Home LMS THMG158 – Chemical Agent Monitors, Part III

THMG158 – Chemical Agent Monitors, Part III

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In this episode, Bob and Mike conclude their discussion of chemical agent monitors. You’ll hear about combustible gas monitors, metal oxide sensors, and polymerase chain reactions.

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Complete Show Notes

3:50 Combustible Gas Indicator (CGI) Monitors

  • Capable of picking up combustible gasses in high concentrations – i.e. the percentage of the atmosphere and the percentage of LEL
  • These are very large numbers in comparison of what we’re picking up with PID monitors
  • Three main types:
    • Wheatstone bridge
      • Brings the sample into the meter and pushes it across a bead that’s plated with a catalyst
      • Catalyst causes a combustible material to oxidize at lower temperatures than it normally would
      • The material basically reacts with the oxygen and oxidizes – this produces heat and causes the entire bead to get hotter
      • This monitor also has a wire that puts a voltage across the system – as the bead and wire heat up, resistance starts to increase
      • This change in resistance allows the meter to detect the presence of a combustible material in the atmosphere
      • Limiting factors:
        • Doesn’t work well in O2 outside of 19.5 – 23.5
        • When there isn’t enough O2, the monitor can’t react in the correct ratio and gives off too little heat
        • When there’s too much O2, the monitor isn’t able to react correctly and gives off too much heat
        • Doesn’t work well above UEL because it gets hot enough where the sensor burns out – some sensors compensate for this by locking out and shutting down the sensor until you move to fresh air
        • Some chemicals have the ability to coat the bead
        • Larger molecules have difficulty entering the bead – they also produce less heat
        • This decreases the sensitivity of the LEL sensor as the size of the molecule gets larger
    • Nondispersive infrared (NDIR)
      • One of the first CGI monitors out there, but is only now becoming well known
      • Shines IR light down a tube to a sensor – looks for carbon-hydrogen bonds as the gas comes in
      • Carbon-hydrogen bonds absorb some of the IR light – the dimmer the light, the more product there is in the air
      • Like Wheatstone bridge monitors, NDIR meters are capable of picking up gasses at high concentrations
      • Limiting factors:
        • If there isn’t a carbon-hydrogen bond, the meter won’t be able to pick up on the material
    • Thermal conductivity
      • ”Dumb” meter used by gas companies
      • Gas companies are dealing with known gasses (like methane), so they already know LEL and UEL
      • Can’t distinguish between different gasses – for example, it reads carbon dioxide and propane as the same
      • Only looks for thermal conductivity (ability to pull heat away from an element), not flammability
      • Ultimately, if you don’t know which gas you’re dealing with, this meter is pretty much useless

21:30 Metal Oxide Sensors

  • Fireman favorite used to measure natural gas and hydrogen sulfide
  • We use them most frequently as confirmation meters or for leak detection with natural gas
  • Also used in home meters to measure CO2
  • Work by reducing or oxidizing gas to read a difference in resistance – this difference is your reading
  • When you use these monitors, you’re dealing with oxidized metals – gas reacts with the oxides on the surface of the metal
  • This changes the voltage across the sensor, and that change in voltage is turned into a signal
  • Some sensors have the ability to convert to a concentration, but they don’t work well outside low concentrations
  • There’s a hold list of different metals that can be bound with O2 to produce a reaction with various chemicals
  • Limiting factors:
    • Most give you a tone pulse where a change in pitch or tone frequency indicates a larger concentration of a substance
    • These meters are very reliable, but they’re not very good at giving a high-range PPM reading
    • Like PID, metal oxide sensors don’t tell you anything about the material they’re picking up – just provides confirmation of the presence or absence of a certain material

27:25 Polymerase Chain Reaction (PCR) Monitors

  • PCR is a technique for amplifying the DNA molecules within a sample
  • These amplified molecules are then compared to known DNA samples to create a match
  • Portable versions of PCR exist that allow very fast and relatively accurate assessment (30 minutes to an hour)
  • Can be used with samples of E. coli, ricin, salmonella, smallpox, Anthrax, and much more
  • Extremely helpful because you can determine (potential) danger on-scene, rather than having to send samples into a lab
  • Limiting factors:
    • Can’t determine whether you’re dealing with multiple DNA samples – has to be a very pure sample
    • Sample has to be in the meter’s library

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