What do LEL%, VOL% and PPM mean in gas detectors?

In order to ensure safe production, gas detectors are widely used in various industries. Gas detectors can monitor the leakage concentration of toxic and harmful gases and flammable gases in the air in real time, effectively preventing the occurrence of various accidents such as fires, explosions and poisonings. When using gas detectors in daily life, users will find that the detection range on the instrument label often shows similar figures such as 0 - 100LEL% or 0 - 2000PPM. Or on the LCD screen of the gas detector, VOL% or PPM may also appear. What do these three units specifically mean and how do they convert among each other?

1. VOL% (Volume Percentage of Gas)

VOL is a physical unit that describes the volume of gas. Expressed as a percentage, it represents the percentage of the volume of a specific gas in the air. For example, 5%VOL methane means that the volume of methane in the air is 5%. The detection range of many gas detectors is expressed in VOL%. For example, if the detection range is 0 - 100%VOL, it means that when this gas detector detects a certain gas, it can detect the proportion of the gas in the air within the range of 0 - 100%. We can also set a certain percentage value of VOL as the alarm point. When the content of a certain gas reaches or exceeds this set value, the gas detector will alarm. This involves another unit, LEL%.

2. LEL% (Lower Explosive Limit)

The concept of flammable gas refers to the gas that can be evenly mixed with air (or oxygen) within a certain concentration range to form a premixed gas, which will explode when encountering a fire source. The lowest volume percentage concentration of this flammable gas in the air that can cause an explosion, that is, the gas explosion lower limit concentration, is LEL%, abbreviated as "lower explosive limit". The gas volume concentration of the lower explosive limit is expressed in VOL%, and its unit is percentage, that is, the lower explosive limit is divided into one hundred parts, and one unit is 1 LEL%.

3. PPM (Parts Per Million of Gas Volume Percentage)

The concept of PPM is similar to that of VOL, except that PPM represents one millionth of the gas volume. For example, 10PPM of carbon dioxide means that there is 10 parts per million of carbon dioxide in the air. Since the PPM unit is a dimensionless unit ("dimensionless unit" is an astronomical term, which includes pi (π), natural constant (e), radian (rad), golden ratio (φ) and relative molecular mass (Mr), etc. The opposite of it is the dimensional quantity, which has units such as length, area and time. Dimensionless quantities are often written as the product or ratio of two dimensional quantities, but after their final dimensions are eliminated from each other, dimensionless quantities are obtained). Gas detectors that can detect PPM levels are used to detect slight gas leaks in the working environment, because slight gas leaks are very dangerous and may cause major accidents after a long time. Therefore, PPM level gas detectors are needed to detect and eliminate the locations of slight gas leaks in a timely manner.

4. 
   Conversion among VOL%, LEL% and PPM

First, the conversion between VOL and PPM is relatively simple. Because %VOL is the percentage of volume and PPM is the millionth of volume, 1%(VOL) = 10,000PPM.

Second, for the conversion between VOL and LEL, it is necessary to first find out the lower explosive limit value of the flammable gas. When the concentration of the flammable gas in the air reaches its lower explosive limit value, we call the explosion hazard degree of the flammable gas environment in this place 100%. For example, the LEL of hydrogen is 4%VOL, that is, when its volume percentage in the air reaches 4%VOL, it will explode when encountering an open fire. Therefore, regarding 4%VOL as 100% danger is called 100%LEL, that is, 4%VOL = 100%LEL, then 1%VOL = 25%LEL.

Third, the conversion between PPM and LEL cannot be done directly. It is necessary to first convert LEL into VOL, and then convert VOL into PPM. Here is a formula for you. PPM = %LEL × LEL(vol%) × 100. Taking methane (with a lower explosive limit of 5%) as an example, to calculate how many PPM is 20%LEL of methane, according to the calculation formula: 20(%LEL) × 5(%VOL) × 100 = 10,000PPM.

Generally speaking, gas detectors that can detect PPM are very accurate, while gas detectors of LEL are generally used in explosion detection scenarios. The order of magnitude of VOL is the largest among them and is also more commonly used.

In conclusion, the relationships among LEL%, VOL% and PPM in gas detectors are interrelated. Through appropriate conversion formulas, we can convert them into each other to meet different measurement needs and application scenarios. In practical applications, understanding the relationships of these units can help us more accurately evaluate gas concentrations and process relevant measurement data. Therefore, the correct use and understanding of these units are of great significance for ensuring the reliability and safety of measurement results.