in volts to the user, but it was not available for this study: instead, it was connected to a voltmeter (see Section 4). pb-502 Ultrasonic/Megasonic Energy Meter The pb-502 manufactured by PPB Megasonics (Lake Oswego, OR, USA) is a hemispherical shaped probe with a 60 mm diameter and an acoustic lens base. The device claims to provide users with an RMS of the cavitation energy intensity by isolating the acoustic signal relating to cavitation implosions. The sensing element detects higher frequencies than hydrophones and it is these megasonic frequencies that are used as an indication of cavitation implosion shockwaves and also gives the probe high spatial resolution [31]. In use, the probe is connected to a rechargeable handheld meter displaying a value on-screen corresponding to the cavitation activity in W/gal, which implies a power density measurand. The manufacturer provides a NIST calibration for the device on request, although it is not clear to which primary standard unit this is traceable. Hygea Ultrasonic Activity Meter Ultrawave (Cardiff, UK) are the manufacturers of the Hygea Ultrasonic Activity Meter, a simple handheld device used to comparatively measure the performance of ultrasonic cleaning baths over time. It has a rugged construction like the CM-3-100, with a rigid metal probe of 13 mm in diameter, housing piezoelectric detection elements. With a working range of 5 kHz to 50 kHz inclusive of typical ultrasonic cleaning systems, the undefined unit of cavitation displayed on the meter is a percentage reading of 10% increments and it is designed to be used for relative checks only [32]. CaviSensorTM

The hollow cylindrical design of the National Physical Laboratory’s CaviSensorTM enables high spatial resolution sensing of cavitation in ultrasonic systems [9, 21, 14]. The outer material absorbs megahertz frequencies produced by cavitation outside the sensor, meaning that signals detected in this range must arise from acoustic cavitation inside the fluid volume enclosed by the sensor. Along the sensor’s cylindrical axis, a combination of the active region of the sensor being approximately 8 mm less in height than the encapsulating cylindrical absorber, together with theoretical modelling, suggests an ‘end correction’ of less than 1 mm. Modelling consisted of the sensor’s response being computed for a MHz-frequency point source moved throughout its volume [21]. The CaviSensorTM is used with a bespoke signal processing unit, known as the CaviMeterTM [14]. In the manner described in Section 2 above, the CaviMeterTM analyses the received signal and displays simultaneously the Direct field (DF) of the ultrasound generated by the system under test, and the Cavitation Activity (CA) in the megahertz frequency range, arising from cavitation implosion shockwaves.

Device

Manufacturer

Detection Method and Parameter

Dimensions

CM-3-100

Alexy Associates Inc.

Acoustic energy

HCT-0310

Onda

Broadband acoustic pressure

pb-502

PPB Megasonics

Acoustic power density

Hygea Ultrasonic Activity Meter CaviSensorTM and CaviMeterTM

Ultrawave

Acoustic energy

Length: 610 mm Diameter: 58 mm (head) Length: 244 mm Diameter: 15 mm

National Physical Laboratory

Broadband acoustic pressure (DF and CA)

Length: 34 mm Diameter: 38 mm (external), 28 mm (internal)

Length: 450 mm Diameter: 12.7 mm Length: 300 mm Diameter: 3 mm

Frequency Range

Sensitivity

20 to 120 kHz

“ Sensitivity is +/- 0.2%”

30 to 300 kHz (300 kHz to 1.2 MHz optional) 0 to 5 MHz

1 V/MPa (typical)

5 to 50 kHz

Relative measurement only

Direct field: 20 - 130 kHz Cavitation field: 1.5 7 MHz

1.6V/MPa (typical)

Table 2 – A summary of the devices used in this study [29, 30, 31, 32, 9]

