Page 35 - Occupational Health & Safety, August 2018

A low concentration O2 may cause a large reading error and give you low gas readings due to incomplete combustion with possible carbon monoxide as a byproduct or unreacted combustible gas molecules.
LEL sensor to still read as high as 2.16% vol before it starts to drop, due to the lack of oxygen in the chemical reaction. When the oxygen becomes lower and lower, carbon monoxide starts to accumulate. This con- firms the incomplete combustion of CH4 inside the sensor, as explained above.
Another interesting phenomenon ob- served in this experiment is that the in- complete combustion happened rapidly when the gas was stopped. During this recovery time, the residue of high concen- tration CH4 inside the gas environment and O2 diffused from the ambient causing both complete and incomplete combustion reac- tions. The CO reading jumped to as high as 110 ppm before it dropped slowly to zero. The LEL peak reading was recorded as high as 3.28% vol during this recovery process.
Now let’s look at another example. We have 4% vol O2 and our LEL sensor reads 40% LEL (or 2% vol) methane. Is this read- ing trustworthy? The answer is probably not. In this case, 4% vol O2 only allows complete combustion of methane at no more than 2% vol (or 40% LEL). If the gas concentration is 3% vol (60% LEL), you may still just read 2% vol because there is not enough oxygen to burn all the gas.
To further investigate this, we designed another experiment: We blended gas cylin- der #1 (10% vol CH4 balanced with N2), gas cylinder #2 (100% N2), and gas cylinder #3 (zero air: 20.9% O2). Gas cylinder #1 and #2 were used to adjust the methane gas concentration down to 5% vol (the maxi- mum range the LEL sensor can read). The blended gas was then again blended with cylinder #3. By adjusting the ratio of the blended gases, a steady mixture of CH4/O2 gas can be achieved. An MX4 gas detector
was continuously exposed to this mixed gas until a steady reading was recorded for each test level.
The chart below shows that the actual CH4 readings match well with the ex- pected CH4 values when the O2 gas con- centration is high enough (over two times of CH4 concentration). The CH4 reading depended on CH4 level in this case. When the O2 concentration was further reduced, the actual CH4 gas readings started to de- viate from its true concentration (expect- ed CH4). Further analysis shows the CH4 readings were close to about 50% of the O2 concentration level, as the combustion was determined by O2 level instead of CH4 under this circumstance. A small level of carbon monoxide (CO) was also observed this time, which supports that incomplete combustion occurred.
Summary
In conclusion, catalytic bead LEL sensors need a certain level of oxygen to correctly read combustible gas up to 100% LEL. A low concentration O2 may cause a large reading error and give you low gas read- ings due to incomplete combustion with possible carbon monoxide as a byproduct or unreacted combustible gas molecules. Therefore, Industrial Scientific does not recommend using catalytic bead LEL sensors in an environment with less than 10% vol O2.
Instead, dilution tubes or infrared (IR) sensors are recommended to use when oxygen levels are less than 10% vol.
Yong Wang serves as global director of sen- sors for Industrial Scientific. Industrial Sci- entific offers IR sensors in the MX6 iBrid® and Ventis® Pro Series.
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