Metal Oxide Semiconductors (MOS) Sensor in detecting CH4
Sensors in the process safety industry play the same role as the human sensory organs by checking on the various process parameters.
CH4 is also known as natural gas. It is odorless and lighter-than-air gas. CH4 rises and accumulates at the higher, stagnant areas of enclosed buildings because it is lighter than air. Without gas detection devices, methane is exceedingly difficult to detect.
In the confine spaces, methane can displace oxygen, resulting in an oxygen-deficient environment. Methane is highly explosive when mixed with air at a volume between its LEL of 5% and its UEL of 15%. At concentrations of 50,000 ppm or more (a threshold of 5%), methane can erupt.
MOS sensor consists of a heater coil produced on an alumina tube. Two Au electrodes are attached to the tube's ends to measure the semiconductor's resistance.
The surface of the tile metal oxide semiconductor is heated to 350 to 400°C by the tile heater coil. The semiconductor maintains a constant resistance with ambient oxygen deposited on its surface in the forms of informs or O and O2. CH4 gas or something similar comes into touch with the surface and becomes chemisorbed by it, before oxidised by O2 ions and separated. The following is a representation of the response that occurs on the sensor's surface:
CH4 adsorbs on the sensor's surface and removes the absorbed oxygen, resulting in an increase in free electrons inside the sensor and a reduction in resistance. The sensor determines the gas concentration by sensing the change in resistance.
A typical electrochemical sensor (EC) sensor life span is about two years in the petrochemical refinery environment. While Semiconductor (SG) sensor is a solid sensor unlike EC sensor which has electrolytic subjected to fast depletion in the outdoor condition. It is because humidity slightly effects the sensitivity, the reading may vary depending on the weather. SG sensor can easily last about five years if properly maintained and the plant doesn't have frequent gas leakages. Compare with the EC, SG is highly resistant to toxicity and severe atmosphere.
GD-70D smart gas detector series sets new performance, flexibility, and versatility standards. It features an unrivalled selection of sensor technologies, including unique options like our hydrogen-specific or LEL variants. The smart sensors save all calibration and sensor-specific data in non-volatile memory, allowing for field hot-swapping without the need for programming. The sensors also store calibration information, allowing them to be calibrated independently of the transmitter, removing the need to take calibration gases to the field.
GD-70D is a stand-alone device that connects to current PLC systems and multi-channel controllers using a variety of communication protocols.
A large, easy-to-read integral LCD display, a tri-color bar graph providing visual warning of alarm status, programmable low and high alarm relays, and a fault relay are all included in all GD-70D transmitters. For maintenance-free operation, the pump flow is self-tuning. The sensors can be swapped among the GD-70D units which giving the user maximum flexibility.
SD-1GH uses MOS to detect combustible gas. It is an ATEX certified device that detect ppm levels in the hazardous atmospheres. The SD-1GH can also measure a wide range of dangerous organic chemicals in a PPM range, including acrylonitrile, ethylene oxide, n-methyl pyrolidone, methylene chloride, and many others.
SD-1 comes with a 4-20mA signal output, dry contact relay and HART communication.
Hazardous Gas Alarm
GD-70D and SD-1GH can communicate with safety system as well as SCADA/PLC. The dry contact in GD-70D is energized when the gas concentration reaches or exceeds the alarm setpoint. The contact activation is reset automatically when the gas concentration drops below the alarm setpoint.
While for SD-1GH contact is activated when the gas concentration reaches or exceeds the alarm setpoint value (only when the alarm is used).
The contact activation automatically resets when the gas concentration drops below the alarm setpoint value.
Written By: Easter Marcia Anak Rijem
About the Writer
Easter Rijem is an Application Engineer from Riken Keiki Malaysia graduated from Universiti Tun Hussein Onn Malaysia (UTHM) with a degree in Electronic Engineering with Honours with Microelectronic.