0.04 V/W.in-2

4. Experimental Protocol Diagram and Equipment

Figure 2 – Schematic setup of the multi-frequency reference vessel and commercial cavitation sensor Reference vessel The ultrasound field used to test the devices was generated using NPL’s multifrequency reference vessel, shown schematically and pictorially in Figures 2 and 3 respectively. Detailed descriptions of the system have been reported previously [26, 33], but in brief, the system comprises a 13 litre stainless steel cylindrical tank, to which 21 Tonpilz transducers are bonded, deployed in three rows of seven sources. Each row can operate at the fundamental and third harmonic frequencies corresponding to the transducer dimensions, such that the vessel has six operating frequency windows, centred around 21 kHz, 35 kHz, 44 kHz, 59 kHz, 92 kHz and 136 kHz. It is operated by a lock-in amplifier drive system, allowing the driving frequency to be tightly controlled, in turn ensuring the repeatability of the acoustic field generated within the cylindrical reactor cavity. The system was designed by NPL and realised by Sonic Systems (Puckington, Somerset, UK). It can be operated at drive levels below or above the inertial cavitation threshold, and in this study, drive levels of 50 and 200 mV were predominantly used. Figure 3 – Photograph of NPL’s multi-frequency reference vessel Each sensor under test was suspended from a 3-axis positioning system over the vessel (LG Motion, Basingstoke, UK), providing positioning capability with a resolution of 5 µm. The positioning system, vessel and acquisition electronics (where used, see below) were all controlled by a PC running in-house software written in NI LabVIEW (USA). Water medium In typical industrial practices, cleaning baths are filled with tap water whose naturally occurring gas content has been shown to be crucial to cleaning action [34]. On the other hand, tap water does not provide a stable, reproducible medium for experimentation. As a result, this study used filtered (95% of particulate >5 µm removal) de-ionized tap water prepared by an ELGA (High Wycombe, UK) Option 30 system [26]. Micro-90 surfactant, 0.2% by volume (Cole-Parmer, UK), was added to the water medium, in line with previous studies [9, 14, 25]. Surfactant was found to improve vessel and sensor wetting and so minimises the number of large bubbles forming on the vessel wall and sensor surfaces which have the potential to affect both the acoustic field and cavitation measurements. Water temperature was monitored by a Hanna Instruments (Woonsocket, RI, USA) HI-98127 before and after each test was performed. The reference vessel was refilled after approximately every five measurement runs (which consisted of any

combination of sensor and drive level, so randomising the tests when day-to-day repeats are also incorporated), each time filling up to a scribed level on the vessel inner wall. Scan dimensions Each measurement device used in this study was geometrically aligned such that the acoustic centre of each was aligned with the experimental origin – defined as the water’s surface on the cylindrical axis of symmetry of the vessel. Alignment was performed using the positioning software with an uncertainty estimated to be ±0.5 mm. Devices were orientated vertically along the central cylindrical axis of the vessel with the exception of the pb-502 whose angled dome design meant its base was aligned parallel to the vessel base. One-dimensional line scans were performed for each sensor, with the acoustic centre (where known) of each device set at a depth of 74 mm below the water surface. This depth was chosen as it is located at a peak in the 21 kHz acoustic field along the cylindrical axis as measured in previous studies [26]. Scans were carried out in the Xdirection across the vessel (see Figure 2), traversing as large as distance allowable by the vessel and the relative size of each device. For the smallest sized probes, Onda’s HCT-0310 for example, this resulted in a line scan from -120 mm to 120 mm in the X-direction in 2 mm increments. Measurement Due to each system employing a different indicator, the measured quantity was not consistent between all meters. Reson’s TC4038, TC4013, Onda’s HCT-0310 and the NPL CaviSensorTM/CaviMeterTM all produce voltages proportional to acoustic pressure, through a BNC output, and so this was measured on a Rohde & Schwarz (Munich, Germany) URE3 voltmeter with a data acquisition link to a PC. For the remaining devices which did not have the option of PC data acquisition, like the CM3-100 and the Hygea Ultrasonic Activity Meter, measurements were read manually. Four measurements were taken in each position at regular two second time intervals, generating an average and standard deviation unit of the device measurand. It is important to note, the vessel remained running during the course of the whole scan to avoid transients. For devices which had a number sensitivity ranges, such as the CM3-100, a conversion factor generated between the ranges by performing comparative measurements at various positions in the vessel. Each scan was carried out at least four times, often on separate days and with fresh water samples to provide meaningful statistics on repeatability. Field conditions For each device, scans were performed using a range of the available frequency windows for the vessel, at up to four drive levels (50 mV, 100 mV, 150 mV and 200 mV, set using the Ametek lock-in amplifier). Table 3 gives some of these drive levels and the voltages delivered to transducers (RMS) as measured by a Tektronix TDS 784D digital scope. The power to transducers was estimated by measuring the voltage output from the E&I power amplifier to a load impedance of 50 Ω from the transducers through a matching circuit. The lower frequency acoustic field of 21.06 kHz was chosen to provide reference conditions for this study as it was the most resistant to perturbation by the presence of the sensors and therefore the most stable and repeatable field. Previous studies using this vessel have found higher frequency

