Comments Off on Parts-per-million or percent by volume measurements versus percent LEL measurements
I was just reading your information on gases produced by lead acid batteries and came across this in the “Ask Bob” blog:
For LEL range measurement, using a standard catalytic combustible gas (CC) sensor with a range of 0 – 100% LEL is a good approach. For situations where you need to take action at a lower concentration, using an electrochemical (EC) toxic gas sensor to measure the hydrogen may be a better approach. The typical range for an EC hydrogen sensor is 0 – 2,000 ppm. (This is equivalent to a range of 0 – 5.0% LEL.)………..Should that be 0–0.05?
This is a good question.
It’s important not to confuse parts-per-million or percent by volume measurements with percent LEL measurements. And decimal places can be tricky!
Hydrogen (H2) can be measured by means of catalytic type percent LEL combustible gas sensors, or by means of substance-specific electrochemical sensors.
Catalytic LEL sensors detect gas by oxidizing or “burning” the gas. Catalytic LEL sensors require the presence of oxygen in order to detect gas. Standard catalytic LEL sensors cannot detect gas if the atmosphere contains too little oxygen. Most of the time measurements for combustible gas are given in percent LEL (% LEL). The reading provides a comparison of the measured concentration against the LEL concentration of the gas used to calibrate the sensor. Deciding which combustible gas or “scale” to use when you calibrate the sensor is an important issue. These issues are discussed in detail in Application Note 1018, “Understanding catalytic LEL combustible gas sensor performance.” The note is posted at the following link: http://goodforgas.com/wp-content/uploads/2013/12/AP1018_Combustible-sensor-performance_6_30_13.pdf
Electrochemical sensors use a substance-specific chemical reaction that causes a change in the electrical output of the sensor that is proportional to the concentration of the measured gas. The electrochemical hydrogen sensors in GfG instruments can be used for extended periods of time to measure H2 in oxygen free atmosphere. Most substance-specific electrochemical (EC) sensors read in percent volume or parts-per-million (ppm) units. GfG offers several different versions of electrochemical hydrogen sensors for different applications.
For low range leak detection we usually use an electrochemical hydrogen sensor with a measurement range of 0 – 2,000 ppm. The readings for this sensor are given in 1.0 ppm increments. The sensor is able to detect changes in concentration of ± 1.0 ppm.
When the application calls for a wider measurement range we use an electrochemical hydrogen sensor with a measurement range of 0 – 4.0% volume. The readings for this higher range hydrogen sensor are given in 0.01 percent volume increments. One percent (1.0%) volume is equal to 10,000 ppm. One hundred percent (100%) volume is equal to 1,000,000 ppm. So, 0.01% volume = 1,000,000ppm X .0001 = 100 ppm.
The higher range sensor cannot detect changes in concentration less than 0.01% (or 100 ppm). If you need to measure smaller changes in concentration, you should choose the lower range version sensor.
According to OSHA, “Lower explosive limit (LEL)” means the minimum concentration of vapor in air below which propagation of a flame does not occur in the presence of an ignition source. When the concentration of gas reaches 100% LEL, it can be ignited if a source of ignition is present. Different combustible gases have different lower explosive limit concentrations. For instance, the LEL for methane (CH4) = 5.0% volume, while the LEL for propane (C3H8) = 2.1% volume.
The lower explosive limit (100% LEL) concentration for hydrogen is about 4.0% volume, which is equal to 40,000 ppm.
So, 1.0% LEL hydrogen = 40,000 ppm X 0.01 = 400 ppm.
So, a concentration of 2000 ppm hydrogen = 2000 ÷ 400 = 5.0% LEL.