This article provides an overview of the operation, accuracy, reliability, maintenance, and configuration of pellistor (catalytic bead) sensors.
How Pellistor (Catalytic Bead) Sensors Work
Pellistor (catalytic bead) sensors consist of a pair of heated metal oxide beads contained inside a flameproof housing. The beads are arranged at opposite ends of a Wheatstone bridge circuit, which measures changes in electrical resistance between the beads. Each bead’s construction impacts its interaction with combustible gases:
- The catalytic bead has a catalyst coated onto the bead surfaces. The catalyst causes combustible gas to burn at lower concentrations and temperatures than an untreated bead in air.
- The reference bead, or compensator is not coated with a catalyst. Gas encountering it will not burn until it reaches the lower explosive limit (LEL).
When combustible gas contacts the catalyst bead, heat is released as result of the gas - bead interaction. The energy produced causes a change in resistance from the catalytic bead, while the reference bead maintains a steady resistance. The variation in resistance is measured by the Wheatstone bridge and indicates the presence of combustible gas.
More information about how pellistor sensors and other combustible gas sensors work is available in the Blackline Safety whitepaper Combustible Gas and its Detection.
Accuracy
When a combustible gas encounters a pellistor sensor, the gas will react according to its specific properties. This means that a pellistor sensor can detect the gas it is calibrated for with a high degree of accuracy.
Pellistor sensors can detect a wide variety of combustible gases. However, since different gases burn at unique concentrations and temperatures, the sensor cannot distinguish between combustible gases, and will display readings that reflect the total concentration of all gases combusting on the bead. This means the user may want to change the calibration gas depending on their particular location.
In addition, to prevent poisoning or inhibition, pellistor sensors can be equipped with filters to limit exposure to molecules that can affect the catalytic bead. While the filters prevent the gases or vapors from materials that can impact the catalyst from impacting the catalyst, the filters also restrict the flow of heavy hydrocarbons (hydrocarbon gases with 6 or more carbon atoms in their chemical makeup). This means pellistor sensors equipped with filters are less responsive to heavy hydrocarbons.
Reliability
Pellistor (catalytic bead) sensor performance is impacted by the following factors:
Poisoning
Contaminant compounds, such as lead, silicones, phosphate, and sulfur-based substances can decompose on the catalyst, forming a solid coating on its surface.
The coating permanently decreases the sensitivity of the sensor and can lead to full sensor failure. Examples of common products containing these substances, include hair care products, lotions, cleaning supplies, and degreasers.
Poisoning can happen gradually, but in extreme cases pellistor sensors can fail after a single event. Poisoning can only be identified by a bump test or calibration.
Temporary Inhibition
Contaminant compounds, such as volatile organic compounds (VOCs) and hydrogen sulfide (H2S), can be absorbed by the catalytic bead catalyst.
The absorption of these compounds creates a blockage in the reaction sites on the catalyst, inhibiting normal reactions, and resulting in a temporary loss of sensitivity.
Readings from an inhibited sensor, will be artificially low. Sensor inhibition can be resolved after a period of exposure to fresh air.
Sensor Overload
When a pellistor sensor is exposed to high concentrations of combustible gas (e.g., above 8% volume percent (%v/v) of methane (CH4) > 100% LEL), the sensor behavior could be impacted in the following ways:
- The sensor may display a significant shift off of the baseline zero value and record a %LEL value for the combustible gas, even in fresh air.
- The sensor may lose sensitivity and be unable to accurately measure the %LEL value for the gas.
- The sensor catalytic beads may also be permanently damaged, and the sensor will no longer be able to detect gas.
Pellistor sensors exposed to high gas levels must be recalibrated to ensure that the sensor is fully functional prior to further use.
Low Oxygen
Because pellistor sensors work by burning combustible gas, they rely on adequate oxygen levels to provide accurate readings of gas levels.
When oxygen levels drop below 11%, complete combustion of the gas is no longer possible. If this occurs, and the catalytic bead can be contaminated with soot, resulting in a permanent loss of sensitivity.
Mechanical Breakage
Pellistor sensors use fine platinum wires embedded in the bead as the source of heat. Mechanical stress, along with the heat of the beads, and exposure to industrial chemicals, can weaken the intimate contact between the wire and the bead surface, leading to potential failure. If broken, the sensor is unable to detect the fault and fails-to-unsafe.
Environmental Conditions
Temperature, humidity, and pressure do not have a significant impact on the accuracy of pellistor sensors.
Maintenance
Blackline recommends that you protect pellistor sensors from exposure to known contaminants.
To ensure their safe operation, Blackline recommends bump testing and calibrating pellistor sensors at regular, scheduled intervals, in addition to immediately after exposure to a known contaminant or damaging event.
If bump testing and calibration determines that performance has been impacted, replace pellistor sensors immediately.
IMPORTANT: Always follow proper bump testing and calibration procedures. For example, do not test the combustible sensor’s response with a butane cigarette lighter, as doing so will damage the sensor.
Calibration Gas Selection
Blackline’s pellistor sensor is factory calibrated to 50 %LEL methane (CH4). If monitoring a different combustible gas in the %LEL range, calibrate the sensor using the appropriate gas.
Pellistor Sensor configuration
Blackline’s pellistor (catalytic bead) sensor (LEL-C) is equipped with a silicone filter. This configuration makes it ideal for use in environments that have known sources of silicone or vapors containing silicon. This sensor is not recommended for use when monitoring diesel, kerosene, jet fuel or other heavy hydrocarbon vapors with flashpoint temperatures above 38°C (100°F).
We’re here to help
Let us know if you have any questions — don’t hesitate to reach out to our Customer Care team.