fields are less reproducible due to their high sensitivity to water level and temperature changes [26]. Table 3 – A selection of reference vessel drive levels used in this study and their estimated resulting powers Drive Level PreAmplification (mV RMS) 50 100 200

Voltage to the Transducers (V RMS) 52.0 105.0 180.0

Estimated Power to the Transducers (W) 3.9 14.6 38.7

5. Results and discussion As described above, the reference vessel can be operated at drive levels which span the inertial cavitation threshold [26, 33]. This enables comparison of the driving field across the devices measured, and also an assessment of their outputs under cavitating conditions. For clarity in comparison, vessel drive levels of 50 mV and 200 mV are mainly considered with selected instances where more drive levels are shown to illustrate sensor responses in more complex fields. Similarly, whilst measurements at four frequencies were made, only results from the lowest frequency window of 21.06 kHz are presented. Our early conclusions of measurements at elevated fundamental frequencies are consistent with the findings from the 21.06 kHz data and may form a future publication. Through monitoring the water temperature both before and after each scan, temperature increase was dependant on the amount of time it took to perform a particular scan. The temperature increase never exceeded 4 ºC during any particular scan and the average temperature of the vessel water across all scans was approximately 22 ºC. In the absence of standardised methods for calibrating cavitation activity, the 'reference' measurement technique for the vessel is considered to be the results obtained from the three calibrated, small-element sonar and cavitation hydrophones. Data obtained using the Reson TC4038 and TC4013 are thus shown in Figure 4(a), along with the results of the pressure calibrated HCT-0310 at the same low drive level for comparison. The 21.06 kHz acoustic field at 50 mV power consists of five peaks with the largest central peak at 0 mm and four smaller peaks either side at -110 mm, 50 mm, 50 mm and 90 mm [26]. The central peak has a maximum peak negative pressure of 120 kPa with the smaller subsidiary peaks around 60 kPa. The TC4038, TC4013 and HCT-310 all agree well, measuring on-axis pressures which are just above the inertial cavitation threshold, as determined in this and other reference systems at NPL [14].

Figure 4 – (a) Pressure measurements generated by the TC4038, TC4013 and HCT0310 of the reference vessel at 50 mV drive, (b) Pressure measurements using the HCT-0310 at 50 mV, 100 mV and 200 mV drive. Plotted error bars are ±1 standard deviation The two Reson hydrophones have a maximum operating pressure of 200 kPa [28, 27] so measurement of the acoustic field at drives above 50 mV was performed using the HCT-0310 only. As shown in Figure 4(b), the field at 100 mV and 200 mV drives retains the same features of the lower drive field with five peaks in the same locations. The HCT-0310 measures a central peak which is broader at 200 mV compared to 50 mV with a maximum pressure of 300 kPa, which may arise from the direct field being scattered by the cavitation bubble cloud at the centre of the vessel. The smaller peaks all have maximum pressures of approximately 160 kPa with the exception of the peak at 80 mm which has a larger pressure of 210 kPa. It is important to note that the quadrupling of drive voltage, from 50 mV to 200 mV, only results in the HCT-0310 measuring a roughly doubling of pressure in the on-axis

region. This observation can be attributed to the acoustic energy at 21 kHz being transferred to higher harmonics and broadband frequencies once cavitation establishes, demonstrating the non-linearity of the cavitation phenomenon [2].

