Fluke Biomedical 35080B - Manual (Page 61)

Appendix
Correcting kVp Measurements For Gain Effects
B
B-1
Section B
Correcting kVp Measurements For Gain Effects
B.1 Introduction
Section 10.4, Calibration, of the Model 35080B kVp
Divider user’s manual details a method for
performing gain corrections by adjusting the
internal potentiometers of the instrument. This
remains a completely acceptable technique. The
purpose of this addendum is to provide an
alternative method for users with sealed units, who
cannot access the adjustment potentiometers.
The calibration mode feature of the Model 35080B kVp Divider provides a method to monitor the gain of
the internal circuitry. When the kVp divider is placed in CAL mode, the specific voltage and kVp stated on
each individual filter pack should be obtained. If the output voltage in CAL mode is not within ± 1% of the
value stated on the pack, kVp measurements should be corrected in one of the following ways,
depending on the device used for measuring the output of the kVp divider. A deviation of less than ± 1%
will result in a maximum kVp error of 0.75% of reading.
B.2 Oscilloscope Measurements
The Model 35080B kVp Divider Instruction Manual describes the technique that should be used to
measure kVp when connected to an oscilloscope (refer to Section 2.4). A slight modification of this
technique will allow for the correction of gain effects. Briefly, this technique is as follows.
1. Insert the filter pack to be used into the kVp divider and connect to the oscilloscope. Set the
oscilloscope gain to an appropriate value depending on the sensitivity of the filter pack.
2. With the function switch of the kVp divider set to OFF, adjust the oscilloscope vertical position such
that the trace coincides with the bottom horizontal line on the screen. This line now corresponds to
the Zero offset kVp given on the pack.
3. Switch the kVp divider to the CAL position. The oscilloscope trace should now be at a level
corresponding to the CAL kVp given on the filter pack. If this is not the case, the gain effects
should be corrected before making kVp measurements in the RUN position.
4. The gain effects can be corrected by adjusting the fine-tuning knob on the Volts/div switch of the
oscilloscope. While still in the CAL position, adjust this knob until the trace on the screen
corresponds to the kVp value expected for the filter pack in use. Measurements made in RUN
mode will now be corrected for gain effects.
NOTE