Fluke Biomedical 10100AT - Manual (Page 52)

10100AT
Operators Manual
3-6
scale of 10 pA and an effective resolution of 5 fA. Similarly, the least sensitive range has a full scale of
100 nA and an effective resolution of 0.5 pA.
3.12 300 Volt Electronic Bias Supply
The 300 Volt ionization potential is generated internally by an electronic bias supply circuit. The supply
eliminates the need for the Model 35050AT Dosimeter to contain high voltage batteries. As explained
previously, the ion chamber collector and guard are positively biased relative to the ion chamber body,
which is maintained at the same potential as the chassis of the dosimeter for operator safety. A readout
of the bias potential is facilitated by a divided bias signal generated within the bias supply block and
connected to the A-D converter.
The output of the electronic bias supply is highly regulated and filtered. In addition, the output energy and
current have been limited should the operator come in contact with the input or guard points while
connecting or disconnecting an ion chamber. As a further safety feature, the bias voltage at the ion
chamber input connector is automatically discharged at power-down. The operator should always power
down the instrument before attempting to make or break a connection of the triaxial system.
3.13 Measurement Technique
The Model 35050AT Dosimeter can be operated in four basic measurement modes. These are
automatically selected by the microcomputer and are differentiated from one another by the use (non-
use) of a Model 35080/80A/80B kVp Divider, and the use of integrating (non-integrating) measurement
units. The subsections that follow contain brief descriptions of the measurement techniques utilized
during operation in these four measurement modes.
3.13.1 High Sensitivity Dose
This mode is activated when “35080/80A Not Used” 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, the
35080 Input is ignored and the microcomputer provides automatic drift and offset compensation, as well
as automatic post exposure display hold. Autoranging across all five-charge 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 rate of change corresponding to an input current of less than 0.25 pA is considered a
leakage drift and correspondingly nulled at the display. The start of an exposure is designated by the first
crossing to above this threshold level. An exposure is assumed to have ended when the signal remains
below the threshold level for 2 seconds.
At the end of each exposure, the integrator will be reset and a new measurement sequence initiated. The
displayed value of integrated exposure will be held until the beginning of the next exposure is detected or
until the operator presses the RESET/MEASURE button.
The instrument, when in this measurement mode, resets the electrometer's integrator whenever 5 pC of
charge has accumulated due to system leakage. At .01 pA leakage, reset would occur every eight
minutes. The reset is performed at this level to preserve the necessary “headroom” on the 10 pC full-
scale range for a true exposure.
During the automatic reset, the instrument is insensitive to any exposure. A very short exposure
occurring during this interval would be completely lost, and a long exposure only slightly reduced in value.
At the .01 pA leakage level, there is less than a 0.2% chance of the reset and the exposure coinciding.
As the leakage current increases, the chance of a coincidence increases proportionately. In the case
where the start of an exposure is detected immediately following the automatic reset, an error message is
displayed. This message will be cleared when the operator presses the RESET/MEASURE button or
repeats the exposure.