Fluke Biomedical 451P - Manual (Page 24)

451P
Operators Manual
14
Theory of Operation
 Caution
To prevent damage to the Product, do not open or disassemble
the Product. The ionization chamber is pressurized and sealed
at the factory, because the high impedance circuits of the ion
chamber are easily contaminated with grease and dirt that
produce electrical leakage. The ion chamber cannot be
disassembled for servicing.
The Survey Meter is a pressurized ionization chamber meter calibrated in
exposure rate units of roentgens/hour (or Sieverts/hour) for gamma and x-
radiation in the energy range of 20 keV to 2 MeV. The Survey Meter responds to,
but is not calibrated for beta radiation. Beta energies that can be measured are
above 1 MeV.
The liquid crystal, supertwist display shows the radiation rate in digital and
analog form with the range multiplier values also showing on the scale. It is a
lightweight electronic device that requires the computational capabilities of a
microprocessor to make it operate. It functions in a multiplex mode called
quadriplex. This mode uses four back-planes to accommodate the 128 elements
of the display.
The microprocessor performs data collection, averaging, and multiplication by
stored calibration factors, range changing, and battery check functions, in
addition to driving the LCD. Between computational periods, it “sleeps” in a low
power mode to conserve battery power. The microprocessor reads stored
information from an electrically erasable memory, EEPROM, which is used by
the program for calibration and display units. The EEPROM will retain stored
data when the Survey Meter is OFF or when the batteries are removed. Data can
be entered into the EEPROM using the RS-232 port.
Collection voltage for the ion chamber is approximately 105 V obtained from five
lithium cells. All internal power for the Survey Meter is supplied by the 9 volt
batteries.
The digital and bar graph displays read directly. The bar graph display update
periods are listed in Table . The digital display updates at 1 second intervals
nearest the current bar display update. The bar graph and digits display do not
always show the same reading because the bar graph is faster than the digital
update. It is more convenient to watch the bar graph when the reading is
changing quickly and to read the value of a slowly changing or static reading by
looking at the digital display.