Fluke Biomedical 35080B - Manual (Page 62)

35080B
Operators Manual
B-2
B.3 Voltmeter Measurements
If the output of the Model 35080B kVp Divider is connected to a voltmeter, a mathematical correction is
required to compensate for gain effects. The first step in this correction process is to determine the
expected and actual voltages when the kVp divider is placed in CAL mode. With the filter pack in place,
switch the kVp divider to the CAL position and read the voltmeter to obtain the actual CAL voltage of the
unit. The expected CAL voltage is the value listed on the filter pack itself. If these values do not agree to
within ± 1%, perform the following correction for all subsequent kVp measurements made using this filter
pack.
Voltages obtained in RUN mode will deviate from correct values by the same ratio as the deviation in CAL
voltages. That is,
Voltage Corrected
mode RUN in Voltage Measured
Voltage CAL Expected
Voltage CAL Actual
=
where Corrected Voltage is the true voltage that would be obtained if the gain were accurate. Therefore,
we can obtain a Corrected Voltage for any exposure by rearranging the above equation.
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
mode CAL in Voltage Actual
mode CAL in Voltage Expected
Voltage Measured Voltage Corrected
The corrected kVp is then calculated as usual by multiplying the Corrected Voltage by the sensitivity and
adding the Zero offset kVp of the particular filter pack in use.
Example: A Model 35080B kVp Divider with a Wide Range Filter Pack (37617) (50-150 kVp) is
connected to a voltmeter and switched to CAL mode. A value of 0.412 V is observed, which is 3% higher
than the expected CAL voltage of 0.400 V for this pack. Therefore, subsequent measurements made
with this pack should be corrected.
The kVp divider is now switched to RUN mode. An x-ray exposure is made, and a voltage of 0.515 V is
measured. A Corrected Voltage is calculated from the previous equation.
0.500V
0.412
0.400V
0.515V Voltage Corrected
= = ⎟
⎠
⎞
⎜
⎝
⎛
Using the sensitivity of 0.1 V/10 kV and 40 kVp Zero offset given for this filter pack, the Corrected kVp is
90. A kVp of 91.5 would have been measured if the above gain correction had not been performed.
B.4 The Model 35050A Dosimeter Measurements
The Model 35050A Dosimeter/kVp Readout automatically converts the output voltage of the Model
35080B kVp Divider to a kVp value, using the sensitivity and Zero kVp offset of the selected filter pack.
Therefore, the method for calculating a Corrected kVp value is slightly different.
The first step again is to determine the actual and expected values with the Model 35080B kVp Divider in
CAL mode. In this case, since the values are in kVp rather than raw voltages, the Zero offset value must
be included in the calculation. The ratio equation in terms of kVp values is
( )
( )
( )
( )
kVp offset Zero kVp Corrected
kVp offset Zero mode RUN in kVp Measured
kVp offset Zero kVp CAL Expected
kVp offset Zero kVp CAL Actual
−
−
=
−
−
where the Expected CAL kVp and Zero offset kVp correspond to the values given on the actual filter
pack. Note that the sensitivity need not be considered since it cancels out of the above equation. The
Corrected kVp can be obtained directly from this equation by rearranging.