Two (2) Model 942A Universal Digital Rate meters are used to process the detector output and display the reading in units of counts per minute (CPM).The first UDR operates in the “gross” mode, displaying the total output of the detector. The second UDR is provided with a single channel analyzer (SCA) option circuit board, set to display the count rate for a specific, control isotope, normally 135I. In the SCA mode, an 135I sensitivity of 1E-4 µCi/cc may be realized to detect fission product activity at the 1% failed fuel element level. Using the attenuator plug, the range may be extended up to 1E+3 µCi/cc, approximately 30 times higher than the expected activity at the 1% failed fuel level. As an option, a single UDR maybe provided, with a front panel mounted Gross/SCA display select switch. In operation, the detector output will be monitored by the UDR located in the control room. An optional Model 942-200-80 Serial Communication Module is available to provide monitor status and historical data via a serial port, for use by the plant computer or a laptop PC.
The radiation value calculation is a measurement of the average radioactivity seen by the detector. The equation for the radiation value is:
CPMnet = (CPM - CPMBKRD) x Ka)
CPMnet net detector output, counts per minute
CPM detector output, gross or SCA, counts per minute. This is the average of the 60 previous counts per second values, updated once per second
CPMBKRD set point, detector output due to ambient background radiation, counts per minute
Ka user enterable radiation value compensation factor. May be used for sample volume pressure compensation, flow compensation, or engineering units conversion
The display is updated once per minute, and is the result of the sum of the last 60, one second values. Longer counting times, up to 20 minutes are available through use of the statistical accuracy jumper options provided on the USR.