The sample is withdrawn from the process flow stream via isokinetic nozzles through the customer’s sample line to the isokinetic splitter. The splitter causes isokinetic flow through the respective motorized selector valve to either the normal or accident range sampling system. (The Normal Range System is described in the Model 940-1 data sheet.)
Under accident range conditions, the sample passes through one of the redundant filter paths and the mass flow controller to the accident range gas sample volume and then through the pumping system and back to the process flow stream. Where isokinetic sampling is required, a bypass flow path for the full normal range flow rate is provided, and a flow rate of 1,000 SCCM is routed through the accident range monitor. Filter transfer may be initiated manually, or automatically, based on gas channel activity. Two lead shielded filter carriers are provided to safely remove the loaded filters and transport the filters for laboratory analysis.
The normal range system must have the capability of providing a control contact which is set to make contact approximately 1 decade below full scale.
This contact initiates the switch-over of the selector valves, closing the sample path to the normal range skid, shutting down the normal range pumping system, starting the accident range pumping system, and sounding the alarm. When the radiation drops below the top of the first decade of the accident range system, the sample flow reverts to the normal range path and resets the alarm.
The accident range detector signal is displayed by the accident range Model 960 Local Control Unit in gross noble gas counts or concentration in µCi/cc, and compensated for changes in pressure at the gas volume chamber. A low flow alarm for the accident range sample flow is also provided.