Page 33 - Occupational Health & Safety, August 2018

O2 (% vol)
2.1
4.2
5.2
7.0
10.5
13.9
15.1
17.2
19.6
20.9
Expected CH4 (% vol)
0.25
0.50
0.63
0.83
1.25
1.67
1.81
2.06
2.34
2.5
CH4 Reading (% vol)
0.19
0.45
0.58
0.79
1.21
1.63
1.78
2.05
2.34
2.5
Reading Error
-24%
-10%
-7%
-5%
-3%
-2%
-1%
0%
0%
0%
Dilution tubes or infrared (IR) sensors are recommended to use when oxygen levels are less than 10% vol O2.
bead surface when the O2 concentration is so low. Hence, not all the CH4 molecules participate in the chemical reaction. This is why the reading is low. So in order to keep the instrument reading of combustible gas accurate enough, in practice, it’s better to operate the LEL sensor in the environment with O2 higher than 10% vol.
What If My O2 is Below 10% Vol?
Should I trust my catalytic bead LEL read- ing? The answer is, “It depends.” It all de- pends on the gas environment, how much O2 is present, and the gas reading you get on your detector. Let’s still use methane as an example. When there is not enough oxygen, the reaction between methane and oxygen becomes complicated. Some gas molecules may still react fully and result in carbon dioxide. Some gas molecules may have incomplete combustion due to the lack of oxygen and result in carbon monox- ide as a byproduct. The incomplete chemi- cal reaction formula is:
2CH4 + 3O2 => 2CO + 4H2O
If you understand this, you should feel confident on how to interpret your data and be able to judge whether the reading is trustworthy or not in an environment with less than 10% vol O2. For example, in an environment with 8% vol O2, your LEL sensor reads 10% LEL (or 0.5% vol) meth- ane (CH4). This gas reading might still be accurate (with error less than +/- 5%), as the complete combustion only needs 1% vol O2. There is enough oxygen to burn this amount of methane gas in this particular circumstance.
We conducted another experiment to demonstrate how an LEL sensor responds to methane gas in an inert (no oxygen) environment. A 2.5% vol of CH4 balanced with N2 was applied to the MX4 multi-gas detector with LEL, O2, and CO sensors. The transient gas readings were recorded by a specifically designed and built instrument data-logging firmware that can record the gas readings every second. The chart below shows the gas responses when the 2.5% vol CH4/N2 was applied and then stopped after approximately 150 seconds. The residual O2 in the instrument and sensor allow the
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