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.
- Before-Operations Checks:[20] During this check, the operator inspects the M8A1 Alarm System internally and externally—including components and ancillary equipment—for cleanliness, damage, or any other defect (e.g., missing parts). Among other things, the operator must check seals for leaks, ensure that the air flow through the detector is within the designated range, check the batteries for sufficient voltage, test the audible signals (or horns),[21] and ensure that the detector is capable of sensing a nerve agent simulant.[22] During before-operations checks, the operator should pay close attention to all operational cautions and warnings in the technical manual or on the data plates to properly prepare the system for operation.
- Start-up and Operational Procedures:[23] After completing the before-operations checks, the operator can place the M8A1 Alarm System into operation using a step-by-step procedure detailed on the exterior data plates of both the M43A1 Detector and M42 Alarm units. If starting cold, the Detector unit can take up to 15 minutes to warm up.
- During-Operations Checks:[24] While in operation, the operator must check the system periodically to ensure that it is operating properly. One periodic check is a 24-hour reservice check. The operator must also conduct another check every time the system is reactivated after an operational alarm. (The operator conducts this check in response to the system’s alarm and to verify the cause of the alarm—i.e., the presence of nerve agent vapor). If the operator suspects a chemical attack, individuals take immediate protective measures as proscribed by doctrine and standard unit procedures (including upgrading the unit’s MOPP and decontaminating the system). If a chemical agent is verified, the system must be decontaminated. Once the operator decontaminates the system, start-up procedures commence again. The operator repeats start-up procedures and during-operations checks until the unit commander determines the area to be clear (usually after two negative M256A1 Chemical Agent Detection Kit results).
- Operations under Unusual or Severe Conditions:[25] Operators must follow special M8A1 Alarm System operational procedures when operating this system under unusual or severe conditions. Unusual or severe conditions include: blowing dust or sand; rain, sleet, or snow; temperatures below -40� F and above 120� F; humidity equal to 3-100 percent relative humidity; fording; and emergency operations undertaken with broken controls or indicators. While operating under unusual or severe conditions, operators may have to change batteries and filters more often than usually required.[26]
- Shut-down Procedures (After-Operations Checks):[27] After shutting down the system—and to prepare it for storage—the operator should make an internal and external inspection of the system (including its main components and ancillary equipment) for cleanliness, damage, and other defects. During these procedures, the operator should pay particular attention to removing, storing, and/or destroying the Detector’s outlet filter as explained in the technical manual.
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