Appendix B Calibration (DiRex System)
HF Series X-ray Generators - Service Manual Revision W
Quantum Medical Imaging, LLC B-43
Ion Chamber Balancing (Three-field Chambers)
On systems using a three-field ion chamber, the chamber detectors must be
balanced prior to performing AEC calibration (refer to the ion chamber manu-
facturer’s instruction manual). When balanced, the mAs will be approxi-
mately the same (±5%) for exposures taken using any of the three
chambers, or with two or more chambers selected simultaneously. This pro-
cedure must be performed on each ion chamber used in the system (for
example, both table and wall ion chambers when equipped with two). Before
proceeding, check that the ion chamber detectors have been correctly config-
ured so when the chamber select buttons on the operator control panel are
pressed, the appropriate ion chamber detector is enabled. If not, refer to
"Table/Wall Ion Chamber Configuration Screens" procedure described previ-
ously in this chapter and re-configure. Proceed as follows:
1. If not already done so, a kVp bleeder tank should be connected in series
between tube and High-Voltage (H.V.) Transformer A17. If using a mAs
meter, connect the mAs meter between pins A17P1-9 and A17P1-10 on
H.V. Transformer A17.
2. Access Generator Settings Screen (see Figure B-9). Make a note of the
Backup MAS setting, then change the Backup MAS to "40".
3. Press the Manual/AEC Mode button on the operator control panel and
select AEC mode.
4. Using a lead sheet, cover the UPPER RIGHT and CENTER ion chamber
detectors.
5. Place 20 CM acrylic over UPPER LEFT ion chamber detector.
6. Set operator control panel for 100 KV, 200L MA, select the UPPER LEFT
ion chamber detector and take an exposure. Note the measured mAs
reading.
7. Repeat above steps with the CENTER ion chamber detector selected and
UPPER LEFT and UPPER RIGHT detectors covered with lead. The mea-
sured mAs reading should be within 5% of the mAs reading noted in the
previous step. If not, adjust the CENTER detector gain potentiometer
located on the ion chamber amplifier (refer to ion chamber manufac-
turer’s documentation for details), then re-take exposure and verify it is
within tolerance. Repeat as necessary.
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