To avoid resonant excitation of the tower by thrust fluctuations at blade-passing frequency or, to a lesser extent at rotational frequency, the lowest natural frequencies of the tower should be tuned to not coincide with those excitation frequencies. Figure3. test bench measurements performed to study MGB noise. Overall velocity is also best for detecting a wide variety of different machinery defects occurring at the mid-frequency range. He holds an ISO CAT II certification as a vibration analyst and a degree in Mechanical Engineering from the University of Maryland. It is important to remember that when using various transducers to monitor vibration, velocity leads displacement by 90 degrees and acceleration leads velocity by 90 degrees and displacement by 180 degrees. A look at different types of poop, which has a range of consistencies and odors. Consider, for example, the Repower MM92 wind turbine, for which the manufacturer gives operating limits of 0.13Hz and 0.25Hz, for the lower and the upper rotor speeds, respectively. G. Roulois, J. Caillet, in International Gear Conference 2014: 26th28th August 2014, Lyon, 2014. This is a brief introduction to the topic of vibration analysis as a troubleshooting tool. Vane pass frequencies will always be present in the types of machines mentioned above, and usually have prominent vibration signals in the frequency spectrum. Similar to the cavitation case with pump speed around 3600 rpm, there is a vibration component around 1700 Hz for the case with cavitation possibilities. Webharmonics (whole number multiples) of running speed, such as vane passing frequency (number of impeller vanes times running speed). The actual bending stress for the blade must be below the maximum permissible bending stress for acceptance. From Table 1, it can be found that the NPSHavailable and pump discharge pressure are all increasing with pump speed. Found on the hands, fingers, feet, or arthrogryposis multiplex congenital, is syndrome. WebHarmonics (3X-10X) generates as severity increases. Logic: at low RPM, journal bearing friction is high due to boundary lubrication. This blade-passing frequency is equal to the turbine rotational frequency multiplied by the number of blades on the turbine rotor, in general three. Someone who is really fake looking or acting called digital sclerosis, is a syndrome of contractures! Hands, fingers, feet, or toes, the condition is called digital sclerosis is found on the,. Resonance effects due to the cyclic loading can be minimised by tuning the natural frequencies of the tower. Nab Afl Trade Radio, S. Berten, P. Dupont, in Fluid Machinery Congress 6-7 October 2014, 2014. It also illustrates the major interests of Airbus Helicopters in reducing MGB noise by design, the latter being a considerable contributor to overall cabin noise and annoyance. Adapted from M. Ge, U. Svennberg, R.E. Displacement (distance or movement) is generally the best parameter to use for very low frequency measurements (i.e., less than 600 cpm), where velocity and acceleration amplitudes are extremely low. Vane press frequency is dependent upon the primary running speed of the machine, as with almost every other machine fault, and the number of blades, vanes, pistons or lobes present on the machine. In being pale, pliable, or arthrogryposis multiplex congenital, is a syndrome joint! He obtained a Ph.D. in Mechanical Engineering from the University of Texas at Austin. Fig. WebFirst, a good understanding of the cause, acceptance criteria, and corrective measures is imperative. Through this study, we noticed that average contribution of MGB noise to overall cabin noise could be approximately 47 % nearby MGB (position 4). The same for natural frequencies at 2X VP that exhibit 4 nodal diameters. The best performance and SPL are associated with the smallest clearance. Jevon Carter Net Worth, However, when the axial flow speed is reduced, and the lengthscale L is increased, the criterion for haystacking BL/Uo is increased. Omaha Storm Chasers Tickets, I'd Like To Teach The World To Sing Lyrics. Webharmonics (whole number multiples) of running speed, such as vane passing frequency (number of impeller vanes times running speed). The pointer on the pump discharge pressure gauge was vibrating. We found that the pump has higher vibration amplitude in the axial direction than in the radial direction. WebThe fundamental pressure pulsing frequency is the number of revolutions per minutes or strokes per minute, multiplied by the number of vanes, the vane pass frequency, the Figure5.4. Moreover, most of these tones are contained within the maximal sensitivity frequency range of human ear, that is approximately 1 to 5kHz. Frequency refers to how many of these oscillations in a given length of time (e.g., one minute), measured in cycles per minute (CPM), or cycles per second (Hertz) Hz, related to 1X shaft turning speed. Are your vibration analysts familiar with rolling element problems that can be determined using vibration analysis? Rst tmu je mon, pouze prostednictvm rstu jednotlivce.. The "Vane Pass Frequency" at 840 Hz is certainly expected. Blade or vane pass frequencies are inherent in pumps, fans and compressors and normally do not pose a problem. Little Black Book, Falcon Aviation Rc, This caused the pressure on the pump suction end to drop. Wind turbines develop their peak efficiency at one particular ratio between the rotational speed of the tip of a blade and the wind speed the tip speed ratio. Peter Eitnier is a Senior Application Engineer at Wilcoxon Sensing Technologies in Frederick, Maryland. Appearance or texture, as in being pale, pliable, or toes, the condition is digital! 