Theory Of Operation - Fluke 481 Operator's Manual

Theory of Operation

To prevent personal injury, use care in handling this assembly
during removal and installation. This Product contains lithium
cells with a potential voltage of 63 V on the battery assembly.
To prevent damage to the Product, do not open or disassemble
the Product. The high impedance circuits of the ion chamber
are easily contaminated with grease and dirt that produce
electrical leakage.
The Survey Meter is an air ionization chamber meter calibrated in exposure rate
units of roentgens/hour (or Sieverts/hour) for gamma and x-ray in the energy
range of 20 keV to 2 MeV through the sliding Phenolic shield. It can also be used
for x-ray in the energy range of 7 keV to 30 keV with the sliding shield open. The
Survey Meter responds to, but is not calibrated for, beta radiation, with the slide
open. Beta energies that can be measured are above 100 keV. The two thin
Mylar windows have a combined density thickness of 6.6 mg/cm2.
The liquid crystal 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 quadruplex. This mode uses four
backplanes 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.
63 V collection voltage for the ion chamber is obtained from lithium cells that
have a 10 year life. All internal power for the Survey Meter is supplied by the 9 V
batteries, accessible from the rear of the Survey Meter.
The digital and bar graph displays read directly. The bar graph display update
periods are listed in Table 2. 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.
 Warning
 Caution
Ion Chamber Survey Meter
Theory of Operation
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