Confined space instruments almost always include a sensor used to measure percent LEL combustible gas. There are two types of widely used LEL sensors; catalytic “Wheatstone bridge” or infrared (IR) sensors. Each type of sensor has both advantages and disadvantages.
Catalytic LEL Combustible Gas Sensors
Wheatstone bridge combustible gas sensors detect gas by catalytically oxidizing the gas on an active bead located within the sensor. Catalytic LEL sensors contain two coils of fine platinum wire, which are coated with a ceramic or porous alumina material to form beads. The beads are wired into opposing arms of a balanced Wheatstone bridge electrical circuit. One bead is additionally treated with a platinum or palladium-based material that allows catalyzed oxidation to occur on the "active" (or detector) bead. The catalyst is not consumed by the oxidation process.
The temperature of the active bead must be high enough for the gas to be oxidized. While other gases can be oxidized at lower temperatures, in order to detect methane the temperature of the active bead must be 500°C or higher. If combustible gas is present, oxidation heats the active bead to an even higher temperature. The temperature of the untreated reference bead is unaffected by the presence of gas. Because the two beads are strung on opposite arms of the Wheatstone bridge circuit, the difference in temperature between the beads is registered by the instrument as a change in electrical resistance.
Catalytic combustible gas sensors are unable to differentiate between different combustible gases. They provide one signal based on the total heating effects of all the gases capable of being oxidized that are present in the vicinity of the sensor.
Catalytic-bead sensors respond to a wide range of ignitable gases and vapors. The heating effect on the active bead varies between gases. For this reason readings may vary between equivalent concentrations of different combustible gases.
An important consideration is that catalytic LEL sensors require the presence of oxygen in order to detect gas. As the concentration of oxygen drops, the ability of the sensor to oxidize gas is reduced, resulting in increasingly inaccurate readings. Most manufacturers specify that their catalytic LEL sensors cannot be used if the concentration of O2 is less than 10%.
Another important consideration is that catalytic LEL sensors can be poisoned by exposure to chemicals that inhibit or poison the sensor. Catalytic LEL sensor poisons include molecules that contain sulfur, phosphorus, or vapors that contain silicones. Catalytic LEL sensors should never be deliberately exposed to vapors that contain silicones.
Infrared (IR) LEL Sensors
Infrared (IR) LEL sensors measure gas as a function of the absorbance of infrared light at a specific wavelength or range of wavelengths. Molecules can be c
onceptualized as balls (atoms) held together by flexible springs (bonds) that can vibrate (stretch, bend, or rotate) in three dimensions. Each molecule has certain fixed modes in which this vibratory motion can occur. Each mode represents vibrational motion at a specific frequency. When a chemical bond absorbs infrared radiation the bond continues to vibrate at the same frequency but with greater amplitude after the transfer of energy. For infrared energy to be absorbed (that is, for vibrational energy to be transferred to the molecule), the frequency must match the frequency of the mode of vibration.
When infrared radiation passes through a sensing chamber that contains a measurable gas, only those wavelengths that match the vibration modes of the chemical bonds in the molecules of gas are absorbed.The rest of the light is transmitted through the chamber without hindrance. The instrument measures the amount of light that is absorbed by the molecules in the sensing chamber. The greater the concentration of measurable gas, the greater the reduction in the amount of light that reaches the active detector when compared to the reference signal.
It is the chemical bonds in the molecules being measured that actually absorb the infrared light. Since larger molecules have more chemical bonds holding the atoms in the molecule together, they provide more opportunities for infrared radiation to be absorbed. Thus, NDIR sensors are actually more sensitive, not less sensitive, to larger combustible gas molecules like hexane, octane, and nonane.
A primary benefit of IR LEL sensors is lower power consumption than catalytic LEL sensors. Another benefit is that IR LEL sensors do not require the presence of oxygen in order to detect gas, and can be used in oxygen deficient or inerted atmospheres. Additionally, IR LEL sensors do not include catalysts or materials that can be damaged by exposure to silicones or other catalytic LEL sensor poisons.
IR LEL sensors are excellent for the detection of common combustible gases like methane and propane. However, IR sensors cannot measure a gas unless the bonds in the molecules absorb IR at the measurement wavelengths.
Hydrogen does not absorb infrared light, and IR LEL sensors cannot be used for measurement of hydrogen (H2). In applications where H2 may be potentially present, the instrument should be equipped with a type of sensor designed to respond to H2, such as a pellistor LEL sensor or an electrochemical sensor capable of measuring H2 in the desired range.
IR LEL sensors used in portable CS instruments are not usually designed to detect acetylene. Depending on the manufacturer, they may or may not be able to detect benzene or other “unsaturated” volatile organic chemical (VOC) vapors. It is important to verify that the IR LEL sensor is able to detect the gas(es) of interest before use.