Fluke Biomedical 10100AT - Manual (Page 24)

10100AT
Operators Manual
2-6
11. Exposure Reset selection. Pressing the up or down button toggles between Automatic and Manual.
When the Exposure Reset option is set to Automatic, Model 35050AT Dosimeter automatically
resets between exposures; exposure start and end detection are based on the exposure rate level
crossing a 0.25 pA threshold. When the Exposure Reset option is set to Manual, this button may be
depressed between exposures to clear the display. A symbol appears in the upper right corner of
the display when the Manual Reset option is active. Exposure start detection is based on the
exposure rate level crossing a 0.25 pA threshold but the exposure measurement is only reset by
pressing the Reset/Measure button. The manual reset feature is only active in the Full-sensitivity
Exposure mode.
12. Exposure Time Units selection. Pressing the up or down button toggles between Seconds and
Pulses. When Seconds are selected as exposure time units, exposure time is measured between
the first and last passage through the kV threshold. The displayed units are milliseconds (ms) or
seconds (s). The threshold is set at 75% of the kVp, or the minimum kVp for the filter pack in use,
whichever is greater. When Pulses are selected as exposure time units, exposure time is displayed
in pulses (P), and pulses are counted by detecting each passage through a threshold on the rising
and falling edges each pulse in the output waveform.
2.6 Basic Measurement Techniques
2.6.1 General Considerations
It will be helpful to the user to understand the presence of several automatic compensations being
performed during measurements and how they could affect the measurements under certain
circumstances.
Automatic Voltage Offset and Drift Compensation
The instrument automatically measures the voltage offset of the electrometer and subtracts the value
from the exposure and exposure rate measurement results.
Automatic Current Offset and Drift Compensation
In the usual situation, this feature is transparent and amounts to system leakage current being nulled from
the measurements. In order to do this, both a short-term history of the leakage current and the
establishment of a leakage/“real” current threshold must be made.
The threshold is set at 0.25 pA and only a few seconds are required for an accurate leakage level
assessment. In the typical case, system leakage current is on the order of 10 fA and reasonably steady.
It may be viewed at any time by going to the OPTIONS and scrolling to the System Leakage screen.
When it approaches the current threshold level and/or becomes highly erratic, the system may improperly
compensate the measurement value.
Another consideration of higher, but suppressed, leakage levels is that the automatic electrometer
integrator reset frequency is raised. This increases the small, but finite possibility, that a reset may occur
during an exposure and thereby lose some of its value. With low system leakage and manually resetting
just before the exposure, this possibility is precluded. In the case where the Model 35050AT Dosimeter
detects the start of an exposure immediately following an automatic reset, an error message will be
displayed advising the user to repeat the exposure.
For several reasons, then, it is good operator practice to regularly check for a low system leakage level.
Experience should serve as a guide to how often this needs to be done and in what circumstances.
Similarly, ten seconds or so should be allowed to elapse between the last triaxial cable or chamber
movement or other disturbances to the high impedance part of the system before commencing an
exposure--so that the “correct” history is applied to the measurement.