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An analysis of electrochemical versus metal oxide sensors

In gas detection, hydrogen sulfide (H₂S) monitoring is critical for oil and gas facilities around the world. A debate persists as to which measuring technique is better—electrochemical or metal oxide sensors. This article evaluates the electrochemical sensor vis-à-vis metal oxide sensors.

Hydrogen Sulfide
H₂S is a flammable, colorless gas with a sweetish taste and characteristic odor of rotten eggs that can be poisonous at high concentrations. Other names for hydrogen sulfide include hydro sulfuric acid, sewer gas, and stink damp. People usually can smell hydrogen sulfide at low concentrations in air, ranging from 0.0005 to 0.3 ppm., however, at high concentrations, people can lose their ability to smell it. This makes hydrogen sulfide very dangerous. Industrial sources of hydrogen sulfide include petroleum refineries, natural gas plants, petrochemical plants, coke oven plants, food processing plants, wastewater treatment plants and tanneries.

Hydrogen sulfide is released primarily as a gas and spreads in the air. However, in some instances, it may be released in the liquid waste of an industrial facility or as the result of a natural event. When hydrogen sulfide is released as a gas, it remains in the atmosphere for an average of eighteen hours. During this time, hydrogen sulfide can change into sulfur dioxide and sulfuric acid. Hydrogen sulfide is soluble in water, and is a weak acid in water.

Exposure to low concentrations of hydrogen sulfide may cause irritation to the eyes, nose, or throat. It may also cause difficulty in breathing for some asthmatics. Brief exposures to high concentrations of hydrogen sulfide (greater than 500 ppm) can cause a loss of consciousness. In most cases, the person appears to regain consciousness without any other effects. However, in some individuals, there may be permanent or long-term effects such as headaches, poor attention span, poor memory, and reduced motor function. No health effects have been found in humans exposed to typical environmental concentrations of hydrogen sulfide (0.00011–0.00033 ppm). However, deaths due to breathing in large amounts of hydrogen sulfide have been reported in a variety of different work settings, including sewers, animal processing plants, waste dumps, sludge plants, oil and gas well drilling sites, tanks such as those used as part of a process on offshore drilling sites, refineries, and cesspools.

Standard Sensing Technique for Hydrogen Sulfide
Paper tapes impregnated with lead acetate have been widely used for air sample measurements of hydrogen sulfide in the field (e.g. Honeywell Analytics Chemcassette®). This method has been improved by impregnating the paper with mercuric chloride or silver nitrate. Mercuric chloride paper tape is sensitive and reliable for measurement of hydrogen sulfide in air with a sensitivity of 0.7 µg/L (MDA Scientific—a Honeywell Analytics gas detection brand). Tapes impregnated with silver nitrate are suitable for determination of hydrogen sulfide concentrations in the range of 0.001–50 ppm. Potentiometric titration with a sulfide ion-selective electrode as an indicator has been used to measure hydrogen sulfide in the air at ppb levels. This method has been shown to have very good accuracy and precision. Passive card monitors has been used to detect hydrogen sulfide in workplace environments. Badges worn in a worker’s breathing zone that change color based on exposure to toxic gases also detect hydrogen sulfide. The sensitivity for the hydrogen sulfide badges is 10 ppm/10 minutes with a color change from white to yellow. Other colorimetric methods for monitoring hydrogen sulfide include hand-held colorimetric tubes. Air is drawn through the tube and a color change indicates the presence of hydrogen sulfide by reacting with a chemical reagent in the glass tube.

How Electrochemical Sensors Work
Substance specific electrochemical and metal oxide sensors are the most commonly used sensors for toxic gases, including hydrogen sulfide. These sensors consist of a diffusion barrier which is porous to gas but non-porous to liquid, a reservoir of acid electrolyte (usually sulfuric or phosphoric acid), a sensing electrode, a counter electrode, and (in three-electrode designs), a third reference electrode (Figure 1). Gas diffusing into the sensor reacts at the surface of the sensing electrode. The sensing electrode is made to catalyze a specific reaction. Dependent on the sensor and the gas being measured, gas diffusing into the sensor is either oxidized or reduced at the surface of the sensing electrode. This reaction causes the potential of the sensing electrode to rise or fall with respect to the counter electrode. The current generated is proportional to the amount of reactant gas present. This two-electrode detection principle presupposes that the potential of the counter electrode remains constant. In reality, the surface reactions at each electrode cause them to polarize, and significantly limit the concentrations of reactant gas they can measure. In threeelectrode designs, it is the difference between the sensing and reference electrode that is actually measured. Since the reference electrode is shielded from any reaction, it maintains a constant potential, which provides a true point of comparison. With this arrangement the change in potential of the sensing electrode is due solely to the concentration of the reactant gas.

