Page 32 - Occupational Health & Safety, August 2018

GAS DETECTION
Why Do You Need 10% Vol Oxygen to Operate a Catalytic Bead LEL Sensor?
Catalytic bead LEL sensors need a certain level of oxygen to correctly read combustible gas up to 100% LEL.
BY YONG WANG
The catalytic bead lower explosive limit (CB LEL) sensor is widely used for combustible gas detection based on its low cost, ease of use, and the ability to detect a wide range of gases. However, for some special applications, such as environments with less than 10% vol oxygen (O2), the CB LEL sensor is not recommended. Here’s why:
Reason #1: 10% vol O2 allows
gas readings up to 100% LEL.
To help you better understand this, let me explain the basic principle of how a catalytic bead LEL sen- sor works. A catalytic bead LEL sensor senses a com- bustible gas through flameless combustion that oc- curs with the help of electrically produced heat and a catalyst material coating on the sensing bead. In other words, a CB LEL sensor detects gas through the actual burning of the gas. This is why it can detect a wide range of gases and can detect multiple gases at the same time. Like three elements of a fire, CB LEL gas sensing requires fuel (combustible gas in this case), heat (by a metal wire coil buried in the sensor bead), and oxygen.
How much oxygen is needed for CB LEL gas sens- ing depends on how much and what type of combus- tible gas is present in the detection environment. Let’s use methane (CH4) gas as an example. When methane burns with oxygen, each CH4 molecule consumes two
oxygen molecules in a complete oxidation reaction, resulting in byproducts of one carbon dioxide and two water molecules. The chemical reaction formula is:
CH4 + 2O2 => CO2 + 2H2O
The lower explosive limit (LEL) for methane is 5% vol. In order to burn this 5% vol (or 100% LEL) meth- ane completely, you will need at least 2x5%=10% vol O2 gas. If O2 is less than 10%, you will not be able to read up to 100% LEL methane due to the lack of O2 in the reaction.
Different gases may have different LEL numbers and require different ratios of O2 to react. In best prac- tice, to keep the meter readings for most gases up to 100% LEL (typical measurement range of a gas detec- tor), 10% vol O2 is the minimum requirement, allow- ing the sensor to measure up to this range. This is one reason why we don’t recommend using catalytic bead LEL sensors in an environment with less than 10% vol oxygen.
Reason #2: 10% vol O2 ensures measurement accuracy.
To study how the gas reading changes under different O2 levels, we conducted an experiment. Two gas cyl- inders, #1 (2.5% vol CH4 balanced with air 20.9% vol O2) and #2 (100% N2), were blended to adjust the O2 gas concentration levels. One thing to keep in mind is that the ratio of CH4 to O2 is kept the same at 2.5/20.9 in this experiment. So in theory, there should be enough O2 to react with CH4. The Ventis® MX4 multi- gas detector (equipped with LEL, O2, and CO sensors) was used to read the blended gas concentration. The table and chart below show the test results.
You can see clearly from the results that the CH4 reading is still accurate enough (error <5%) as long as the O2 level is high enough (over 10%). As the O2 level gets lower and lower, the reading accuracy starts to decrease (reading lower than the real concentra- tion). When the O2 level is at 2.1% vol, the reading error can be as large as 24%. Two reasons explain this deviation. One is that the sensor is calibrated at a con- dition of 20.9% vol O2. The larger deviation of the O2 level results in a bigger error of the CH4 reading. The other reason is that not all O2 molecules are reach- ing and impacted by catalyst material on the sensing
28 Occupational Health & Safety | AUGUST 2018
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