Home Podcasts THMG014 – Metering Overview and Basic Sensors

THMG014 – Metering Overview and Basic Sensors

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In this episode, Mike and Bob discuss the enormous amount of information on the topic of metering and sensoring. They also explain the most common technologies and techniques used in monitoring.

Complete Show Notes

5:50 Why Metering Matters

  • Metering is vitally important to hazmat – it determines the atmosphere we’re entering and the types of harms we expect it to impose
  • Helps us gauge isolation distances and PPE – monitoring is dynamic and constantly evolving

6:55 Why Do We Monitor a Scene?

  • Establish zones and exposure potentials for us and the public
  • Locate and confine a substance or issue
  • Establish lines of ILDH (immediately dangerous to life and health)
  • Determine whether something is flammable, oxygen rich/deficient, or toxic
  • Confirm that our actions are beneficial to the operation
  • Each monitor reads a different thing that we’re interested in – you can’t just use one meter
  • Meters bridge the gap between our senses and what’s truly happening in an area – in some cases, our senses don’t alert us to danger until well after a chemical has a problematic effect on us or our environments

11:30 Things to Meter For

  • 1. Radiation – this is one of the first things you want to meter for because it can touch you from a long distance away
  • 2. Flammability – directly impacts PPE selection and whether we’re dealing with toxics
  • 3. Oxygen
    • Influences PPE and SCBA selection – is there enough oxygen for you to breathe in that environment?
    • Oxygen can really screw with your meter readings
    • Can affect flammability, reactivity, and change the products of reactions
  • 4. Toxicity – affects you (not just the environment), so this is a really important thing to meter
  • 5. Corrosives – very easy to monitor, so it’s something you should be monitoring at all times; use fluoride paper for easy identification; can ruin your meter
  • 6. Reactivity – whether a reaction is happening that’s going to be endothermic or exothermic

17:05 Terminology

  • Response time – time the meter needs to get a sample and put a number on the screen/move the needle
  • Two types of collection methods that meters use
    • Pass-by/diffusion – sample naturally passes by a sensor and gives a reading
    • Pump-through – pump (hand or electronic) used to get substance to the sensor
  • Sensitivity – can be equated to the LEL and UEL of flammability; the more sensitive something is, the quicker the meter will pick it up
  • Selectivity – how accurately a meter can monitor either a single gas or group of gasses; don’t want interference from other compounds, as they can be lumped together in the reading
  • Operating range – accuracy range for a given concentration; in some cases, if the range is smaller, the accuracy gets better
  • Amplification – the meter’s ability to magnify a small quantity and give a reading; outside interferences may come into play
  • Accuracy – the relationship between the true reading and the meter’s reading; may have to use the correction factor to arrive at a correct value
  • Reliability – how consistently a meter will give you a reading
  • Calibration – calibration sets the sensor reading to a known value, so it adjusts its zero reading to comply; meters need to be calibrated periodically because they expire
  • Bump tests
    • Field tests where a small sample of a known gas is used to express a response from the meter
    • Can be the agent or a known cross sensitivity
    • Tests alarms and notifications
    • Different than a fresh air setup

25:25 What is Relative Response?

  • Meters are calibrated against a known gas – typically one that’s in the middle of the range we’re looking for
  • Meters think they’re looking at that specific gas all the time and reads accordingly
  • Use a correction factor chart where 100% equals 1.0
  • The reading on the meter should be multiplied by the correction factor for the actual gas you’re reading, which gives you a true reading

29:40 Cross-Sensitivities and Interferences

  • For the most part, chemical reactions drive these meters
  • Cross-sensitivities occur when other substances act like the chemical you’re looking for with your meter
  • These chemicals are usually known substances from the manufacturer
  • Important to keep the paperwork that comes with your meter – manufacturers do testing to give you a baseline of the substances the meter detects
  • An interference is something that will give you an out of true reading – examples include temperature and humidity
  • Different interferences destroy meters – i.e. carbon dioxide ruins oxygen meters

32:10 Action Levels

  • Very straightforward – it’s the point at which you must perform an action
  • Everybody has different action levels depending upon what their job is
  • Most meters allow you to put in high and low alarms based upon your action level
  • Actions include evacuation, calling for more resources, putting your SCBA on, getting another meter, etc.

