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THMG145 – NDIR Overview


In this episode, Bob and Mike unpack non-dispersive infrared technology, or NDIR.

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Complete Show Notes

4:15 What is Non-Dispersive Infrared Technology (NDIR)?

  • Non-dispersive means light goes straight through the substance without scattering or changing direction
  • If we looked at a diagram of the sensor, we’d see a tube with a light source on one side and a sensor on the other
  • The infrared part of the light source isn’t scattered and lands within the infrared spectrum of light
  • Simply put, infrared is the light source that our thermal imaging camera picks up
  • We can’t see infrared light with our eyes – if we look at the electromagnetic spectrum, we have all different frequencies and wavelengths
  • We see the entire spectrum when we start learning about radiation – there’s a narrow band of wavelengths labeled visible light, along with a rainbow
  • The infrared region is just prior to red – we can see it from about 390 to 700 nanometers – this huge spectrum allows NDIR to work so well

11:15 History of NDIR

  • First developed by the military in the 1930s, but was classified until the 1940s
  • NDIR has undergone only around 5 different technology changes since the first sensor was created in the 30s
  • The first chemical sensor was designed for medical use and was an end title CO2 detector – in other words, it measures the amount of CO2 being exhaled
  • In the 1970s, NDIR was used to understand and fight against sensor drift – this was a major issue for the next 30 years
  • In the 1990s, scientists discovered the biggest issue with sensor drift was the source
  • This discovery and the advancements in computing power allowed the use of NDIR to expand
  • At this point, NDIR sensors are cheap and more specific than electrochemical sensors

15:20 What Can We Pick Up with NDIR?

  • We can pick up a lot, but the most common gasses we look for are methane, propane, CO, CO2, SF6, and almost all halogenated hydrocarbons
  • NDIR works a little differently than catalytic bead sensors (for LEL)
    • In catalytic bead technology, an air sample is passed over a bead that produces an oxidation (AKA burns it)
    • This heat is picked up by a change in the resistance of the wire or the voltage and is then correlated to a percentage of the LEL
    • With an LEL meter, the focus is on things that burn – mostly organic compounds
  • On the other hand, NDIR sends out one of two wavelengths of light – we’re looking for the carbon-hydrogen bond
  • The process is the same – the IR light is shone down a tube; the less light gets to the other side, the more product the meter calculates as being in the air

23:30 NDIR vs. Catalytic Wheatstone Bridge

  • Wheatstone bridge is an oxidation reaction – this means it requires oxygen to complete the reaction
  • For example, in order to oxidize 1 methane molecule, we need two O2 molecules for a 2:1 ratio
  • NDIR is only looking to see how much of the light is absorbed in the tube to translate that into a concentration
  • NDIR doesn’t require O2 to work properly – you’ll get a reading of the LEL of the atmosphere whether oxygen is there or not
  • It’s important to remember that there’s a correction factor with the Wheatstone bridge
  • The catalytic bead can pick up a wide range of flammable compounds – remember, it’s picking up anything that causes a change in the voltage
  • Change in voltage is directly related to the resistance of the wire, which has a few different variables – material, thickness, and temperature
  • Since the material isn’t changing and the thickness of the wire remains the same, the only other effect on resistance is the temperature
  • During an oxidation reaction, heat is going off – keep in mind that every molecule gives off a very specific amount of heat during the process
  • Larger molecules (like hexane) give off less heat than methane, and therefore have a different effect on the temperature of the wire (and therefore the resistance)
  • With NDIR, light isn’t absorbed by the molecule – it’s absorbed by the carbon-hydrogen bond
  • As the number of carbon-hydrogen bonds increases, NDIR becomes more sensitive – on the other hand, the catalytic bead becomes less sensitive
  • As we move away from saturated hydrocarbons into organic molecules with double bonds in the carbons, things become more complicated because NDIR LEL is looking for a carbon-hydrogen bond – these double bonds reduce the number of carbon-hydrogen bonds
  • Keep in mind that the presence of double or triple bonds changes how the bonds behave (i.e. acetylene)

34:15 NDIR and Sensor Degradation

  • Catalytic bead poisoning is a very well-known and well-understood issue, especially when dealing with response to certain chemicals
  • These chemicals include certain derivatives of hydrocarbons, hydrogen sulfide, and silicone vapors and compounds
  • However, NDIR doesn’t present these issues
  • NDIR also uses significantly less power than the catalytic bead
  • The catalytic bead can run 8 hours or more, so time becomes a factor when deciding between NDIR and the catalytic bead

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