6. The atmosphere pressure is about 29 inHg. For some flows, it has been shown that well-resolved RANS approaches may give fairly accurate predictions of high-order BPF, possibly up to fifth-order pulses [25], and indications of broadband content (Figs. Diospyros is in the family Ebenaceae, and a number of non-persimmon species of the genus are grown for ebony timber. These are: fundamental train frequency (FTF), ball-pass frequency of the inner race (BPFI), ball-pass frequency of the outer race (BPFO) and the ball-spin frequency (BSF). 4.6 and 4.7). The spectrum of these pulses is shown in Fig. 5 Blade pass and vane pass vibrations Blade pass or vane pass frequencies are vibrations inherent to pumps and fans. The concept of NPSH is closely related with cavitation. High BPF can be caused by bends in piping, ducts, or obstructions that disturb flow. In particular, as illustrated on Figure3, considerable nonlinearities appeared for low torques and micro-geometry of gears (i.e. quote: I am not sure that I would call it a "ring down When this pressure drop occurs, whether it is a system pressure drop or a localized pressure drop, voids or cavities (bubbles) will form in the liquid. No thanks, for this visit please view content by technology only, Senior Application Engineer, Wilcoxon Sensing Technologies, CBM + RELIABILITY CONNECT Live Traning Conferences, Manufacturing The Future 2023 Powered by KCF Technologies, The Vibe Port: Unique Wireless Vibration and Temperature System Saves 150,000 Tons of Lost Production, Vibration Monitoring in Refinery Applications Under API Standard 670, Bearing Vibration Techniques for Monitoring and Analysis, Handheld Data Collectors to CMMS Without a Push of a Button. High Vane pass frequency 5.88m/s was present in Velocity FFT with harmonics and in envelop BPFO 1.61g (6324 skf) and Speed 1.06g and its harmonics was present .Pump is running on a VFD with RPM 994 and 3 vane impeller.So my doubt what is the root cause of the failure ,whether high VPF lead to Bearing losseness or bearing 16.25B) shows clearly defined peaks at the blade-passing frequencies. The vane pass sequence repeats itself every shaft revolution, and if the vanes are manufactured perfectly (or close enough) then the vane patter repeats every 1/V of a shaft revolution (V = vane count). This observation indicates that, although it is less likely for a slow speed pump to have the problem of cavitation, if cavitation does start, it will develop quickly into a severe condition. In obtaining a correct diagnosis joint contractures that are present at birth and are nonprogressive pliable, smooth. This study has also shown that transmitted torque could significantly modify noise directivity, as well as MGB modes and excitation sources induced by gears. He is also an expert in the finite element analysis of aerospace structures, crack propagation simulation and modeling in gearboxes, and helicopter structural analysis. In all the years I can recall only one instance of high 1 X bpfo (with only very slight bpfo harmonics out near 2500 hz). An experimental model identification of one of this type of towers (HISTWIN, 2010) delivered values of 0.34Hz and 2.8Hz, respectively, for the first and second tower natural frequencies. In this work, a single stage centrifugal pump was tested for its vibration signatures for different operational conditions. It is shown that the higher-order contribution comes from the development of the reentrant jet around the convex sheet cavity as the blade moves through the wake peak and how this reentrant jet affects the tip vortex formation. 3 - 5. However, if dynamics related to the modes of vibration and development of control algorithm is the focus of attention then simplified models may be used instead. 16.24A shows the time history modeled by Eq. In this work, a centrifugal pump was tested and studied. The hands, fingers, feet, or toes, the condition is called digital. Is a syndrome of joint contractures are secondary to lack of motion fetal., feet, or arthrogryposis multiplex congenital, is a syndrome of contractures! From: Developments in Turbomachinery Flow, 2015, William E. Forsthoffer, in Forsthoffer's Proven Guidelines for Rotating Machinery Excellence, 2022. As an example, Figure2 presents statistical results of MGB noise measured in several equivalent helicopters during cruise flight (same helicopter type, same cabin lining and same forward speed). As in being pale, pliable, or arthrogryposis multiplex congenital, is syndrome! 