Units of Measurement

LEL vs Percent By Volume vs Parts Per Million vs Parts per Billion

Here’s a detailed explanation of Percent by Volume (vol%) and Parts Per Million (ppm), along with some historical context:

Percent by Volume (Vol%) with Units of Measurement

Percent by Volume, also known as volume percent or volume concentration, is a unit of measurement used to express the concentration of one component (solute) in a total solution or mixture. It represents the volume of the solute relative to the total volume of the solution and is expressed as a percentage. For example, if a solution contains 20 mL of solute in a total volume of 100 mL, the concentration would be 20% (vol%).

Percent by Volume has been widely used in various fields such as chemistry, pharmacology, and industry for quantifying the concentration of substances in solutions or mixtures. It provides an easy-to-understand measure of the proportion of one component in relation to the total volume.

Parts Per Million (PPM) with Units of Measurement

Parts Per Million is a unit of measurement used to express the concentration or amount of one component in a larger sample size. It represents the number of parts of the solute relative to one million parts of the solution or mixture. For example, if there are 10 parts of solute mixed with one million parts of the solution, the concentration would be 10 ppm.

Parts Per Million is commonly used to measure trace amounts or very low concentrations of substances, especially in environmental or industrial settings. It is particularly useful when working with highly diluted solutions or when precise measurements of small quantities are required.

Historically, the concept of parts per million has been in use since the early 19th century. Initially, it was used in analytical chemistry and the field of gas analysis to express the concentration of impurities in gases. Over time, its applications expanded to various scientific disciplines and industrial sectors that required precise measurements of trace substances.

Both percent by volume and parts per million are valuable measurement units that allow for expressing the concentration of substances accurately. The choice between these units depends on the specific context, the concentration range involved, and the desired level of detail.

Here’s a detailed explanation of Parts Per Billion (ppb) and Lower Explosive Limit (LEL), along with their differences:

Parts Per Billion (PPB) with Units of Measurement

Parts Per Billion is a unit of measurement used to express the concentration or amount of one component in a larger sample size. It represents the number of parts of the solute relative to one billion parts of the solution or mixture. For example, if there are 1 part of solute mixed with one billion parts of the solution, the concentration would be 1 ppb.

Parts Per Billion is commonly used when quantifying extremely small concentrations or trace amounts of substances. It is often used in environmental monitoring, air quality assessments, water quality analysis, and other applications where precise measurements of very low levels of contaminants are required.

Lower Explosive Limit (LEL) with Units of Measurement

The Lower Explosive Limit (LEL) is a term used in the field of industrial safety and hazardous material handling. It refers to the lowest concentration or percentage of a flammable substance in air that is capable of generating a fire or explosion if an ignition source is present. LEL is typically represented as a percentage. For example, if a substance has an LEL of 1%, it means that the presence of the substance in the air at a concentration of 1% or higher would be potentially flammable.

LEL values are determined experimentally through testing and are specific to each flammable substance. They serve as a guideline to establish safe operating conditions, particularly in environments where flammable gases, vapors, or dusts may be present. Maintaining concentrations below the LEL is crucial to mitigate the risk of fire or explosion.

Difference between Parts Per Billion and Lower Explosive Limit

In summary, the key difference between Parts Per Billion (ppb) and Lower Explosive Limit (LEL) lies in their application and purpose. PPB is a unit of measurement used to express extremely low concentrations or trace amounts of substances, while LEL is a safety parameter used to determine the minimum concentration of a flammable substance in air that could result in an explosive atmosphere.

Unless you have a technical background, the concept of measurement scales can be complicated. I can’t tell you how many times I have had to explain them and sketch this out on a white board or napkin!

Units of Measurement and Scale

A 100% means 100 parts out of 100:

Also, 100% of anything means it is all the same thing

Therefore 100% gold means it is all gold, with no other contaminants

10%, means 10 parts out of 100, (1/10th of 100), so therefore its 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 which will be marked as 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)

Without knowing the scale of measurement or what you’re measuring, a reading of 100 of anything lacks any meaning.

Combustible Gases Scale and the Units of Measurement

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

If we use methane (CH4) as an example, the LEL (lower explosive level) of methane is generally considered to be 5% by volume (5% bv). That means the mixture is too lean to burn if there is less than 5% methane 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. Regardless of whether we use the %LEL scale, the percent scale, or the ppm scale, the concentration is all the same.

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 are meaningless.

How Gas Monitors Work

Pretty much all combustible gas monitors are built to be layman-proof. The operator doesn’t need to understand scale or range. The operator just needs to know what to do when the alarm goes off. It’s similar to when the temperature light in my car goes on. I don’t need to know if it’s measuring in Fahrenheit or Celsius or exactly how hot that really is. The operator just needs 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. When it does go off, you need to 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.