Home Podcasts THMG071 – Meter Reading, Sensor Response, and Measurement Metering Ranges

THMG071 – Meter Reading, Sensor Response, and Measurement Metering Ranges


In this episode, we continue to discuss metering and how to read and understand them.

Complete Show Notes

4:35 Meter Reading

  • Communication on a hazmat scene is a very important part of an effective operation – make sure you understand and utilize the basics of metering terminology, because it’s the foundation of our job
  • Our equipment gives us 4 basic types of meter readings – these individual “languages” need to be expressed properly and with the correct units to be universally understood
    • PPM (parts per million) and PPB (parts per billion)
      • Carbon monoxide, sulphur dioxide, hydrogen sulphide, and PID (VOC) readings are all expressed in PPM
    • Percentage of LEL – lower explosive limit
      • Lower explosive limit is expressed as percentage of LEL – no actual units
    • Percentage by volume or percentage of atmosphere
      • Carbon dioxide and oxygen are percentage of volume or percentage of atmosphere
    • Qualitative – no actual measurement (i.e. hot vs. cold)
  • Remember that some normal readings are zero, while others aren’t
    • Carbon monoxide, sulphur dioxide, hydrogen sulphide, and VOC readings are all normally zero
    • Carbon dioxide is normally present in air at around 0.04% by volume
    • Oxygen reading is normal at 20.8%
    • There is no “normal” LEL
    • With radiation, you should have an idea of what the normal background reading is for your location – it varies
    • Other than slight, infrequent cosmic blips, there is no normal neutron reading

16:15 Calibration and Correction Factors

  • Imagine that all flammable gasses are different dialects of the same language
  • The calibration gas is the dialect the meter knows perfectly, and it recognizes the others to varying degrees
  • The most important factor is the trend, which is where the LEL is increasing or decreasing

17:30 What Your Meter “Sees” and Cannot “See”

  • It’s important to be aware of what your meter can and can’t “see” and how small of something it can “see” (or how much of something has to be present for the meter to react
  • You also have to know how small of a change will cause the readings to increase or decrease (resolution) – this is because some instruments are designed to detect small amounts of something (i.e. leak detection), while other meters will miss those small amounts entirely
  • Knowing the response range of your meters is also important during the process of meter selection
    • For example, a small meter will react quickly and accurately to the presence of something, but only for a small incident in a narrow range
    • It sees small amounts at high resolution, but once the level rises above a certain amount, the meter is over range or “maxed out” and no longer useful
    • A catalytic bead sensor may not have the ability to “see” anything below 500 ppm for a particular something, while a semiconductor style sensor will “see” it at 5 ppm – the range is very important here
    • When you’re dealing with leak detection – which is typically a small amount in a specific location – the range is less important and the resolution is more important
    • An actual measurement isn’t as important as a simple qualitative, low range, high-resolution reaction when you’re dealing with leak detection

22:40 Overwhelmed Meters

  • Your meter can become saturated, or your environment can simply overwhelm the range of the sensor
  • This is dangerous because you can no longer monitor how the atmosphere is changing
  • In an oxygen-deficient atmosphere, some sensors will skew or read incorrectly
  • Ambient temperature, humidity, and dust can all adversely affect your meter and cause readings to skew
  • Catalytic bead sensors are usually designed so that if they become saturated, the meter will automatically shut down to keep them from burning out – in this case, the meter stops reading

26:30 Low and High Alarms

  • A low alarm can be silenced, while a high alarm can’t
  • However, once your meter exceeds its high alarm level and reaches saturation (over range), it stops providing readings
  • it’s very important for you to know how your meter will react next
  • Latching alarms
    • A latching alarm is an over range condition that can be reset by simply removing the meter from the contaminated atmosphere
    • Once you’re in cleaner air, the meter begins to clear and provides readings again
  • Locking alarms
    • A locking alarm is an over range condition that can only be reset by removing the meter to a cleaner atmosphere, shutting it down, and restarting it

29:15 Radiation Detectors

  • It’s also important to know how response range and resolution function are related to radiation
  • In this area – unlike anything else discussed on this episode – size matters, and looking at the physical size of the detector can give you a hint
  • A large detector will have a higher resolution and will more accurately detect and measure low levels of radiation
  • A small detector is designed to detect and measure higher levels of radiation
  • However, it’s still important to know when to switch from one size detector to another or from one meter to another
  • If your meter is “maxed out” at 25 mR/hour, you don’t know if you are being exposed to slightly or significantly more radiation
  • All high radiation alarms are latching alarms and will reset once they’re removed from the field

Have a question? Send an email to feedback@thehazmatguys.com or leave a message on our Haz Mat Guys comment hotline: 843-628-1484

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