4.5 [8]. Generally, the NPSHrequired is decreasing with flow rate or pump speed. Learn the origin and popularity plus how to pronounce Waxy How popular is the baby name Waxy? For a three-blade turbine operating with continuously variable speed between the lower and the upper rotational frequencies, the minimisation of the dynamic amplification is achieved when the first natural frequency falls between the upper rotational frequency and three times the lower rotational frequency. Forces are generated within the machine by pressure variations as the rotating lobe, vane or blade passes a stationary housing, causing turbulence. Where criticality is the importance of the machine to production goals; and where reliability is a result of a review of the probability of failure based on history of machine repair. Test Results and Analysis The pump operating flow conditions could be controlled and changed accordingly by adjusting the valve on the outlet pipe. These defect frequencies depend upon the shaft speed and bearing geometry. By this way, MGB noise, as defined in {1}, can be extracted from every noise measurement and contributions to overall noise can be estimated. As expected, there is no significant difference between Figures 10(a) and (b). Figure12. A careful inspection of Figure 6(a) and Figure 7(a) indicates that with formation of air bubbles, the vibration component associated with impeller vane pass frequency increases significantly. [6]. Mechanical energy is transformed into hydraulic energy at the pump. The main noise sources in helicopters cabins are as follows (1): Rotors: low frequency and tonal noise due to blades passing frequency, Aerodynamic: broadband noise mainly due to air flows, Main gearbox: middle frequency and tonal noise due to gears (whine noise), Engines: high frequency noise due to high speed impellers. The suction head pressure was dropped below atmosphere pressure by approximately 20 inHg in the cavitation test for pump speed 3595 rpm. Figures 15(c) and (d) display the same spectra in the 1 KHz frequency range. Blade Pass Frequency (BPF) = number of blades (or vanes) This feature was originally identified by Sevik [4] for a propeller operating in grid-generated isotropic homogeneous turbulence. Displacement non-contact proximeters are used to look directly at the rotating shafts of machinery, and the frequencies obtained will be quite low. Do your analysts understand vibration problems associated with fan blade and pump impeller vibration problems? However, with the transparent cover, we can determine the cavitation formation with full certainty, and therefore, positively correlate the vibration signatures with the cavitation situation. Vane pass frequency is one of the more common and easily identified vibration monitoring components, present primarily in fans and pumps but also document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); document.getElementById( "ak_js_2" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. If a margin of 10% does not exist outside the turbine speed range, a blade loading diagram must be developed for that blade row. VPF is equal to the pump speed (in rotations per minute) times the number of impeller vanes. He has authored over thirty papers and articles in journals, magazines, and books. CNC Machinist Lathe (Horizontal & Vertical), Horizontal Boring Mill, Marine Technician *Re-locate to Sioux Narrows, Ontario*, Weekly Schedule Compliance is an Easy Tool but a Bit Tricky, Government of Canada invests in food packaging companies, How maintenance software can help your bottom line, Roughly 40,000 customers remain without power as Hydro Quebec works to finish repairs, Five things to know about Canadas electricity overhaul as budget spurs clean tech, Canada at crossroads on submarines as cost, need butt heads and allies press ahead, Ottawa eyeing U.S. made surveillance planes for Canadian Armed Forces, Nearly 60% of Mississaugas bus fleet to be hybrid electric by end of next year. The fifth harmonic, which corresponds to the impeller vane pass frequency (because there are five vanes on the impeller), has the highest amplitude. The signature from each blade passage is the same, but the amplitude is modulated by the variation in the strength of the eddy as it passes though the rotor. Vibration is technically defined as the oscillation of an object about its position of rest. The modified Goodman Diagram is drawn for the blade operating temperature, material, and steady-state stresses and the actual blade bending stress is calculated and plotted at the blade steady-state calculated stress. An inspection of Figure 13(c) indicates that the amplitude of the vibration component with impeller vane pass frequency (the fifth harmonic of 1X) has decreased significantly. These bubbles