AE05.pdf

Measurement of intensity assumes a 40 db preamplifier

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measurement of intensity assumes a 40 dB preamplifier. True Energy. True energy is a measure of the acoustic emission hit and is measured in attojoules. Absolute energy is derived from the integral of the squared voltage signal divided by the reference resistance (10 k Ω ) over the duration of the acoustic emission waveform packet. The range is from 9.31 × 10 –4 aJ to 1.310 25 × 10 6 aJ. Resolution is 9.31 × 10 –4 aJ per count (at 1 MHz or greater sample rate). This feature is available in both the hit data set and the time driven data sets. As a hit feature, it reports the true energy of the acoustic emission hit. As a time based feature, it reports the energy in the time driven data rate interval, which must be specified by the operator. As a time driven feature, this parameter is very good for monitoring continuous signals because it is independent of hit based activity. Threshold. The threshold feature records the value of the threshold at the moment of a hit. This threshold is needed only when an adaptive technique such as a floating threshold makes it important to know the threshold value at the moment of a hit. Counts to Peak. Counts to peak is a measure of the number of acoustic emission counts between the hit start and the peak amplitude of the hit. There are three counts to peak in the example of Fig. 5. Root Mean Square. Root mean square is a measure of the continuously varying signal potential into the acoustic emission system and is measured in millivolts. Root mean square here refers to the rectified, time averaged acoustic emission signal measured on a linear scale. Averaging time must be specified in terms of an acoustic emission hit feature, the instantaneous value of root mean square voltage is collected and processed along with the other features when a hit is detected. Average Signal Level. Average signal level is a measure of the continuously varying and averaged amplitude of the acoustic emission signal in decibels. This measurement is similar to that of root mean square in that it is an averaged reading and the averaging time must be specified. Parametrics. External signals such as load, strain gages, temperature and pressure might be related to the stressing of the item under test. They might be important acoustic emission hit and time driven parameters although they are not called features . Parametric information can provide the cause and effect relationship needed to identify a source mechanism and is very useful in pattern recognition and analysis of acoustic emission signals. The parameter is recorded in exact units, such as temperature in degrees celsius, pressure in pascals and load in newtons. Time Driven Features The time driven acoustic emission features are collected and recorded on a timed basis, referred to as the time driven data rate. Time driven data report on how the background or continuous acoustic emission activity is changing during time, showing trends in the analysis of process changes, leaking or faulty components in rotating machinery or continuous processes. Absolute energy is also a
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