Noise background

The results of over 1 year of continuous, on-line acoustic emission (AE) structural surveillance of high temperature / high pressure steam headers, gained on 2 M-scale 600MW supercritical multifuel ENEL power units in normal operation, are presented in the paper. The influence of background noise, the correlation between plant operating conditions (steady load, load variations, startup / shutdown transients) and AE activity and the diagnostic evaluation of recorded AE events are also discussed. 

Different plant operating conditions (steady load, load variations, startups / shutdowns) have been encountered during the monitoring period. Electrical load, steam pressure and steam temperature values vs time have been acquired and stored during the entire period. At the same time, the RMS values of the acoustical background noise were have been continuously checked and stored, thus providing a quick check of proper instrumentation condition and a correlation between variations of plant parameters and the acoustical behaviour of the components. 

Figure 6 shows the histogram of localized AE events vs axial position for the same time period as in fig.5. The location of the AE source corresponds, within source location errors (< 10-15 cm), to one of the welds under surveillance. The weld was known by ultrasonic examination to be affected by internal discontinuities. However, the position of the source could also correspond to one of the hangers. The steps observed in EA event accumulation have taken place during steady load operation, which normally corresponds to very low background noise conditions. This type of event, however, has not been observed afterwards. 

At steady electrical load the background noise is normally low and fairly constant along the SH headers and with time no AE sources come up on the Unit 3 header, while the very few localized events recorded on Unit 4 are spread out over the whole length of the header. 

During electrical load variations the background noise is low and constant along the SH headers, it increases with load variations. AE sources appear during load variations, but their position are again uniformly scattered over the length of the headers. 

The chemical shift of the N—H proton of amides appears m the range 8 5-8 It IS often a very broad peak sometimes it is so broad that it does not rise much over the baseline and can be lost m the background noise... 

Background noise in a meter obtained by measuring signal over time in the absence of analyte. 

Detection limit. The detection limit of an instrument should be differentiated from its sensitivity. The detection limit reflects the smallest flow of sample or the lowest partial pressure that gives a signal that can be distinguished from the background noise. One must specify the experimental conditions used and give the value of signal-to-noise ratio corresponding to the detection limit. 

Concerning the sound pressure level, when a noise generated by an HVAC system or an air-handling unit increases the ambient background noise by a dB, the noise increase is just perceptible. On the contrary, an increase oi 5 dB or more is clearly perceptible. 

Remember also to take background noise into your calculation. Too frequently, specifications are made which ignore this, with the result that equipment is applied to a more rigid design than is absolutely necessary. 

Federal, state, and local regulations require serious attention be paid to noise levels within the plant. Vibration analysis can be used to isolate the source of noise generated by plant equipment as well as background noises such as those generated by fluorescent lights and other less obvious sources. The ability to isolate the source of abnormal noises permits cost-effective corrective action. 

Figure 13.6 Carbon-13 NMR spectra of 1-pentanol, CH3CH2CH2CH2CH2OH. Spectrum (a) i a single run, showing the large amount of background noise. Spectrum lb) is an average of 200 runs.

Detection limit The detection limit is the smallest sample flow that provides a signal that can be distinguished from background noise. 

The accuracy with which a wavefront sensor measures phase errors will be limited by noise in the measurement. The main sources of noise are photon noise, readout noise (see Ch. 11) and background noise. The general form of the phase measurement error (in square radians) on an aperture of size d due to photon noise is... 

The limit of detection is the smallest amount of an analyte that is required for reliable determination, identification or quantitation. More mathematically, it may be defined as that amount of analyte which produces a signal greater than the standard deviation of the background noise by a defined factor. Strictly for quantitative purposes, this should be referred to as the limit of determination . The factor used depends upon the task being carried out and for quantitative purposes a higher value is used than for identification. Typical values are 3 for identification and 5 or 10 for quantitation. 

The selectivity of a detector is often related to its limit of detection, i.e. the more selective it is, then the lower the background noise is likely to be, and consequently the lower the limit of detection. 

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