Fluke Biomedical 35080M - Manual (Page 40)

Mobile Filter Pack Plus Option (37946)
Introduction
7
7-1
Section 7
Mobile Filter Pack Plus Option (37946)
7.1 Introduction
The Mobile Filter Pack Plus (37946) is a specialized filter pack designed primarily for use on mid- to high-
frequency generators (
≥ 1 kHz) with substantial kV ripple (≥ 10%). For single-phase and 3-phase
generators, or other low-frequency generators (< 1 kHz), use of the Mobile Filter Pack Plus (37946) is not
recommended. The Wide Range Filter Pack (37617) should be used for these applications.
Measurements below the 50kV operating range of either the Mobile Filter Pack Plus (37946) or the Wide
Range Filter Pack (37617) should be made with the Low Range Filter Pack (38237).
When an invasive kV divider is connected to high frequency, high-ripple generators for calibration, the kV
waveform is often substantially changed due to the capacitance of the added high voltage cables. When
the invasive divider is subsequently removed for normal operation of the X-ray system, the kVp at any
given generator setting may be significantly higher than was measured with an invasive divider. The
Model 35080M kVp Divider with the Mobile Filter Pack Plus (37946) provides an easy and accurate non-
invasive method for measuring the kVp of these systems.
The operating instructions for the Mobile Filter Pack Plus (37946) are the same as given in Section 4 for
the Wide Range Filter Pack (37617) with minor exceptions as detailed below. The recommended focal
spot to kVp divider distance is 12 inches, rather than the 22 inches suggested for the Wide Range Filter
Pack (37617). The long axis of the Model 35080M kVp Divider should still be perpendicular to the X-ray
tube axis to avoid “heel effect”, as shown in Figures 2-2 and 2-3.
The formula used to calculate the kV is the same as for the Wide Range pack:
kV = Model 35080M voltage x 100 + ZERO kV
where the ZERO kV is 40 for both the Mobile and Wide Range Filter Packs (37946C and 37167).
However, when the kVp divider is placed in the “CAL” mode with a Mobile Filter Pack Plus (37946)
inserted, an output voltage of 0.600 V ± 1% should be obtained, corresponding to a CAL kV of 100.
The kVp Divider has a special circuit intended to prevent the presentation of a distorted waveform at low
intensities. Section 3.5, Intensity Cutoff, discusses the effect of this circuit for single-phase waveforms.
For generators with substantial kV ripple, for which the Mobile Filter Pack Plus (37946) is intended, this
same effect may be noticed. As the kV waveform decreases, the output suddenly switches to the filter
pack’s baseline, which corresponds to 40 kVp with the Mobile Filter Pack Plus (37946). During the rising
portion of the next kV cycle, the output will switch back to and follow the actual waveform, as shown in
Figure 3-3 for the single-phase example. This is normal operation of the Model 35080M kVp Divider for
large ripple signals.
The filtration and linearity correction curves for the Mobile Filter Pack Plus (37946) are given in the next
two sections. Two linearity curves are shown. The solid line on the linearity graph is the standard
linearity curve for an X-ray generator with a 2 kHz waveform with 20% observed ripple. The dashed line
is normally not used, but shows the linearity curve for generators with DC to 720 Hz ripple components.