33:30 Monitoring Techniques

  • When you’re metering, you always have to consider whether the gas will rise or fall
  • Do this by referencing the NIOSH book to determine the chemical and physical properties of the gas
  • Remember that some gasses are affected by temperature and wind currents that may invalidate your data
  • All areas should be monitored for true readings – these provide the greatest accuracy

34:45 The Nitty Gritty

  • Many different types of sensors out there
  • Oxygen meters
    • Low oxygen is 19.5%
    • Low oxygen skews readings on CGIs because you need oxygen to take flammability readings
    • High oxygen is 23.5%
    • High oxygen increases flammability
    • Easily affected by temperature – huge variations in temperature seasonally
    • These meters read by chemical reactions that create electrical impulses
    • Altitude can also have an effect on your readings
  • Combustible gas indicator (CGI) meter
    • Can be used as a pass-by or pump-through
    • Has a chamber with a filament coated with some kind of catalyst
    • Wheatstone bridge inside the meter – electronic circuit that has a sample circuit and a known circuit – measures the distance between the two to take a reading
    • Temperature affects this meter – high temperatures give you higher readings
    • Normal oxygen levels are required to read this meter properly
  • Metal oxide meters
    • Used to measure natural gas and hydrogen sulfide – also used in home meters to measure CO
    • Measures by reducing or oxidizing the gas to read a difference in resistance, which is your reading
    • Very reliable meter, but not very good at giving a high range PPM reading
    • Many metal oxide meters give you a tone pulse – a change in pitch or tone frequency indicates a greater concentration of a substance
  • pH meters and strips
    • There are meters that perform the same action, but they’re typically not field-friendly
    • Require a buffer solution to be used before the reading
    • The most common way to determine pH is the pH strip – it provides a color change based on the amount of H or OH
  • Colorimetric methods
    • Two main types in use – the CDS and the CMS
    • CDS is a colorimetric glass tube that requires a pump
    • The CMS is the electronic equivalent of the CDS and uses a chip that measures digitally on a screen
    • They can be very specific in what they’re reading or can read a range of gasses (i.e. halogenated hydrocarbons)
    • These work by passing gas through a tube with filters, desiccants, and chemicals that react when the gas is drawn through the tube
    • The amount of pumps aid in determining the concentration
    • Keep in mind that humidity and temperature may affect readings
    • These meters take a long time to read – sometimes up to 100 minutes – and you may need to pump the trigger dozens of times
    • Always read the instruction manual prior to use – each tube may have its own individual instructions
  • M8, M9, and M256 kits
    • M8 paper looks like pH paper and is dipped into the substance – it indicates using a color change that can be compared to a chart on the box
    • M9 is a tape that detects the droplet of an agent – it gets splotchy, which indicates a presence
    • M256 kit is a rather complex method – it uses ampules of reagent and colorimetric tubes to detect nerve agents, blister agents, and blood agents; very time-consuming and requires 2 people; can register well below the required regulatory limits
  • Radiation detectors
    • The human body has no way to sense radiation exposure – it shows eventually, but by then, it’s too late
    • There are several kinds of radiation meters
    • Geiger Mueller
      • This is a gas-filled tube that senses the amount of ions
      • Sealed from outside air
      • Good meter for alerting you that something is there, but it doesn’t have a wide range
      • Low and high energy uses specific probes
    • Ion chamber
      • These have a tube that utilizes ambient air to detect ionized gasses
      • Beneficial because their readings are directly related to the intensity of the radiation
      • When you have radiation of varying intensities, this reflects a true reading, rather than an average
    • Scintillation detectors
      • These use a crystal that interacts with radiation – crystals can be made of sodium iodide, cesium iodide, or zinc sulfide
      • Causes a brief flash of light in the crystal – the meter then amplifies the flash of light thousands of times
      • May have a Mylar film over the sensor to protect from debris, temperature, and humidity
      • Most useful when reading small amounts of radiation
    • Dosimeters and badges
      • Typically worn by a user to measure the dose that individual has taken
      • These are self-reading, which means they don’t need to be sent to a lab – you can get your dose by looking at a screen
      • Personal dosimeters typically only read gamma radiation – not alpha and beta

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    Author: The HazMat Guys