Fluke Biomedical 956A-201-M2 - Manual (Page 45)

3-19
If the current reading (R) is greater than the full-scale value (FSV) the output is limited to the MAX. value
(typically 10 Vdc or 20 mA).
As an example:
Assume: LSV = 1E-1, FSV = 1E7, R = 1E3, IMax = 20 mA, and IMin = 4 mA,
Then:
P = [log (1E3/1E-1) ] / [ log (1E7) - log (1E-1)]
= log (1E4) / log (1E8)
= 4/8 = 0.5
and: I = 0.5 (20 mA - 4 mA) + 4 mA
= 0.5 (16 mA) + 4 mA
= 12 mA
To aid in calibrating the analog output, the high scale or low scale setpoint may be set to force the output
high or low. If the low scale setpoint is set above the displayed value, the analog output will be forced to
low scale. If the high scale value is set below the displayed value, the analog output will be forced to full
scale. The procedure for calibrating the analog outputs is contained in Section 4.
3.12 SOFTWARE CALCULATIONS
The 956A-201-M2 UDR requires setpoints to be entered which are inherent to each particular detector.
The following explains the calibration constant and resolving time.
Conversion Constant
The first of these setpoints is the conversion constant. The conversion constant converts the detector
pulse rate into an mR/h value, using the following equation:
D = (CPM x K)
Where: D = the calculated value in mR/h (used for alarm setpoint limit checks)
CPM = the current, corrected, count rate in CPM (the sum of the 60 most recent one (1) second
values, corrected for Tau)
K = the conversion constant in mR/h/CPM
The value of K is supplied on the detector calibration data sheet.
Resolving Time (Dead Time)
This constant is a correction for the resolving time of the detector. As the radiation field that the detector is
viewing increases, the detector cannot count every pulse, because some are in coincidence or are so
close together that two pulses may look like one. To correct for this non-linearity, the resolving time is
corrected for by the following equation:
CPM = Ro
1 – (Ro x Dead Time)
Where: CPM = the true count rate
Ro = the observed count rate (the sum of the 60 most recent one (1) second values)
Dead Time = the resolving time in minutes/count
The value of Dead Time is supplied with the detector and is found on the detector calibration data sheet.
This value identified as TAU on the data sheet.