Figure 5 – A comparison of normalised cavitation level as measured by the commercially-available cavitation meters at 50 mV drive. Plotted error bars are ±1 standard deviation. Note: the HCT-0310 error bars are too small to be visible on this plot Figure 5 shows the comparison of the commercial cavitation meters at the 50 mV drive level. Due to each meter differing in measurement unit, the data is plotted as a normalised cavitation level relative to the central point at 0 mm. The agreement in the spatial variation across the vessel is, on first inspection, quite encouraging, although it is clear that the CM-3-100 profile shows an axial peak with a -6 dB width which is less than half of the other systems. This suggests that the displayed value of Cavins represents a parameter proportional to the square of the acoustic pressure, i.e. energy, or intensity. It is important to note that the point at -120 mm for the CM-3-100 plot was removed as it was greater than the central point to which all plots are normalised. However, the peak to the far left of this line scan is still represented by the point at 115mm and is a feature not measured by the other meters but is persistent throughout repeats. The reason for this is unclear but may relate to electrical pick-up from the transducer located on the far left wall. The CM-3-100 also differs from the other cavitation meters in only registering peak in Cavin levels at the central axis and at 80 mm, which suggests it has a lower signal-to-noise ratio performance. The pb502 shows strong agreement with the HCT-0310 which in turn correlates well with the pressure distribution of the vessel as shown in Figure 4(a). The offset in peak position seen in the pb502 data, particularly the 15 mm offset of the peak at 35 mm, can be attributed to the large sensor dimensions outlined in Section 3. The 58 mm diameter probe head is likely to have a significant spatial averaging and field perturbation effect. This may also account for the asymmetric cavitation level response of the pb502 at 35 mm in comparison to the HCT-0310. The Hygea Ultrasonic Activity Meter by Ultrawave did not register a cavitation level with 50 mV drive at all positions along the line scan and has been plotted as such. A possible explanation for this is a difference in threshold performance, signal-to-noise or this device having a lower sensitivity in comparison with the other cavitation meters.

Figure 6 - A comparison of normalised cavitation level as measured by the commercially-available cavitation meters at 200 mV drive. Plotted error bars are ±1 standard deviation For the 200 mV drive level results (Figure 6), the agreement across most of the devices is also generally good, with each device picking out the main features across the vessel. At 200 mV, Hygea Ultrasonic Activity Meter now registers cavitation activity in the vessel, although it appears to have low special resolution in comparison with the other devices. The Hygea detects elevated levels towards the left side of the

line scan in a similar manner to the CM-3-100 at 50 mV (Figure 5) which also may relate to electrical pick-up from the transducer. As before, the CM-3-100 profile shows a narrower axial peak: the -6 dB width of the central lobe is comparable to the 50 mV results. The CM-3-100 loses the -120 mm feature as seen at 50 mV but retain the two peaks at 0 mm and 80 mm. The pb502 result again shows heightened values for the ancillary maxima, and a lower overall dynamic range. Care was taken to ensure that the meter values were not saturating during the test, and so it is considered that this difference is due to the device itself, perhaps because of spatial averaging, or to non-linearity in the device detection electronics. As suggested above, the design of the pb502 is likely to have a significant perturbation effect. To a first order, comparing the results at two drive levels suggests that the data may simply be scaled by a suitable factor to predict one data set from the other. Such an approach may be suitable for acoustic pressure determination (although non-linear effects must be considered, as described above) but the key parameter of interest, see Section 2, is the cavitation generated by the pressure field. This is the claimed measurand by many of the devices tested, although detailed device or data processing specifications are not necessarily provided in most cases.

