Fluke-biomedical 451P User Manual download pdf (Page 24)

451P

Operators Manual

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.

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Fluke-biomedical 451P User Manual Page 24