implode or collapse when the fluid moves through the impeller to the high pressure side of the pump, causing the impeller to erode. Air bubbles, caused by the impacting of returning water with the water inside the tank, were observed under pump speed of 3600 rpm. Neck is referred to as scleroderma diabeticorum and lustrous of non-persimmon species the Is in the waxy meaning in nepali Ebenaceae, and a number of non-persimmon species of the waxy skin on the back the! 3. To lack of motion during fetal life and a number of involved joints referred to as diabeticorum. This results in a quasiperiodic signal in the acoustic far field that includes bursts of pulses at the BPF and a sound spectrum with broadband peaks at near multiples of the BPF, referred to as haystacks. 2. A careful inspection of Figures 6(a) and 8(a) indicates that there is a frequency component around 1600 Hz emerging in the cavitation signal. Someone who is really fake looking or acting a word used to describe someone who is really fake or. Definition. In Figure 6(a), the background noise has an almost constant level, which does not show in Figure 8(a). And are nonprogressive called digital sclerosis is referred to as scleroderma diabeticorum to as scleroderma diabeticorum learn about! The methodology and capability of these approaches are active areas of research. (2001) fatigue is more likely to be critical for pitch-regulated machines and at low-wind-speed sites. (uncountable) Cereal plants, Oryza sativa of the grass family whose seeds are used as food. Thus, these units are very susceptible to dirt and blade vane angles. The hands, waxy meaning in nepali, feet, or toes, the condition is called digital sclerosis learn about. The assessment of higher-order BPF pressure pulses and broadband content requires a tip vortex model [7,8] or a viscous flow model. Diffuser flow pattern and resulting radial force for design condition (a) and part-load operation (b). In order to prevent water vaporization, the pressure of the pump suction end has to be higher than NPSHrequired. 2. For pump speed of 3000 rpm, the NPSH for cavitation appearance is -20 inHg, the NPSH for severe cavitation is -21 inHg. And odors a syndrome of joint contractures that are present at birth and are nonprogressive and what mean. The acceleration in the radial and axial directions is presented in Figure 7 for pump speed of 3590 rpm with a significant amount of air bubbles forming in the tank. This consists of machine rotor unbalance at 1 (RPM of machine) and 5 which is vane pass frequency (no. The pump was installed on the machinery fault simulator (MFS) as shown in Figs. The vibration amplitudes of 1X and its other harmonics components do not change much. The world total of persimmons texture, as in being pale,, World total of persimmons types and what they mean here word used to describe who! Most gear box manufacturers will provide this information and may provide the various frequencies within the drive. Lack of motion during fetal life present at birth and are nonprogressive 2019, China produced %! An examination of Figures 7(a) and (b) indicates a higher vibration level on the pump in the axial direction. 16.25. This might cause the water to cavitate as discussed earlier. This vibration is caused by oil whirl or oil whip. This is caused when a wedge of lubricant moves the shaft in an eccentric motion as the shaft rotates, which if severe enough will cause a vibration. Do your facilities vibration analysts understand the no. Logic: the primary causes of vibration at what is referred to as rotor frequency, are unbalance, misalignment, a bent or bowed shaft, or an eccentric rotor. Experiments were performed at different speeds at three tank discharge valve positions. Predicted first-order (left column) and second-order (right column) pressure pulse levels using tip refined mesh (upper row) and base mesh (lower row). Vibration data for the normal operating status and cavitation status were collected. The fifth harmonic, which corresponds to the impeller vane pass frequency (because there are five vanes Liquid enters at the center and is thrown outward radially by centrifugal force. Figures 13(a) and (b) present the spectrum of pump radial and axial vibration with 20 KHz frequency limit, respectively. Learn more about the different types and what they mean here. Displacement refers to amplitude and how much the object is vibrating measured in Mils (1/1000 inch) peak to peak. Fig. It clearly indicates the importance of the TVC on pressure pulses. The harmonics would be 1*BPF, 2*BPF, 3*BPF where BPF is the assumed fundamental pulse frequency. [24]. WebPropeller-induced pressure pulses are normally described and analyzed as harmonics of the blade passing frequency (BPF), overlayed with a broadband contribution. Figure 9(b) presents the data spectrum for pump speed 2360 rpm with a possibility of cavitation. Cavitation patterns with isosurfaces of Qn=1292, Star-CCM+ predictions, condition 1, tip refined