Figure 7 – A graph of high frequency filtered data of the HCT-0310 and a CaviSensorTM at 50 mV drive. Plotted error bars are ±1 standard deviation To determine the spatial variation of inertial cavitation across the same x-axis direction, two measurement sets were carried out using the CaviSensorTM and CaviMeterTM [14], measuring the average RMS voltage over the range 1.5 MHz to 7 MHz, as a way of quantifying the cavitation field. This acoustic emission frequency range has been shown previously to be comparable to secondary effects of cavitation [9, 13], and so is considered a robust measure of inertial cavitation. The HCT-0310 was also able to be used with the CaviMeterTM, and the results obtained at 50 mV and 200 mV drive levels are shown in Figures 7 and 8, respectively. The 50 mV drive setting, which is close to the acoustic pressure threshold shows clear differences when compared to Figure 5: there is a clearly-defined central peak, but no other obvious features. This demonstrates that at this drive level, inertial cavitation is only achieved around the central lobe, and the size of the HCT-0310 error bars suggests that its level varies strongly. At 200 mV drive level, inertial cavitation occurs across the vessel, with a structure more in-line with the pressure distributions seen in Figures 5 and 6. Comparing the two devices shows that at the 50 mV drive, the HCT-0310 detects a slow ramping in the cavitation level towards the centre, whereas the CaviSensorTM is more distinct. This is due to the differing spatial resolution characteristics of the two devices: the CaviSensorTM's cylindrical absorbing construction means that the megahertz signals measured are specific to the central axis of the cylinder, whereas the HCT-0310 detects signals over a wider locus, and is hence less able to resolve the local structure. Again, elevated levels on the left side of the scan could relate to electrical pick-up from the transducer which is in close proximity, in a similar manner to the data from the CM-3-100 and Hygea presented in Figures 5 and 6, respectively. At 200 mV, both sensors detect the ancillary maxima, although they appear to be

more prominent for the HCT-0310 measurements. The reasons for this are unclear, although may again be related to the less well-defined spatial resolution of the HCT device, such that the apparent peak at x = 80 mm is formed of contributions from a wider spatial range of the vessel.

Figure 8 - A graph of high frequency filtered data of the HCT-0310 and a CaviSensorTM at 200 mV drive. Plotted error bars are ±1 standard deviation Figures 9(a) and 9(b) also displays the high frequency data shown in Figures 7 and 8. Here, acoustic pressure measurements are presented against the cavitation level recorded by the same device - Onda’s HCT-0310, both with and without CaviMeterTM high frequency filtration. In both figures, the background noise-equivalent cavitation signal of 0.002 has been subtracted from the cavitation plots and the profiles have been scaled to the maximum signals recorded in the measurement run. With the stochastic nature of cavitation, there is an uncertainty in doing this, although, at the low drive level of 50 mV seen in Figure 9(a), there is some suggestion the width of the measured cavitating region is slightly larger than the cavitating region indicated by the pressure measurements. Under the 50 mV drive conditions, we know that inertial cavitation is only occurring at a location close to the central axis of the reference vessel. Elsewhere in the vessel, the acoustic pressure is less than 60 kPa, and there should be no inertial cavitation occurring, nevertheless, the high frequency signal from the Onda HCT-0310 is non-negligible. This signal probably arises from the Onda device detecting high frequency emissions from the acoustic pressure maximum (x=0), but it may also originate from high frequency emissions generated by stable cavitation, whose signals fall within the filtered measurement bandwidth. In Figure 9(b), it is clear that, across the majority of the reference vessel, the acoustic pressure is above the 100 kPa inertial cavitation threshold [14], such that most of the vessel is cavitating inertially. Agreement between the two sets of measurements, in terms of the spatial profile, is particularly noteworthy around the central peak and the off-axis peak shown at +80 mm, with the HCT-0310 measuring elevated cavitation levels over the same region as high peak negative acoustic pressures. Good agreement in the profiles may arise because the device is located within regions of the vessel where inertial cavitation is high, which dominates the high frequency signal. In contrast, differences between the two measurands are found in the rest of the line scan where acoustic pressure measurements are generally not found to be a good indicator of localised cavitation level. This is probably because, due to the spatial resolution of the Onda device, its high frequency output signal (cavitation) represents a convolution of its response to cavitation events occurring through the vessel. The directional response of the device at MHz frequencies may also be relevant. s It should also be noted that the acoustic pressure values may themselves be contaminated by high frequency signals arising from cavitation events, although these are generally dominated by the fundamental driving frequency of the reference vessel field. Figure 9 – (a) Comparison of acoustic pressure and cavitation level measured by the HCT-0310 at 50 mV drive level, (b) Comparison of acoustic pressure and cavitation level measured by the HCT-0310 at 200 mV drive level. Plotted error bars are ±1 standard deviation. Note: the pressure error bars in (a) are too small to be visible on this plot

