Fluke Biomedical 10100AT - Manual (Page 53)

Theory of Operation
Measurement Technique
3
3-7
3.13.2 High Sensitivity Rate
This mode is activated when “35080/80A Not Used” is selected from the Detector Select screen and the
operator has selected non-integrating exposure units such as A, R/s, or Sv/min. During operation in this
mode, the 35080 Input is ignored and the microcomputer again provides automatic drift and offset
compensation. The display is updated at a rate of once each second. Autoranging across all five current
ranges provides a span covering from low-level image intensifier input measurements to the
measurements of unattenuated direct beams.
When operating in this mode, the rate of change of the integrator output level is monitored at 1-second
intervals. Any change corresponding to an input current of less than 0.25 pA will be considered as
leakage drift and correspondingly nulled at the display. Any value above 0.25 pA will result in the display
of an offset corrected value. If the charge stored during any 1-second measurement interval reaches a
level of 1.2 pC, the integrator feedback capacitors will be automatically reset prior to the beginning of the
next interval.
3.13.2.1 Very Low Dose Rate
This mode is the same as High Sensitivity Rate, except the measurement threshold of 0.25 pA is not
used. This effectively turns off leakage suppression, so any low level input current (including leakage) will
be displayed. This mode is activated when “35080/80A Not Used” and an ion chamber displaying VLDR
in its description are selected from the Detector Select screen and the operator has selected non-
integrating exposure units such as A, R/s, or Gy/min.
3.13.3 kVp/Dose/Time
This mode is activated when one of the Model 35080/80A/80B kVp Divider filter packs is selected from
the Detector Select screen and the operator has selected integrating exposure units such as C, R, or Sv.
During operation in this mode, direct beam exposure levels are assumed. Thus, the automatic drift and
offset compensation are not required and autoranging is limited to the upper three sensitivity ranges. In
this mode, kVp and exposure time measurements are performed along with the basic exposure
measurement. Automatic reset and display hold is provided following each exposure.
While operating in this mode, the burden placed upon the microcomputer by the kVp divider data
acquisition task inhibits its ability to continuously monitor the rate of change of the integrator output level,
and thus prohibits the use of a 0.25 pA start/end exposure threshold. Instead, the start of an exposure is
designated as the first time the kVp divider output’s crosses to above the minimum kV level for the filter
pack in use. Furthermore, the end of the exposure has been designated as the last time the kVp divider’s
output level drops below this threshold and remains there for a minimum of 100 ms. At the end of each
exposure, the display is updated with new kVp, dose, and time measurement values; and the
electrometer is reset in preparation for the next exposure. The display values are held until the end of the
next exposure, or until the operator presses the RESET/MEASURE button.
The displayed kVp value is derived by scanning a 50 ms wide segment of kV data. The data is first
smoothed by a digital filter using a 1 ms wide rectangular filter function. The digital filter is disabled when
using the Mobile filter pack. The smoothed data is then scanned for its peak value. This peak value is
corrected for the gain and offset of the filter pack in use, and then displayed as the kVp.
The position of the 50 ms wide segment of kV data is selected based on a delay from the start of the
exposure. This delay is fixed at one-tenth of the exposure time.
The exposure time calculation is based on the first and last crossings of the kV signal above the 75% of
the kVp value. In cases were the 75% value is less than the minimum kV for the filter pack in use, the
minimum kV for the filter pack is used.
3.13.4 kVp/Rate
This measurement mode is activated when one of the Model 35080/80A/80B kVp Divider filter packs is
selected from the Detector Select screen and the operator has selected a non-integrating exposure units