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940-7 Duct Monitor

940-7 Duct Monitor

The 940-7 Duct Monitor is a single-channel Geiger-Mueller (GM) area radiation monitoring system capable of operating over the range of 10-2 to 103 mR/h.

The system monitors gamma radiation over a 5-decade range and provides indication when the radiation level decreases below a fail threshold, exceeds a warn set point, exceeds a high set point or exceeds an overrange set point.


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Key Features

  • Range: 5 decades between 10-2 and 103 mR/h
  • Energy response: 15% from 80 keV to 1.5 MeV
  • No external power needed at detector location
  • Preamplifier integral with detector
  • Ratemeter may be remotely located up to
  • Single cable between ratemeter and detector 8 µCi 36Cl check source
  • Life expectancy: up to 105 rads
  • Replaces high maintenance off-line monitoring system
  • Class IE qualified
  • NRC approved


The over-range feature provides two significant benefits. First, it prevents the system from displaying an on-scale, but inaccurate, reading should the detector become saturated. Second, it lessens the risk of damaging the detector by disabling it during an over-range condition. Relay outputs are available to activate alarm annunciators. Analog outputs are available for trend display on a strip chart recorder or computer. In addition, the monitoring system has an integral isotopic check source to verify operational integrity. The system consists of a Model 897A series GM detector with integral preamplifier and the Model 956A Universal Digital Ratemeter (UDR). The detector is bolted to a mounting flange that matches the contour of the exhaust duct.


The 940-7 Duct Monitoring system is designed to be used in any size or shaped duct. One application where this type of monitor is used is the Control Room Intake Monitor at the Rochester Gas and Electric Ginna Station. There, the detectors are mounted in a 42 inch diameter duct. Since the detectors are mounted directly in the duct, there is no need for an expensive or high maintenance off-line type of monitoring system. These monitors can be used in a single channel configuration or grouped together as part of a multi-channel monitoring system.


Universal Digital Ratemeter

The Model 956A-201 UDR, when connected to a Model 897A series GM detector, comprises a monitoring system which operates over a five decade range. The UDR provides display, control, and annunciation functions for the monitoring system, and will display readings in the range of 10-2 to 105 mR/h. Standard features for the instrument consist of a three digit display of the radiation value and a multicolored barograph indicator which covers the entire range of the UDR. The barograph will change color in the event of an alarm condition (green for normal, amber for warning, and red for high). Front panel alarm indicators and rear panel relay outputs for alarm annunciation are also included. Front panel pushbuttons are provided to apply power, display alarm limit set points, acknowledge alarms, and activate the check source. Analog outputs of 0 to 10 VDC (1) and 4 to 20 mA (2) are provided for recording and computer monitoring. The outputs may also be used to drive a remote meter or a local (i.e. near the detector) indicator. All electronics required to interface with a Model 897A series detector are included within the Model 956A UDR. The electronics consist of a high voltage power supply, low voltage DC power supply and the hardware/software required for UDR operation. The system also includes an over-range indicator to preclude the possibility of on-scale readings when the radiation field is beyond the range of the detector.


The Model 897A Series detectors use a thin walled energy compensated GM tube to detect ionizing radiation. Each Model 897A series detector has a GM tube, a check source and a preamplifier. The check source is a low-level radioactive source actuated by a + 15 VDC meter movement. For positive operation, the check source is electrically driven in both the ON and OFF positions. The preamplifier provides the pulse conditioning and cable driving capability necessary to drive a Model 956A UDR. All Model 897A Series detectors are functionally identical. They differ only in housing material, tube type and range. The Model 897A series detectors operate in the voltage range between 500 to 650 VDC. The Model 897A Series detector is designed to operate with the Model 956A UDR or with other readouts. Each detector measures a five decade range. Three measurement ranges are available: low, medium and high. The low range covers 10-2 to 103 mR/h, the medium covers 10-1 to 104 mR/h, and the high covers 100 to 105 mR/h. The Model 897A Series detector may also be used with the Model 960 Digital Radiation Monitoring System (DRMS) equipment or the Model 856 Analog Readout.

Models & Accessories

Model Name


940-7 Duct Monitor, includes: GM Detector with integral preamplifier (897A Series) Universal digital ratemeter (956A)

Miscellaneous Accessories


Rack Chassis

Rack Chassis (part# 948-1)
Part number: 948-1
Model number: 948-1
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