Fluke Biomedical 4000M+ - Manual (Page 27)

A Appendix
Detector Filter Graphs
A-1
Appendix A
Detector Filter Graphs
A.1 Detector Filter Graphs
The curves in this Appendix can be used to determine allowed mA vs. kVp values for each filter position
on the detector as follows:
1. Locate the kVp setting on the generator.
2. Read up the mA axis. The mA setting on the generator should fall within the lower and upper
curves for the filter range position.
a. If the mA setting falls on or below the lower curve, the signal from the Model 4000M+ detector
may not be sufficient to cause data to be loaded.
b. If the mA setting falls on or above the upper curve, the signal may overrange (OVER Error
message).
NOTE
Each of the solid curves corresponds to a particular
filter position as indicated in the Figure; dashed
lines indicate areas of overlap between curves.
Keep in mind the following guidelines:
• The curves are valid for source-to-detector distances (SDD) of 46, 66, and 102 centimeters (18, 26,
and 40 inches).
• If a distance not listed is used, correct the curve to determine the allowable mA values (mA
1
) using
the inverse square law as follows:
mA
1
= mA
0
x (SDD
1
/SDD
2
)
2
where:
SDD
1
is the actual measurement distance,
SDD
2
is the distance represented by the curve,
mA
0
is the allowable mA for (SDD
2
), determined from the curve in the Figure.
For example, to use an SDD of 51 centimeters (20 inches) and a kVp of 80:
a. From Figure A-1, the graph for 46 centimeters (18 inches), it can be determined that the mA range
is approximately 20 to 1500 mA for the 70 – 120 filter range.
b. Adjust from 46 to 51 centimeters (18 to 20 inches) by multiplying the mA limits by (20/18) < 1.235 to
yield a new mA range of 24.7 to 1852 mA.
• The curves are derived from instantaneous dose rates and are equally valid on single-phase
and three-phase machines.
• The curves are for instantaneous kV; if an extreme overshoot exists in the x-ray machine being
tested, the Model 4000M+ may display an OVER Error message under conditions that would
normally be within limits.
• It is good measurement practice to use consistent parameters, although distance and
field size are not critical to the measurement being made.