Up Time Magazine- August/September 2008

ultrasound upload Which Format and Why? Choosing the Right Method for Recording Ultrasound by Allan A. Rienstra, with contributions from Thomas J. Murphy, Eng T oday, ultrasound data collectors play a more prominent role than ever in the early detection and diagnosis of failures that can leave your plant crippled. Ultrasound instruments are a proven and useful ally for trouble shooting complex gearboxes, and indicating the correct intervals for re-lubrication of bearings as well as the correct amount of lubricant to be applied. Where traditional diagnostics have fallen short for predicting slow speed bearing failures, ultrasound has helped fill the void by providing a fast and easy solution. Safely inspecting enclosed electrical cabinets for conditions that can cause deadly arc flash is yet another reason to rely on ultrasound as a fundamentally important data collection tool. If you have used an ultrasonic data collector, you already know it’s relatively easy to listen to a bearing and determine if its condition is rough, smooth, or otherwise. But the decision to pull a machine out of production needs to be based on data stronger than “it sounds rough”. Advanced ultrasound instruments lend accuracy to the mix with digital decibel metering, but this is really useful only if baseline data is available for the machine. So recording and analysis of Acoustic Vibration Monitoring (AVM) signals is the growing trend amongst predictive maintenance practitioners who see value in gleaning information from ultrasound in much the same manner as their vibration analysis program. AVM involves capturing the heterodyned signal from your ultrasound detector’s headphone output and then analyzing the data in the time and frequency domain with analysis software. This article’s focus is on the first part; capturing the ultrasound signal in a way that is both correct, and provides useful data. That sounds simple enough, but for best results some understanding of sound recording basics will prove useful in helping you obtain the best results possible. Who wants to record ultrasound files and why is this practice useful? Well, those who want to record ultrasound files usually come from reliability and predictive maintenance backgrounds. They are inspectors and technicians looking for first, second, and third opinions about the state of their machines. They are people relying on vibration analysis, oil analysis, infrared, and ultrasound data to interpret when and why breakdowns occur. Their purpose is to use all the reliable methods at their disposal to analyze, consult, and arrive at a consensus decision. Recording and analyzing sound files represents the latest destination for ultrasound testing. Doing so allows us to compare, trend, consult, analyze, and diagnose. It allows us to seek second and third opinions, and it provides a means by which to train the ears of future inspectors. We have a few options for recording ultrasound files. I’m going to go over each of them in brief detail and rank them in terms of their usefulness and the ends they serve. The four main options for recording ultrasound files are: • • • • Analogue to Magnetic Tape Digital MP3 (MPEG) Digital WAVE Direct to Vibration Data Collector Magnetic Tape While analogue to magnetic tape represents one of the first commercially available means for consumer recordings, it poses far too many problems for capturing ultrasound files. First, there are too many noise sources in the recording process. There is noise from the speed and instability of the medium, which we also know as “wow and flutter”. Capstan noise produces a low level rumble that may not be totally discernible to the ear but will definitely show up on a time waveform. Tape hiss is another problem that would compete with the very ultrasound noises we are trying to capture. Those problems aside, transferring the recorded sound file to a PC has to be done in real time which is not very convenient or practical. Analogue to magnetic tape is my least favorite method for recording ultrasound files, especially if those files will be used for time and spectra analysis. Digital MP3 (MPEG) MPEG is the acronym for Moving Picture Experts Group. This group developed compression systems used for video data. For example, DVD movies and HDTV broadcasts use MPEG compression to fit video and movie data into smaller spaces. The MPEG compression system includes a subsystem to compress sound, called MPEG Audio Layer-3. We all know it by its abbreviation, MP3. august/september 2008 58

Table of Contents Feed for the Digital Edition of Up Time Magazine- August/September 2008

Up Time Magazine- August/September 2008 Contents Upfront Upclose Information Technology Lubrication Infrared Maintenance Management Motor Testing Precision Maintenance Reliability Ultrasound Vibration Upgrade

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