Home Podcasts THMG069 – Knowing and Understanding Units of Measurement

THMG069 – Knowing and Understanding Units of Measurement


In this episode, we delve into units of measurement (both U.S. and metric/SI) and how they help you stay out of trouble.

Complete Show Notes

5:00 U.S. Customary Units

  • Distance – inch, foot, yard, mile
  • Weight – ounces, pounds
  • Volume – fluid ounces, cups, pints, quarts, gallons
  • Temperature – degrees Fahrenheit
  • Pressure – psi (pounds per square inch) or atm (atmosphere)
  • Force – pound force
  • In the U.S. system, volume and weight are connected – “a pint’s a pound the world around”

6:00 Metric and SI (International System of Units) Units

  • Distance – meter
  • Weight – gram
  • Volume – liter
  • Temperature – degrees Celsius and degrees kelvin, which uses absolute zero as its starting point
  • Pressure – bar or torr
  • Force – newton

7:35 Comparisons and Observations

  • The United States uses one system, while pretty much everywhere else in the world uses the other one
  • Changing a scale in SI units is much easier than the U.S. system, since all you have to do is move a decimal place
  • The entire SI system is pretty much based on powers of 10
  • U.S. units are more cumbersome because you have to remember conversions within the system that aren’t standard
  • It’s important to know the meanings of different prefixes, like milli-, centi-, hector-, kilo-, etc. – tells you how many zeros there are and where they’re located
  • Common SI units we see every day (both on and off the job) include megabytes, gigabytes, liters, gallons, and more
  • The U.S. uses rads to measure radiation, while the rest of the world uses sieverts (or grays)

10:00 Measurement Conversions

  • There are plenty of resources to help you – online, phone and tablet apps, books, quick reference guides, etc.
  • PPM (parts per million) and PPB (parts per billion)
    • This isn’t part of either system, but it’s important to know
    • This is simply a ratio of one substance to another – we use it as a ratio of something in the air
    • It’s like a percentage, except we break the total into one million (or one billion) pieces, rather than 100
  • Percent concentration vs. percent LEL
    • These aren’t the same – and are only remotely related
    • LEL varies based upon specific gasses
    • Percent LEL gives you an idea of whether there’s a flammability risk and how close to boom you might be
    • Percent concentration measures the atmosphere, while percent LEL is an expression of zero to boom
  • Radiation and auto-scaling
    • This can also be confusion – we’re told to read the screen of our meter as “U” or “M” and the number
    • The “U” is actually “Mu” – looks like a small “u” with a little tail on the front
    • Mu is the SI symbol for “micro,” which we use as microrem – the smallest unit we use to measure radiation
    • “Micro” and the “milli” are relevant when measuring radiation because the meter shows you the number changes as the readings change
    • The meter changes the scale of the number on the screen to keep you from having to read a bunch of zeros – this is auto-scaling
  • Milligrams per cubic meter (or micrograms)
    • We use this when measuring mercury content in the air – back to the metric system
    • Your metering device tells you the density of mercury vapors within a particular space in milligrams or micrograms

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