As an additional comparison of the cavitation meters, scans were performed at lower drive levels of 10 mV and 20 mV which are below the on-axis cavitation threshold pressure of the reference vessel. With high frequency filtering from the CaviMeterTM, both the CaviSensorTM and HCT-0310 detect no cavitation above the noise floor at 10 mV drive. This is in line with bubble clouds not being observed in the fluid at 10 mV and no audible emissions from cavitation occurring within the vessel. The CM-3-100 also registers no cavitation at all positions along the x-scan at this lowest drive level, which may mean that it does respond specifically to cavitation, or the sensitivity of this device is too low to indicate a Cavin reading. As with the results of the Hygea Ultrasonic Activity Meter at 50 mV, this device also did not register a cavitation reading at 10 mV drive level, again potentially resulting from a difference in threshold or signal-to-noise performance. The limited bandwidth of the device (see Table 2) suggests that it would not detect the broadband emissions from cavitation. Spectral analysis of the acoustic emission at 20 mV drive reveals the presence of noninertial (stable) cavitation at the centre of the vessel, as indicated by harmonics and sub-harmonics of the drive frequency. This central activity is registered by both the CaviSensorTM and HCT-0310 with high frequency filtering. The CM-3-100 also detects cavitation activity at this drive, however it is found in all regions along the xscan, not just localised to the on-axis peak, and so this strengthens the view that the device does not respond specifically to cavitation. The distinction between measurements of inertial and non-inertial cavitation is not made by the majority of meters in this paper and will be an interesting topic for future studies. It is clear from comparing Figure 5 and Figure 7, that at the 50 mV drive level, all of the devices except the HCT-0310 and the CaviSensorTM (both with HF filtered data) respond only to the low frequency acoustic pressure field variation, and not the inertial cavitation. The good agreement between devices, even with differing bandwidths, is because the acoustic field is being dominated by the 21 kHz component. At 200 mV, Figure 6 and Figure 8 seem to show a better agreement, but this is likely to be a consequence of the direct field acoustic pressure exceeding the inertial cavitation threshold throughout much of the scan. This is an important finding from this carefully controlled study, as users of some of the meters tested here should be aware that for cavitation applications which generate acoustic pressures only marginally above the threshold, their meter may suggest a greater region of the fluid is cavitating than is actually the case. Hence, to give the most accurate depiction of inertial cavitation occurring in a fluid, a meter which responds specifically to the cavitation phenomenon is required, with the particular characteristics identified in Section 2. The results presented here suggest that progressing towards standardisation can be addressed on two specific aspects: a) Developing methods and parameters to determine the spatial distribution of the direct field acoustic pressure generated within cavitating vessel. b) Developing devices, methods and parameters specifically for determining cavitation activity, and within this, to potentially discriminate between inertial and non-inertial types.

To a significant extent, the devices required to address the first aspect are readily available, as are the standards (from Working Group 8 of IEC TC87) which specify their calibration. The second aspect is the topic of ongoing research.

6. Conclusions This paper has presented a study of commercially-available cavitation meters which can be accessed by industry to characterise the levels of cavitation within ultrasonic cleaning baths. Through comparison of each device’s performance in a stable, well characterised, cavitation reference vessel operating at 21 kHz, at spatial locations where the acoustic pressure is above and below the inertial cavitation threshold, the study establishes that four of the six devices tested actually respond to the direct applied acoustic pressure, rather than the resultant cavitation. This approach has the potential to overestimate the level of cavitation in a vessel, with the implication of users believing that their cleaning baths are generating ‘useful’ cavitation over a wider region of fluid than they may do. To provide more useful measurements of ultrasonic cleaning vessels for industry, cavitation meters should ideally deliver more specific indicators of cavitation other than acoustic pressure. Additionally, the ideal sensor must have a good spatial resolution of the field and possess a construction and design which minimally perturbs the acoustic field which they are measuring.

7. Acknowledgements The authors gratefully acknowledge the collaboration of Onda Corp. with the loan of measurement devices used in this study, and the financial support of the National Measurement Office of the United Kingdom Department for Business, Innovation and Skills.

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Highlights of “An objective comparison of commercially-available cavitation meters” • • • •

First objective comparison of commercially-available cavitation meters Most of the tested meters actually responded to direct applied acoustic pressure Users of these devices could overestimate the level of cavitation in their vessels Cavitation meters should deliver more specific indicators of cavitation