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| Product Tip of the Month | |||||
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| September/October 2001 | |||||
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Model 2020 | ||||
| Amplitude Attitude | |||||
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Amplitude is the magnitude of
dynamic motion or vibration. Amplitude is expressed in terms of peak-to-peak,
zero-to-peak, or rms. For pure sine waves only, these are related as follows: rms = 0.707
times zero-to peak; peak-to-peak = 2 times zero-to-peak. Analyzers generally read rms for
spectral components. This is where some users of spectrum analyzers get bit. As with any piece of modern test equipment, data is either printed or transferred to a personal computer for storage, trending, viewing or manipulation in one form or another. When the software is capable of changing parameters of collected data the results can be confusing to say the least. The other problem is in configuring the analyzer to take data. As an example, with the first paragraph above in mind, let’s say you collected a vibration survey with an analyzer you configured. The OEM bases the pass or fail of the engines condition on a maximum amplitude of 1 IPS/Peak. That’s one inch per second of measured peak velocity. For clarity, let’s assume the actual vibration to be .75 IPS/Peak. If you, the user, are setting the configuration of the analyzer, your choices are normally peak, peak-to-peak, or RMS. In this case, if you configure the analyzer to collect the vibration in peak-to-peak by mistake the amplitude reading is going to be twice the amplitude of a peak survey, 1.50 IPS. Peak-to-peak is often referred to as "double amplitude" for this very reason. In this case the survey would indicate to you that the engine has failed the survey test, when in reality it was still 25% below the maximum amplitude. On the other hand, had you configured the analyzer to collect the survey in RMS but compared it to the limits of the OEM, you might deduce that the engine is still very well below the limits (0.53025 IPS) when in fact it was 0.75. This becomes a critical mistake when the IPS/Peak level is above the actual limit, but you read it as within the limit. RMS = 0.707 times peak, so if the actual vibration was 1.2 IPS Peak, you would mistakenly believe it to be 0.8484. For these reasons, if you are configuring the analyzer to collect vibration surveys DOUBLE CHECK your settings. If you transfer the data to a computer where the software is capable of changing these parameters, DOUBLE CHECK them prior to making any decisions based on the results you see. |
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