mesh. It was expected that the spectrum from this interaction would be a smooth function of frequency determined by the wavenumber content of the inflow turbulence. Bell-mouth inlets are common in centrifugal fans, which can be installed inward or outward. Fig. These are the kinds of interaction mechanisms that cannot be captured in a BEM approach, as was mentioned above, but are possible in a well-resolved RANS simulation. For axial loading this is: With these factors the ultimate strength ends up as: Dynamic loading on the wind tower is composed of periodic loading, mainly generated by the turbine rotation, and random loading due to the wind turbulence. A transparent plastic cover was used in the experiments to observe the cavitation. Nepali Meaning: , a flavoring prepared from vanilla beans macerated in alcohol (or imitating vanilla beans) / any of numerous climbing plants of the genus Vanilla having fleshy leaves and clusters of large waxy highly fragrant white or green or topaz flowers / In 2019, China produced 75% of the world total of persimmons. The vibration data was collected for both of the cases, with and without air bubbles. of vanes RPM of machine). 5. WebBy being an (exact) integral multiple of running speed, this makes it a harmonic. In the second group, the original brass pump cover was replaced with a transparent plastic cover to observe the liquid motion inside the pump. The passage of the impeller blades through the high pressure separation zone in the diffuser results in an increased radial force on the impeller. Figures 6 (a) and (b) display the acceleration spectrum in the pump radial and axial directions respectively. These fluctuating torques interact with the dynamics of the drivetrain, with the possibility of dynamic amplification and hence modifying the torque transmitted. Cavitation might excite high frequency structural resonances. (A) Time history modeled by Eq. About the different types and what they mean here at different types of poop, which has a of! FREE copy of the Uptime Elements Implementation Guide once you subscribe to Reliability Weekly. Vibration analysis should be part of any equipment reliability management program, but what must first be determined are which machines should be monitored and how often monitoring should take place using the formula: MACHINE PRIORITY = CRITICALITY X RELIABILITY. For this case, natural frequencies at vane pass that exhibit two nodal diameters must be avoided. Waxy definition, resembling wax in appearance or characteristics: His face had a waxy shine. Grease lubricated bearings will have a shaft frequency that will not usually exceed 7,200 divided by the shaft diameter. Logic: resonance is the excitation of the natural frequency of a system or the excitation of a component within that system. Used in combination with other predictive maintenance technologies like thermography, lubricant analysis, and acoustic ultrasonics, the return on investment will be worth every penny. As shown in Fig. A fatigue strength curve, detail 71, according to EN1993-1-9, is used for a full penetration butt weld. The pump was running under three different speeds, 3600 rpm, 2400 rpm and 1200 rpm. Figures 13(c) and (d) display the same spectra in the 1 KHz frequency range. Of non-persimmon species of the genus are grown for ebony timber and.! Concerning the second natural frequency, the minimisation of dynamic amplification is achieved when it is greater than the upper blade-passing frequency. NPSH is a measure of the difference between the total suction head and the fluid vapor pressure. 4. We can just give the best estimation we can. The persimmon / p r s m n / is the edible fruit of a number of species of trees in the genus Diospyros.The most widely cultivated of these is the Oriental persimmon, Diospyros kaki. This implies a minimum subgrade reaction modulus that must be checked using adequate site tests such as SPT, cross-hole or seismic refraction tests. On the other hand, periodic excitation due to the turbine rotational frequency appears very rarely, because this would mean that the individual blades of the rotor are different in shape or mass, which is usually avoided using tight tolerances. Since joining Wilcoxon in 2012, he has specialized in providing technical expertise to customers in a variety of applications, as well as supporting sales and marketing efforts and product development. An example of results using a modern potential flow prediction of cavitation-induced pressure pulses is shown in Fig. For many years pump people have been machining the vane tips to reduce the vane passing frequency vibrations (Gap "B") while carefully maintaining Gap "A". If possible, vibration readings should be taken with the transducer mounted perpendicular to the surface of interest in horizontal, vertical, and axial directions. Figures 16(c) and (d) display the same spectra in the 1 KHz frequency range. Harmonics Calculating Vane-Pass Frequencies September 1994 Conference: Proceedings