Pellistors are solid-state devices used to detect combustible vapors and gases in air, in concentrations close to the explosion range. In this article, pellistors are used in a catalytic mode where the gas of interest is burnt and the heat produced is determined by the sensor.
These devices can also be utilized for detecting gases whose thermal conductivities are relatively different than air, for instance helium. This type of mode is called thermal conductivity mode. In this mode, up to 100% volume of the gas can be quantified.
The catalytic sensor includes a coil of thin platinum wire which is supported in a refractory bead (Figure 1). This bead is deposited with a layer of catalyst. Upon passing a current through the coil, the catalytic material is heated to approximately 500°C. At this temperature, the catalytic material is capable of burning the target gas and generates heat. The heat thus produced results in increased temperature which is sensed by the coil, wherein the resistance increases. This active bead is called the detector bead.
Figure 1. Diagram of catalytic sensor
To offset pressure, temperature and other environmental factors, the active bead is matched with a compensator or inactive reference bead. These beads are usually run in a Wheatstone bridge circuit. The signal created is proportional to the concentration of the gas up to the lower explosive limit (LEL). Although the sensor can detect all combustible vapors and gases, the reaction to higher concentrations of hydrocarbons may not be high.
The thermal conductivity sensor includes two beads. One of the beads is integrated within a chamber containing air i.e. the compensator, while the other is exposed to the target gas i.e. the detector. As before, both beads are heated and run in the same kind of circuit as the catalytic sensor. Upon exposing the detector bead to a gas whose thermal conductivity is considerably different than that of air, the rate at which the heat is lost from the bead will change along with its resistance. This change thus quantified is evaluated against the compensator bead. The sensor is usually utilized to detect low molecular weight gases, such as helium and hydrogen, which have greater thermal conductivities when compared to air and thus give the highest response.
The sensors are capable of operating at more than 400°C temperatures and hence can ignite gas mixtures of which are flammable. In order to prevent this problem, if the sensor has to be used in an environment where combustible gases are present, the pellistor beads can be enclosed into a certified flameproof head. This set up will expose the beads to the target gas without the risk of ignition.
e2v offers sensors that come both as beads and as beads within a certified head. However, in the former version, customers must take into account whether the beads address the certification needs of their own flameproof head.
There are certain chemical compounds, which can severely impact the performance of catalytic pellistors. This is usually seen either as a drift in the zero, loss of sensitivity, or both. The effect can differ with respect to the type of chemical and the length of exposure, but the overall effect can be classified as follows:
- Poisoning (e.g. with organic silicones), where the effect is significant and everlasting, for instance with a complete loss of sensitivity
- Inhibition (e.g. with CFCs and H2S, CFCs), where the loss is reversible and less prominent
The types of sensors supplied by e2v have been specifically designed to withstand the effects of poisoning, but exposure to poisons over long period of time will ultimately lead to sensor failure.
Since catalytic pellistors can determine gases in the LEL range of 0 to 100%, they are normally equipped in instruments utilized in applications where combustible gases can be found. Methane is on type of gas that is often encountered and measured in the field. It is present in coalmines, sewers and other areas where organic matter is decomposing. The gas is also a primary constituent in natural gas. Possible sources of methane are also measured in cramped areas linked to telecommunications. Most of the apparatus include portable instruments equipped with multiple gas sensors.
Monitoring of kerosene spillages and monitoring of hydrocarbons are some of the major non-methane applications. The latter application is generally used in the petrochemical industry, both offshore and onshore. These instruments are usually calibrated for pentane and butane compounds. In addition to portable systems, a sizeable market exists for fixed gas detection instruments. Applications for other kinds of gases include ammonia in refrigeration facilities and hydrogen in battery-charging areas
Thermal Conductivity Sensors
Coalmines and monitoring of natural gas are the major application for methane calibrated (TC) sensors. Methane concentrations of more than 50% by volume can be found in coal seam fissures. Other applications of TC sensors include the monitoring of hydrogen, helium and other light gases. Helium is present in a wide range of industrial processes and hydrogen is used in battery charging and fuel cell applications.
Pellistor gas sensors can detect combustible vapours and gases in oxygen-rich atmospheres. They are used in a wide variety of applications, ranging from hydrocarbons and kerosene spillage monitoring to methane, helium and hydrogen monitoring.
About SGX Sensortech (IS)
SGX Sensortech is a market leader in innovative sensor and detector devices that offer unrivalled performance, robustness and cost- effectiveness.
SGX have been designing and manufacturing gas sensors for use in industrial applications for over 50 years, offering excellent applications support for an extensive range of gas sensors and the expert capability for custom design or own label.
As an independent OEM supplier of gas sensors, we pride ourselves on providing customers with unrivalled product reliability and personal product support via specialist engineers.
SGX gas sensors are built to the highest standards with all pellistor and infrared gas sensors achieving ATEX and IECEx certification, SGX gas sensors are also UL and CSA approved.
Our product portfolio has continued to expand in technology and detectable gases used in a wide range of applications including:-
- Oil and gas
- Confined space entry
- Indoor air quality
- Industrial area protection
- Leak detection
This information has been sourced, reviewed and adapted from materials provided by SGX Sensortech (IS) Ltd.
For more information on this source, please visit SGX Sensortech (IS) Ltd.