Fluke Biomedical 4000M+ - Manual (Page 19)

3 Theory of Operation
Basic Definitions
3-1
Section 3
Theory of Operation
3.1 Basic Definitions
Kilovoltage (kV)
NOTE
Kilovoltage is measured during the first 300 ms of
the exposure.
During the x-ray exposure, the tube kilovoltage varies with time, producing the kV waveform. The size
and shape of the waveform (including number of peaks in a given time period) depends on the design of
the machine (e.g., single phase or 3-phase, full-wave or half-wave, etc.).
For example, in a 16-millisecond interval, on a single-phase, full-wave generator, there may be two
peaks; whereas, on a 3-phase, full-wave generator, there may be six peaks in the same time period.
During data analysis, the peak values are calculated and stored in memory. These are instantaneous
values and are waveform independent. Three kV values are displayed:
• kVp MAX
• kVp AVG
• kVp EFF
kVp MAXIMUM
The displayed kVp MAX is the maximum peak value measured during the first 300 ms of the exposure.
Since all of the peaks could be slightly different, the Model 4000M+ searches through the data and finds
the peak with the highest value. This could be an overshoot or simply a minor variation in the peak value.
kVp AVERAGE
The displayed kVp AVG is the average of all of the peaks. The Model 4000M+ determines this value by
adding the values of all peaks and dividing by the number of peaks.
kVp EFFECTIVE
The displayed kVp EFF value has the same physical meaning as the quantity measured by the kVp test
cassette method; i.e., the two detector waveforms stored in memory are integrated independently (as film
does), and then the kVp EFF is calculated from a ratio of the two sums.
Two sets of calibration coefficients exist, one for the calculation of single-phase effective kVp, the other
for calculation of 3-phase (or constant potential) effective kVp. During the analysis, the phase of the x-ray
machine is determined from the waveform, and the coefficients used to calibrate the kVp EFF are then
automatically selected. Even with the calculation compensating for phase dependence, there is still some
waveform dependence on this number.