Parts Per Million or Parts Per Billion - What do you use and where?
Gas Detectors are sensitive devices, but what do any of the numbers actually mean? Sometimes you measure them as a percentage, other times as ppm and still others as ppb. The measurement used will depend on the industry doing the measuring. It will also depend on the substance you are measuring.
Since we’re talking about gases, shouldn’t the unit stay the same? If you’re measuring distance you don't measure miles in one industry and kilometers in another. Why do we do this for gases?
How we measure
It turns out we actually are using the same measurement for gases. Each of these “units” is describing how many molecules of a gas are in the air. What is changing is the scale.
You wouldn’t measure the distance between houses in a city in kilometers if they were right next to each other. You also wouldn’t measure the distance between cities in meters. You’d use meters (or a similar unit) for the houses and kilometers for the cities.
Oxygen measures in at a beefy 21% of air mass. Carbon dioxide ranks a lowly 0.05%, and a deadly dose of carbon monoxide is at about 0.0001%.
When you start to see numbers this small it becomes obvious that a new scale is necessary. Particles per 100 (percentage) doesn’t work for most things. When you are working with smaller numbers it becomes difficult to see much change.
For this reason we use ppm (parts per million) and ppb (parts per billion) as well as percent. We measure most gases in the ppm range. They are neither as common as oxygen but they also don’t tend to be dangerous at lower ppb levels.
Measuring at ppm we can see that Carbon Dioxide is now described as being 5000ppm instead of 0.05%. Incremental changes in the amount of carbon dioxide in the air will show up much better.
What about LELs?
LEL is a different unit – sort of. LEL stands for Lower Explosive Level. It applies only to combustible gas detection. We define it as the lowest amount of a substance required for an explosion.
The LEL doesn’t take into account whether an ignition source is present. It only looks at if the substance could explode if it were ignited. An LEL of 100% indicates the minimum level of gas for an explosion is present.
Methane, for example has an LEL of 4.4%. This means that when Methane makes up 4.4% of the total air mass, or 44,000 ppm it can potentially explode.
Thankfully conversion between these units is rarely necessary. The devices will always measure in the same units the regulations use.
With this in mind hopefully it will be a little easier to approach the myriad of numbers and measurements used in gas detection.