Fluke Biomedical 945A - Manual (Page 25)

Section 4 - Operation/Functional Description
Operation
This section describes the operation and set up of an area monitoring system consisting of a Model
946A-200 UDR and a Model 977-210 Detector/Preamplifier interconnected via a serial communications
interface. The detector preamplifier provides the electronics interface between the detector and the UDR.
It consists of a high voltage/communications interface printed circuit board and an electrometer printed
circuit board. The preamplifier measures the current output of the ion chamber, generates the high
voltage supply for the detector, performs periodic diagnostic checks, and performs an electronic check
source function on demand from the UDR. The current measurement is converted to a dose rate and
this, along with the operational status of the preamplifier and detector, are transmitted to the UDR.
The 946A-200 UDR uses this information to generate the dose rate display, error codes, alarms, and
analog outputs.
Operation of the 946A-200 UDR consists of operator functions and configuration functions. All operator
functions are performed using the front panel shown in Figure 4-1. Configuration functions are performed
using internal switches and/or jumpers, which are accessible with a partial removal of the UDR from the
mounting enclosure (refer to Table 4-6). Calibration involving trimpots are described in Section 6.
Detector Operation
The Model 977-210 wide range ion chamber detector is an eight decade, linear reading radiation monitor.
The chamber, cables, and preamplifier are sensitive to moisture; therefore, the detector and preamplifier
are sealed. The preamplifier box contains a desiccant package to remove moisture from the interior of
the preamplifier. The desiccant has a color indicator to show when it should be replaced.
The detector electrometer converts the current from the ion chamber to a voltage which is measured by
the analog to digital converter of the preamplifier. Under program control, this measurement is made
every 50 milliseconds on the first five decades and every 100 milliseconds on the upper three decades.
The electrometer is auto-ranging and auto-zeroing. An internal coarse zero control (factory adjusted) is
provided. The microprocessor in the preamplifier performs data collection, averaging, multiplication by a
stored calibration factor, range changing, high voltage check, detector cable integrity check, and
communication with the 946A-200 UDR.
The high voltage check and integrity check are run, under program control, every 256 seconds. The high
voltage check is a measurement of the high voltage to ensure that the value is sufficient for 100%
collection at full scale.
4-1