Fluke Biomedical 451B - Manual (Page 25)

Ion Chamber Survey Meter
Theory of Operation
15
There are 20 bars between each major division. The numerical values of the five
major divisions change appropriately for the range in which the Survey Meter is
operating. For instance, the first major division would have the numeric value of
1, 10, or 100. The minor divisions are worth 0.05, 0.5 or 5. The incremental
nature of both the digital reading and the analog bar graph provide greater
accuracy for reading in different portions of the scale. For example, on the
0 mR/h to 5 mR/h (0 µSv/h to 50 µSv/h) range, with a digital reading of 2.0 and
above, the analog bar graph can be read more accurately than the digital display.
Below a digital reading of 2.0, the digital display is more accurate because it
consists of three significant digits. The stated precision of the digital display is
accurate only above a reading of 0.20 on the 0 mR to 5 mR (0 Sv to 50 Sv)
scale.
Note
The same analysis applies to the all the other ranges because the
number of significant digits or active bar elements are independent
of the position of the decimal point or the units multiplier.
There is a small hysteresis built into the range changing circuit so that the Survey
Meter does not keep changing scales if the reading is at the threshold of range
change. It is important when calibrating the Survey Meter that the calibration
coefficients track from range to range because an oscillatory condition can occur
if the calibration on a given range is low and the coefficient for the next more
sensitive range is high.
In addition to FREEZE, the other message that can appear on the display is
LOW BAT. When the LOW BAT message appears, the batteries should be
changed within 1 hour.
The display will indicate HI for good chamber voltage bias condition.
The program in the Survey Meter ROM is proprietary to Fluke. The firmware
version appears in the digital part of the LCD display (prior to the “all elements
on” display) when the Survey Meter is turned on. The firmware program consists
of three main parts: operation, communication, and monitoring.
The operation portion of the firmware performs all of the control functions needed
to read and control the electrometer and range change amplifier, calculate
radiation rate, and display the calculated values on the LCD Display. In addition,
the measured data are smoothed and displayed in an exponential manner with
time that simulates the rise and fall time of an ordinary meter display. Range
changing is performed automatically. If a large increase in signal is detected, the
range changing skips to higher ranges bypassing exponential rise with time to
get to the new reading quickly. The Survey Meter continually integrates the
detected radiation signal and saves the accumulated amount that may be read
by the operator at any time. The operator may also reset the integration process.