How Metal Oxide Semiconductor Sensors Work
Metal oxide semiconductor (MOS) sensors are also used for toxic gas monitoring. Sensitivity of the sensing element to a particular gas may be altered by changing the temperature of the sensing element. MOS sensors are so-called broad range devices designed to respond to the widest possible range of toxicity. This non-specificity can be advantageous in situations where unknown toxic gases may be present and a simple go / no-go determination of the presence of toxic contaminants is sufficient. Since sensitivity of the sensing element to a particular gas is mathematically predictable, a commonly used strategy is to pre-program the instrument with a number of theoretical specific response curves. If the exact nature of the contaminant is known, an identification code can be entered, and readings of the sensor will be adjusted to reflect the expected sensitivity of the sensor to the contaminant being measured (Figure 2).

Electrochemical Sensor Performance Improved by Using Multiple Electrolyte Reservoirs
Traditional electrochemical cell design is still not found to be very robust for extreme environmental condition like high humidity and very low or very high temperature (Figure 3). New developments in electrochemical technology try to address the temperature and humidity deficiency of traditional electrochemical sensors. One such new development is a patented design that incorporates two electrolyte reservoirs, which allow for the “take up” and “loss” of electrolyte that occur in high temperature/high humidity and low temperature/low humidity environments (Figures 3 and 4). This design of dual reservoirs reduces the effect of humidity on cell performance, preventing cell bursting or leakage, and improving the temperature coefficient and speed of response (Figure 5). It also has improved filtering, which reduces cross interferences (Figures 4 and 5). Speed and drift comparisons can be seen in Figures 6,7,and 8)

Further Comparisons
Electrochemical sensors are stable, reliable, fast, require very little power and are capable of resolution (depending on the sensor and contaminant being measured) in many cases to 0.1 ppm. Most substancespecific electrochemical sensors have been carefully designed to minimize the effects of common interfering gases. They are designed to respond only to the gases they are supposed to measure. The higher the specificity of the sensor the less likely the sensor will be affected by exposure to other gases which may be incidentally present. For instance, a substance-specific hydrogen sulfide sensor is deliberately designed not to respond to other gases which may be present at the same time, such as carbon monoxide or methane. Even though care has been taken to reduce crosssensitivity, some interfering gases may still have an effect on sensor readings.

The interfering effect can produce readings that are higher than actual, or lower than actual. MOS sensors offer the ability to detect low (0 - 100 ppm) concentrations of toxic gases over a wide temperature range. The chief limitations in the use of this kind of sensor are the response time, sleeping syndrome (where the sensor become insensitive to any H₂S gas presence for period of time), and difficulty in the interpretation of positive readings, and the potential for false positive alarms. If a user keys in the preprogrammed response curve for a contaminant that is highly detectable by the sensor, but actually encounters one that is less detectable, the result may be erroneously low readings. This is the reason critics of metal oxide sensors argue that these sensors have more electronic output rather then actual sensor output.

Conclusion
When sensors are compared, there are apparent design weaknesses in the MOS sensors as compared to the electrochemical sensors. The biggest drawback is response time. It is impractical and dangerous to wait 120 seconds for a detector to respond if there is a gas leak in a certain area of operation. Preprogramming a MOS sensor for H₂S sensitivity makes it more dependent on its electronics when compared to electrochemical sensors that rely only on their cell to respond. Traditional electrochemical sensors too have disadvantages, but new developments in electrochemical technology has removed many of these shortcomings.

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References:

• C.Kaminski, A. Poll. Electrochemical or Solid State H₂S Sensors: Which is Right For You? InTech, ISA (1985) p. 55.
• Gas Book – Honeywell Analytics (A free copy of Gas Book, an 84-page guide to the principles and practices of gas detection, can be obtained by request at
  detectgas@honeywell. com)
• J.P. Smith, S.A. Shulmaney. An Evaluation of H₂S Continuous Monitors Using Metal Oxide Semiconductor Sensors. Appl Ind Hyg, vol. 3. No. 7 (July 1988)

Mustafa Siddique works for Honeywell Analytics, 405 Barclay Blvd., Lincolnshire, IL , 60069, as a specialist on gas detection. With a Master’s degree in Engineering from Birla Institute of Technology, India, and a Master’s of Business Administration from Rotma n School of Management, University of Toronto and over twelve years experience in Industrial gas processing and detection, Siddique ha s been closely associated wi th the development of advanced gas detection technologies and methodologies in the Middle East, India and North America. He can be reached at 416-458- 6571 or Mustafa.siddique@honeywell.com

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