MOS Sensors Meet ISA H2S Standard
By Ardem Antabian, General Monitors

ISA Performance Standard ISA S92.0.01, Part 1 – 1998, replaces ISA-S12-15, Part 1 – 1990 and implements stringent performance requirements on toxic gas detectors. This is currently the only performance standard for toxic gas detection instruments. In fact, General Monitors’ Controllers, Smart Sensors and Intelligent Sensors using MOS technology are the only H2S gas detectors tested and verified to meet these standards by Factory Mutual (FM) to the best of our knowledge.

A consortium of individuals created this ISA standard, including but not limited to: H2S gas manufacturers, approval agencies and leading companies in the oil & gas industry, such as Chevron, Honeywell, Fluor, etc. This standard was first published in February 1990, and only covered H2S gas detection.

The purpose of the new standard is to provide a universal performance requirement for electrical instruments used in detecting toxic gases. The ISA standards specify that accuracy and repeatability shall be within 2 ppm or 10 percent of applied gas.

These standards ensure that the sensors are extremely accurate at the TLV-TWA Value of H2S (10ppm). They also allow for practical accuracy at the higher concentration levels due to the effects of humidity in H2S detection. The new ISA standard builds on the ISA S12.15 Standard requirements with added procedures that test repeatability, recovery and tighter tolerances.

MOS Sensor diagram

H2S Sensing
In the early 1970s, General Monitors developed and introduced the industry’s first thin film metal oxide semiconductor (MOS) sensor for the detection of H2S. The MOS film is deposited onto a substrate between two electrodes. With no gas present, the measured resistance between these two electrodes is very high (in the mega-ohms). As H2S adsorbs onto the film, the resistance between the two electrodes decreases (to kil-ohms). This decrease in resistance is logarithmically proportional to the concentration of H2S that is present.

The process of H2S adsorbing onto the MOS film is most effective at an elevated temperature. On the outer edge of the substrate is a heater ring. The temperature of this heater ring is measured with a thermistor and kept constant by a circuit located inside the body of the sensor.

MOS sensor operation

As H2S adsorbs onto the film, electrons move from one electrode to the other. This is represented as a decrease in resistance. The process of H2S adsorbing onto the MOS film is completely reversible. As the concentration of H2S desorbs, the resistance between the electrons will increase.

Toxic Effects of H2S
H2S is best known for its rotten egg smell because it is a colorless gas. This chemical compound results from the decomposition of organic matter and metal sulfides—typically found in petrochemical exploration, production and refining. It is also a problem in other drilling operations and mining, as well as sewer and wastewater treatment operations.

H2S is highly toxic. Low-level exposures produce eye and mucous membrane irritation. High exposures at 700 to 800 ppm cause sudden death by paralyzing the respiratory system. It affects all organs and prolonged exposure even at lower levels can cause serious health problems. H2S is highly explosive and flammable as well. Workers who may encounter H2S on the job need to be well trained in hazard recognition and safe work practices. Plants where the potential for H2S exposure is routine generally required a fixed gas detection monitoring system.

H2S Safety Monitoring
With its precision MOS sensor and microprocessor-based transmitter, the Model S4000TH is an intelligent fixed gas detector from General Monitors that meets the latest ISA performance standard for H2S detection. It features one-person calibration and can virtually self-calibrate by simply activating a magnetic switch and applying gas. The S4000TH detects H2S in parts per million with a repeatability of +/- 2ppm or 10% of the applied gas (whichever is greater).

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