Fluke Biomedical 07-494 - Manual (Page 14)

Victoreen 07-494
Operators Manual
3-2
3.3 Waveform Effects
Beam spectrum changes also occur with different waveforms. Single-phase waveforms display different
readings than three phase waveforms, depending on the position of the range selector switch. Correction
for waveform effect is made by selecting the appropriate position (i.e., single phase or three phase) using
the front panel phase selection switch.
3.4 Focus to Detector Distance (FDD)
The Focus to Detector Distance (FDD) affects the mAs requirements as discussed in Section 3.5, mAs
Requirements. Table 3-2 lists FDD settings generally used to keep the mAs requirement between 20 and
100. The FDD does not affect the accuracy of the kV results.
Table 3-2. FDD Settings
Selected Range kVp FDD
50 – 90 kVp
50 to 70
70 to 90 18 in. (45 cm)
36 in. (30 cm)
80 – 150 kVp
80 to 100
100 to 150
18 in. (45 cm)
36 in. (30 cm)
3.5 mAs Requirements
The curves in Figure 1-2 (see Section 1-2, Specifications) are provided as guidelines for selecting the
mAs required for a particular kVp measurement. Any mAs beyond the required amount is not used in the
measurement.
To determine the mAs required:
1. Locate the kVp to be measured on the horizontal axis.
2. Read the mAs from the selected FDD. It may be necessary to interpolate between distances using
the inverse square law.
As examples:
For kVp = 70 and FDD = 18 in.,
the mAs = 20.
For kVp = 100 and FDD = 30 in.,
the mAs = (30 in. /36 in.) (30 in. /36 in.) 90 = 62.5
The curves in Figure 1-2 (see Section 1.2, Specifications) are derived from single-phase measurements.
Three phase machines generally require 70% of the mAs determined from the curves.
Specific mAs requirements depend on the following factors:
• Detector sensitivity
• Tube filtration
• Radiation waveform
• FDD
• mAs errors in the x-ray machine