Fluke Biomedical 451P - Manual (Page 25)

Ion Chamber Survey Meter
Theory of Operation
15
The bar graph 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.
Table 2. Bar Graph Display Update Periods
Range Update Period
5 R/h (50 mSv/h) 0.05 second
500 mR/h (5 mSv/h) 0.05 second
50 mR/h (500 μ Sv/h) 0.05 second
5 mR/h (50 μ Sv/h) 0.05 second
500 μ R/h (5 μ Sv/h) 0.15 second
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
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 5 % of the full
scale.
Note
The same analysis applies to 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 unit’s 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 in calibration of 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.
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.