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The second limitation of this pickup is illustrated by an example. Acceler-ation of one g at .5Hz represents a displacement of 1  inches. It is obvious that in spite of its wide-band response (sometimes .1 15 kHz)， it is severely limited at the low end by a poor SjN ratio.
The transducer type used should be matched to the machine being analyzed. A knowledge of the types of problems normally encountered willbenefit this selection. Forinstance， the noncontacting shaft displacement probe helps to correct misalignment and balancing problems but is inappro-priate in analyzing gear mesh problems and blade passage frequencies.Also，if signal integration or double integration is to be carriedout， the lowpassfilters used to attenuate high-frequency spectra also have a highpassfilter， which effectively creates a lower frequency limit (often as high as five Hz).As mentionedbefore， one main criterion in deciding which transducer to use is the frequency range to be analyzed. Figure 16-3 shows the frequency limitations placed on the three types of transducers discussed previously.
.ynamic .ressure Transducers
The use of dynamic pressure transducers gives early warning of problems in the compressor. The very high pressure in most of the advanced gas turbines cause these compressors to have a very narrow operating rangebetween surge and choke.Thus， these units are very susceptible to dirt and blade vane angles. .ynamic pressure transducers are used to obtain a spectrum where the blade and vane passing frequency are monitored. As the compressorapproachessurge， the second order of the blade passingfrequency (卻 . number of blades . running speed) approaches the magnitude of the first order of the blade passing frequency. The early warning provided by the use of dynamic pressure measurement at the compressor exit can save ma.or problems encountered due to tip stall and surge phenomenon.
The use of dynamic pressure transducer in the combustorsection， espe-cially in the low NOx combustors ensures that each combustor can is burn-ing evenly. This is achieved by controlling the fuel flow to each combustor can till the spectrums obtained from each combustor can are close to being identical. The dynamic pressure transducers when used in this application must be mounted so that the probes are not exposed to the full combustor temperatures. This can be done by the use of buffer gases. This technique has been used and found to be very effective and ensures smooth operation of the turbine.
Taping.ata
For many reasons， it may be inconvenient to take the spectrum analyzerto the field each time an analysis is to be made.Often， several machines areto be analyzed at various locations.Also， a hostile environment may exist atthe testsite， which might result in damage to the analyzer. A way of over-coming these problems is offered by data taping.認ith atape， a permanent record is made. Since each channel of the tape offers a place for data to bestored， this record may be a condensation of several inputs either from different transducers or from the same transducer at various locations. Acontinuous tape monitor is very beneficial. In the event of machinefailure， an analysis of the playback will help diagnose the problem.
The choice of what kind of tape recorder to use is an important decision. AM tape recorders are much less expensive than FM recorders and usuallyhave a voltage saturation limit of卻 or more volts. An FM recorder may be saturated by as little as one volt. A drawback to AM recorders is a rather high roll-off frequency of about 5 Hz (3   rpm). .ata below the roll-off frequency is attenuated and appears to be lessened in magnitude. An FMrecorder has no lower frequency limit. however， it may require careful signal conditioning (attenuation or amplification) to prevent tape saturation.Usually， if the problems lie at the high frequencies， an AM recorder is thebest selection. Regardless of the recordertype， a calibration of input signals is recommended using a known oscillating signal and is usually best done by following manufacturer"s instructions.
　
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