Fluke Biomedical 942A-200L-M4 - Manual (Page 59)

Operation
Analog Output
3
3-19
As an example:
Assume: LSV = 1E-1, FSV = 1E7, R = 1E3, IMax = 20 mA, and IMin = 4mA.
Then: P = [log (1E3/1E-1)] / [log (1E7) -log (1E-1)]
= log (1E4) / log ( E8)
= 4/8 = 0.5
and: I = 0.5 (20mA - 4mA) + 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 zero to force the
output high or low. If the low scale setpoint is set to zero, the analog output goes to low scale. If the high
scale value is set to zero, the analog output goes to full scale. If both are set to zero, the output goes to
full scale. Error code E0004 will be displayed if either setpoint is set to zero. The procedure for
calibrating the analog outputs is contained in Section 4.
3.15 Software Calculations
The 942A-200L-M4 and 942A-200L-M5 UDRs requires setpoints to be entered which are specific to each
detector and sampling geometry. The following explains the calibration constant and resolving time.
Calibration Constant
The first of these set points is a calibration constant. The calibration constant converts the detector pulse
rate into a CPM value, using the following equation:
D = (CPM- BKD) x K
Where: D = the calculated value in CPM (used for alarm setpoint limit checks)
CPM = the current, true, count rate in CPM (the sum of the 60 most recent 1
second values, corrected for Tau, updated once per second)
BKD = the optional operator entered background subtract set point, CPM
K = the optional user calibration constant in CPM
The value of K is a user-supplied calibration constant. The normal value is 1.0.
Resolving Time (Tau)
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____ (or Ro/1-(Ro x Tau)
1 - (Ro x Tau)
Where: CPM = the true count rate
Ro = the observed count rate (the sum of the 60 most recent 1 second
values) Tau = the resolving time in minutes/count
The value of Tau is supplied with the detector and is found on the detector calibration data sheet.