Home LMS THMG158 – Chemical Agent Monitors, Part III

THMG158 – Chemical Agent Monitors, Part III


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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