Laserfiche WebLink
• <br />illustrated in the fi_ure below. It is therefore only an approximation of the frequency of the Peak <br />Particle Veloci[v. <br />Waveforms may have the same Peak Particle Velocities but different Zera Crossing Frequencies <br />depending on the shape of the waveforms involved. With reference to the fi_ures above and <br />below; both waveforms have the scone Peak Panicle Veloci[ies however their Zero Crossing <br />Frequencies differ. In the figure above, the zero crossing frequency uses the I2 period indicated <br />by T,. In the figure below, the zero crossing frequency uses the I/2 period indicated by T.. Notice <br />that T, is less than T_ because of the different waveform shapes, therefore the Zero Crossing <br />Frequency in figure above is greater than the Zero Crossing Frequency in the figure below. It is <br />for this reason, the Zero Crossing Frequency may differ for peaks having the same Peak Panicle <br />Velocity. <br /> /Peak Panitle Veloriry <br />MgRC~ <br /> n.,,. <br /> T= Zen Crossing <br />Figure 5.11 Zero Crossing Frequency Calculation <br />Limitation. <br />• (3) Sample Rate Error <br />The Zero Crossing Frequency requires the period of a wavelength before it can calculate the <br />waveleng[h's frequency using the formula 1/period. A sampling error occurs for higher <br />frequencies when wavelength periods become relatively small and [he sampling rats begins to <br />miss zero crossing points. In other words, the vvaveleng[h periods occur much fas[er than a <br />BlastMate III can sample and use in the calculation. <br />At hieher frequencies there are fewer sample points per cycle and therefore greater error. The <br />follovvins table illustrates how error increases wi[h frequency. <br />Zero Crossing Frequency Sample Rate Error <br />Frequency Range Recording Rate <br />- Standard (1024 Hz) - Fast (2048 Hz) <br />0-30 Hz neelieible error ntelieible error <br />31 - 50 Hz up to ~ Hz ertor up to ?.5 Hz a«or <br />i I - 70 Hz up to 3 Hz error up to ~l Hz a«or <br />71 - 90 Hz up to I S Hz e«cr up to 9 Hz rrror <br />91 - I50 Hz up to >0 Hz t«or up to ?5 Hz error <br />The BlastMate III does not calculate frequencies above 100 Hz because of the high error level at <br />10?4 samples per second. The message "> 100 Hi' displays. Furthermore if a waveform is very <br />• complex, or if it contains a large offset value, the zero crossings may lie outside an acceptable <br />window. Whenever a frequency cannot be calculated the message "<I Hz" displays. The message <br />5-$ BlastMate III Opera[or Manual <br />