Anyone working in industrial automation soon learns there are many ways to measure a particular variable. At least that’s the impression. As a case in point, for measuring liquid flow there are at least a dozen kinds of instruments, each with their particular capabilities and drawbacks. Most applications could probably use any one of several instruments and it would probably work just fine.
Other more specialized variables do not have as much choice, which can make the selection process more critical. Detecting and measuring toxic gases in a spot location really only has two options, so the choice has to be made carefully. That is the topic of Shing Yenn Tan’s article in Industrial Automation Asia Aug-Sep 2019, Electrochemical vs Semiconductor Gas Detection – A Critical Choice. As he discusses, both technologies can do the job, but only within their respective limitations. Deciding which is better for your specific application requires some thoughtful analysis, given how important the function is to the plant and its people.
Semiconductor sensors are versatile and can be used to detect a wide variety of gases. They are also durable and can provide service life up to 10 years. At the same time, they can exhibit poor selectivity, potentially tripping an alarm due to the presence of a different gas. Operationally, they have a higher cost due to the power required to run the internal heating element continuously and maintain the sensor at a temperature between 100-500 degrees Celsius. They also require regular maintenance to ensure baseline shifts do not reduce sensitivity.
So, if you are evaluating this approach, one consideration is the potential presence of other gases that might be benign but able to cause a false alarm. Looking at the other technology, how about the electrochemical approach?
Electrochemical sensors are highly sensitive, consume low power and have good specificity to target gases. Another advantage is their direct linear relationship of current output to gas concentration, allowing measurement of a real zero. One practical feature is their capacity for miniaturization. Sensitivity, accuracy and linearity are largely independent of size since sensitivity depends on the number of reactive sites of the electrode and the size of the gas inlet.
There are naturally drawbacks as well:
On the other hand, responsiveness is determined by factors affecting the chemical reaction. The speed of reaction, for example, decreases with decreasing temperature, so the temperature range of electrochemical cells tends to be narrow, reducing their ability to operate in very cold weather. Additionally, the presence of alkaline metals and silicone vapors can reduce sensitivity and accuracy.
Emerson’s Rosemount 928 Wireless Gas Monitor uses electrochemical Rosemount 628 Universal Gas Sensors. These sensors can detect hydrogen sulfide (H2S), carbon monoxide (CO), and oxygen depletion. A number of improvements have reduced the impact of some of the traditional drawbacks of electrochemical sensors.
For example, the temperature range and stability of some electrochemical sensors can be extended by using ionic liquids as the ion conductor. Such improvements have extended the ability of electrochemical sensors to operate effectively in low-humidity environments with longer service life. Additionally, their high degree of selectivity helps them overcome the effects of sensor poisons and interfering agents.
Clearly, making such a choice requires appropriate analysis. Shing goes into greater detail in the article, and Emerson has a white paper with more food for thought.
You can find more information like this and meet with other people looking at the same kinds of situations at Emerson Exchange and in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Flame & Gas Group, and other specialty areas for suggestions and answers.
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