Fluke Victoreen 451P, Руководство оператора

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Theory Of Operation
Introduction
3
3-1
Section 3
Theory Of Operation
3.1 Introduction
The user is cautioned about indiscriminately
opening and disassembling the instrument. 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 Model 451P is a pressurized ionization chamber instrument 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
Model 451P 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 (4) 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 instrument 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 5 lithium cells. All internal
power for the instrument is supplied by the 9-volt batteries.
The digital and bargraph displays read directly. The bargraph display update periods are listed in the
table below. The digital display updates at one second intervals nearest the current bar display update.
The bargraph and digits display do not always show the same reading because the bargraph is faster
than the digital update. It is more convenient to watch the bargraph when the reading is changing quickly
and to read the value of a slowly changing or static reading by looking at the digital display.
The bargraph display is a digital presentation, programmed to appear as a linear analog meter display. It
is also referred to as the analog display throughout this manual.
NOTE