Combustible gas detector ( LEL gas detector) poisoning occurs when a sensor's core sensing element (usually a catalytic combustion sensor) is irreversibly damaged by certain substances, resulting in a permanent decrease in sensitivity or even complete failure.
This is similar to how a person's olfactory nerves are damaged by strong chemicals, making them unable to smell anything.
1. Core Principle: Catalytic Combustion type gas sensor,LEL gas sensor
The most common combustible gas sensor (used to detect methane, propane, hydrogen, etc.) is the catalytic bead sensor. Its core components consist of a "sensing element" (usually a thin platinum wire coil coated with a catalyst (such as palladium or platinum) and a carrier material) and a "compensating element" that serves as a reference.
When combustible gas diffuses onto the surface of the sensing element, the catalyst causes the gas to undergo flame-less combustion (catalytic combustion) at a temperature far below its normal ignition point.The heat generated by the combustion causes the platinum wire coil's resistance to change. The circuit measures this resistance change to calculate the concentration of the combustible gas.
2. How does EX gas detector poisoning occur?
Poisoning substances attack and destroy the catalyst on the sensing element.
Permanent poisoning: Certain substances (such as silane, hydrogen sulfide, and halogen compounds) react violently and irreversibly with the catalyst, forming new, inactive compounds that permanently cover or alter the catalyst surface. Once poisoned, the sensor can no longer effectively catalyze the combustion of combustible gases, and sensitivity can drop significantly or even reach zero.
Common poisons: Silicon-containing compounds (such as silicone sealant volatiles, hairspray, and lubricants), sulfides (such as hydrogen sulfide, which smells like rotten eggs), lead-containing substances (such as leaded gasoline exhaust), and phosphate esters.
Temporary inhibition: Certain substances (such as high concentrations of halogenated hydrocarbons) can temporarily occupy the active sites of the catalyst, preventing the combustible gas from coming into contact with them. If the inhibitory substance concentration is low and exposure is brief, sensor performance may recover slowly after removal. However, if exposure is prolonged and concentrations are high, inhibition can turn into permanent poisoning.
Common inhibitors: Some cleaning agents (such as trichloroethylene), refrigerants, and flame retardants.
3.Symptoms of "Poisoning"
(1). Slowed Response: The EX sensor reacts more slowly to the same gas concentration.
(2). Decreased Sensitivity: The sensor reading is significantly lower. For example, in an environment filled with a standard concentration of test gas, the reading is significantly lower than expected.
(3). No Response: The LEL sensor shows no response at all, even in an environment with high concentrations of combustible gas.
(4). Calibration Failure: The EX sensor cannot be calibrated using normal calibration procedures.
4.How to Prevent LEL Sensor Poisoning?
(1). Understand the Environment: Identify the chemicals the sensor may be exposed to and avoid using standard catalytic combustion sensors in environments containing toxic substances.
(2). Use a Physical Barrier: Install an anti-poisoning filter (such as PTFE membrane) in front of some sensors. This can block most large silicone molecules and other substances, but it cannot completely protect against all poisons.
(3). Choose poison-resistant sensors: For specialized industries (such as wastewater treatment and chemical plants), specially designed poison-resistant sensors are available. These sensors utilize special catalysts or protective technologies, resulting in longer lifespans.
(4). Regular inspection and calibration: Regularly testing and calibrating sensors using standard gases according to the manufacturer's recommendations is the only reliable way to detect sensor performance degradation (including poisoning).
(5). Proper installation**: Avoid installing sensors near potential sources of poison release (such as ventilation duct openings, areas where silicone sealant is used, or near vehicle exhaust vents in garages).