100% means 100 parts out of 100:

  • 100% of anything means it is all the same thing
  • 100% gold means it is all gold, with no other contaminants

An illustration of various chemical molecules10% means ten parts out of 100 (1/10th of 100), so therefore it’s 90% something else.

1% means 1 part out of a 100:

  • 2% butterfat milk means that 2% is fat and 98% is something else, compared to skim milk is 1% butterfat or less

Eventually, we go down to 0.1%, 0.01%, 0.001%, but these numbers get kind of cumbersome, so it becomes convenient to change scale.

100% also means 1 million out of a million.

Therefore, 10% is the same as 100,000 parts of a million (ppm).

1% is the same as 10,000 parts per million.

This one is important: 1% = 10,000 ppm.

Let’s look at some common examples:

  • Oxygen in the air we breathe is 20.9% (or 209,000 ppm)
  • The time-weighted average (TWA) for carbon monoxide is 25 ppm (the same as saying 0.025%)
  • TWA for hydrogen sulphide is 10 ppm
  • Carbon dioxide in the air we breathe is around 500 ppm
  • Carbon dioxide in our exhaled breath is several thousand ppm

Eventually, as the numbers get smaller, it is convenient to switch scales again – this time, to parts per billion.

  • 1% = 10,000 ppm
  • 0.1% = 1000 ppm
  • 0.01% = 100 ppm
  • 0.001% = 10 ppm
  • 0.0001% = 1 ppm = 1000 ppb
  • 0.000001% = 0.1 ppm = 100 ppb
  • 0.0000001% = 0.01ppm = 10 ppb
  • 0.00000001% = 0.001 ppm = 1 ppb = 1000 ppt (parts per trillion)

A reading of 100 of anything lacks meaning without knowing the scale of measurement or what you’re measuring.

When measuring combustible gases, we use a slightly different scale.

Let’s use methane (CH4) as an example. We generally consider its LEL (lower explosive level) to be 5% by volume (5% bv). The mixture is too lean to burn if less than 5% methane is present. But at 5%, we can burn or explode if there is an ignition source.

So we call 5% bv methane the same as 100% LEL. 5% methane is 100% of the explosive concentration.

Most portable gas monitors will alarm at 10% of the LEL. The concentration is all the same whether we use the %LEL scale, the percent scale or the ppm scale.

  • 100% LEL = 5% methane = 50,000 ppm methane
  • 10% LEL = 0.5% methane = 5000 ppm methane
  • 1% LEL = 0.05% methane = 500 ppm methane

So here we have three numbers at the alarm level: 10 and 0.5 and 5000. Without scale, they’re meaningless.

Pretty much all combustible gas monitors are built to be layman-proof.

The operator doesn’t need to understand scale or range; they just need to know what to do when the alarm goes off.

It’s similar to when the temperature light in a car goes on. You don’t need to know if it’s measuring in Fahrenheit or Celsius or exactly how hot that really is; I just need to know to stop my car quickly. It’s my mechanic that needs to know all that other stuff.

If you’re in a position of authority, you may need to know all about units of measure, scale and range. If you’re working the tools, you just need to understand what to do when the alarm goes off, and trust that the alarm is real.

So generally, you need to know the units of measure to make an informed decision about the concentration, unless you have a device that gives you an “idiot-light” that screams DANGER, and you get out.

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  1. Jay Kay
    Jay Kay
    September 13, 2021 at 2:16 pm

    How does one calculate an LEL or UEL for a single solvent vapor, in air, at an elevated temperature?

    • Levitt-Safety
      September 15, 2021 at 3:54 pm

      Hi Jay,
      These are generally established limits for any given substance and these values can be found on the SDS sheet. For example, for Acetone, the UEL (upper explosive limit) and LEL (lower explosive Limit) can be found on the SDS sheet under section 9 – Physical and Chemical Properties. https://www.labchem.com/tools/msds/msds/LC10420.pdf

      The LEL will decrease as temperature increases. How much it decreases depends also on the size of the containment. The following link gives you a flavour of the somewhat experimental nature of calculating this. http://www.icders.org/ICDERS2005/abstracts/ICDERS2005-254.pdf

      If you need to calculate an LEL at an elevated temperature, as inside some kind of process containment, you’ll need to consult with a qualified engineer. Hope this helps!

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Paul Kroes, B.Sc.

Instrumentation Specialist

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