How the M43A1 Detector Unit works[13]

The Detector unit detects nerve agent vapor through a process of ionization (Figure 3).[14] As a pump draws air and any contaminants through the cell module, the air and contaminant molecules pass over a radioactive source and break up into charged pieces called ions. These ions then travel into the baffle section where the lighter and less stable air ions filter out. The collector then senses the current given off by the heavier ions formed from any nerve agent vapor. An electronic module, which monitors the collector, triggers the alarm when it senses a current change that matches the critical concentration of nerve agent.[15]

Figure 3. M43A1 Detector Unit’s Ion Cell Module[16]


When a military unit arrives in an area it plans to occupy (or is already in place at a Chemical/Biological High Threat Area during increased readiness postures), the M8A1 Alarm System operator should immediately put the system into place—determining its exact positioning after first considering wind speed, wind direction, terrain, and the tactical situation. The number of detectors and alarms used will vary depending on the type of unit and the tactical situation. A company-size unit will usually deploy with five M8A1 Alarm Systems.[18] Unless unusual circumstances or limitations exist (e.g., severe terrain features, a shortage of wire, etc.), the operator should place the M43A1 Detector unit(s) a maximum of 150 meters upwind from the farthest unit position. Since the alarms will go off in less than two minutes after a detection, placing this system at least this far away will allow enough time for personnel to take appropriate protective measures (e.g., putting on all Mission Oriented Protective Posture garments and mask, and verifying the alarm with two M256A1 Chemical Agent Sampler Detectors).

Figure 4. Placement of M8A1 Alarm System[19]

To minimize the possibility of nerve agent vapor drifting through gaps in the detector array, operators should place the M43A1 Detector units no more than 300 meters apart. To ensure that the electric signal remains strong enough to activate the alarms, operators should place the Detector units no more than 400 meters from the M42 Alarm units (Figure 4). To ensure proper positioning and proper maintenance, operators should receive training through formal classes or a correspondence course (e.g., Army Subcourse CM 7105).

A typical Air Force base requires adequate detection for the immediate 3x5 kilometer area. In order to achieve 5 minutes of warning with a 90% detection certainty, operators use a "Dice 5" pattern—laying out approximately 35 detectors 750 to 1500 meters apart. (The "Dice 5" pattern mimics the dot pattern on the fifth side of a die.) To protect personnel from chemical agents within key facilities, detectors are also used inside the perimeter.


To ensure the proper and continued operation of the M8A1 Alarm System, its operators must perform preventive maintenance checks and services (PMCS). PMCS is a series of mandatory equipment inspections and operational procedures, specified in the M8A1 Alarm System’s technical manuals. Operators perform these inspections before, during, and after operating (including troubleshooting) the system, its components, and ancillary equipment. When operators fail to properly inspect and maintain the M8A1 Alarm System, they increase the chance of false alarms from clogged filters, low batteries, or improper set up. Operators should note any observed deficiencies on the appropriate maintenance forms to ensure that these deficiencies are corrected. This paper will not discuss the step-by-step details of PMCS, nor its start-up and shut-down procedures, but will provide a general overview of each phase of PMCS and certain operations.


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