Fluke-biomedical 451P User Manual download pdf (Page 25)

Ion Chamber Survey Meter

Theory of Operation

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 μSv/h) 0.05 second

5 mR/h (50 μSv/h) 0.05 second

500 μR/h (5 μSv/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.

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Fluke-biomedical 451P User Manual Page 25