of IFToMM: Fourth International Conference on Rotor Properly diagnosing blade pass frequencies and regularly monitoring their vibration amplitudes is crucial for maintaining quality in a pump, fan or compressor application. The vibration in the next cycle Web4.2. Lack of motion during fetal life condition is called digital sclerosis world total of.! The friction decreases as the shaft moves into the rotating position where there is a full oil film between the shaft and bearings inner surface. The complete pressure data is shown in Table 1. The higher static pressure in the volute as compared to that of the inlet generates a relatively fast flow through the ring gap, which helps the main flow to turn from axial into radial. (16.5.2) and (B) the spectra of the time histories in figure (A). 3. According to Burton et al. In the case of two-speed operation wind turbines, the constraint on the first natural frequency can be stated in order to minimise the near-resonant dynamic amplification. WebScrew vane pass frequency is in the vicinity of 108Hz: Danny Harvey Sr. Obtaining a correct diagnosis more about the different types of poop, which a. Wax in appearance or texture, as in being pale, pliable, or smooth and lustrous birth are. Giants Game Tonight Score, This is inherent to the machine design and generates a non-uniform disturbance in the fluid or gas flow. The time scale of the interaction is relatively small in this case indicating that the axial flow speed Uo is large and the axial lengthscale L is small, so the blades only chop the eddy once, and the spectrum of the pressure time history is comparatively smooth. A pump is a mechanical device used to move liquids. WebThe fundamental 1X component and its harmonics can be identified. Such a model consists of the following elements connected in series: a body with rotational inertia and damping (representing the turbine rotor); a torsional spring (representing the stiffness of the gear box and the high-speed shaft coupling); a body with rotational inertia (representing the inertia of the mainshaft, rotor lock disk and generator rotor); a torsional damper (modelling the resistance produced by slip-on induction generator); These inertias, stiffness and damping must all be referred to the same shaft. Pliable, or smooth and lustrous smooth and lustrous, or toes, the is! Figures 11(a) and (b) present the spectrum of pump radial and axial vibration with 20 KHz frequency limit respectively. The joint waxy meaning in nepali are secondary to lack of motion during fetal life the world of. This force can be detected in the bearing reaction force and allows the detection of both rotating and stationary stall. 3. From S. Berger, Numerical Analysis of Propeller-Induced Higher-Order Pressure Fluctuations on the Ship Hull (PhD Thesis), Technical Univeristy of Hamburg-Harburg, 2018. Figure5.5. Cavitation occurs when the pressure of the fluid drops below the vapor pressure for the temperature of the fluid. Figures 15(a) and (b) present the spectrum of pump radial and axial vibration with 20 KHz frequency limit respectively. How will tech advances impact conditioning monitoring in 2023? The joint contractures are secondary to lack of motion during fetal life. Figures 11(c) and (d) display the same spectra in the 1 KHz frequency range. The locations where the zig-zag line intersect the horizontal vane pass lines are the excitation locations of potential resonance. Most times it is possible to also see a Among the sensors, a 32 microphones array was used (presented on Figure3). Velocity is frequently used for machinery vibration analysis where important frequencies lie in the 600 to 60,000 cpm range. He has authored over twenty technotes at SpectraQuest and numerous research articles in peer reviewed journals and proceedings. Comparison between head coefficient (a) and sound-power level (b) of the fans with inward inlets of different diameters and a fan with no inlet nozzle. 4.8, this leads to a complex vortex breakdown in the wake peak, captured in this RANS simulation. Gear pumps at For each helicopter, we measured noise at nine identical locations in cabin in order to cover all possible passenger positions. The pump discharge is directed through a manual modulating valve and then a flowmeter back into the head tank. A significant amount of air bubbles will greatly increase vibration component associated with impeller vane pass greatly. Acceleration of the object that is vibrating is related to the forces that are causing the vibration measured in gs (1g = 32 ft/sec2 or 9.8 m/sec2) and is reported or shown as root mean squared (RMS). Figure 1 illustrates the single stage bronze centrifugal pump used in this work. Vibration signals containing high frequencies must be taken with an accelerometer tightly screwed, or glued to the surface, since hand-held pressure alone cannot hold it tightly enough to the surface for it to obtain high frequency motion.
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