Electrochemical vs. Semiconductor Gas Detection—a Critical Choice

 From time to time, humans are forced to choose between two competing technologies, each with its own appeal: Mac vs. PC; iOS vs. Android; and if you’re old enough, VHS vs. Beta. Industrial users face similar dilemmas when selecting toxic gas detectors. This is the topic of an Emerson Automation Solutions white paper, Electrochemical vs. Semiconductor Gas Detection—a Critical Choice

When a facility requires monitoring the air for the presence of hydrogen sulfide, carbon monoxide, or other toxic gases, either approach can usually do the job. However, there are operational tradeoffs which can tip the balance to one side or the other.

Let’s look first at solid-state semiconductor offerings, which are known for their versatility, durability, and long service life, but with some specific drawbacks. As noted in the white paper, many solid-state sensors are highly cross sensitive to gases found in plant environments. Examples are organic thiols used as odorants in natural gas, chlorine, other halogens and NOx compounds. The cross sensitivity contributes not only to unreliable readings but in severe cases can lead to false trips. Solid-state sensors also require a fair amount of power to operate. With power consumption of approximately 300 mW, end users have to find ways to solve power supply challenges when placing solid-state devices in facilities with no nearby power source. They must either use heavier gauge wire to support long distances to reach a supply, or install a dedicated power supply closer to the instrument if it is equipped with an isolated analog output signal. Not surprisingly, operators usually try to avoid using solid-state gas monitors in remote, isolated operations or facilities where power supplies are far from detector installations.

One severe limitation of solid-state sensors is baseline shift. If not exposed to the target gas regularly, sensors desensitize over time, leading to tradeoffs between the cost of maintaining the equipment against degraded performance.

Like solid-state sensors, electrochemical devices have their advantages and limitations. They offer good specificity and are highly sensitive. In contrast to solid-state sensors, electrochemical devices require little power to operate and their direct linear output to gas concentration make for reliable measurements over wide concentration scales.

Electrochemical sensors require routine proof tests and calibration, and are not fail-safe, meaning they are unable to detect when their sensing is impaired. They also have restricted operation in cold environments, usually rated to a low limit of -40 °C.

The Rosemount 928 Wireless Gas Monitor uses Rosemount 628 Wireless Electrochemical Gas Sensors to detect toxic gases and oxygen levels and is equipped with WirelessHART. The fully wireless device reduces the need for cabling and shortens installation and commissioning time of gas detection equipment. The white paper compares these two detection technologies in greater detail but suggests that on balance, electrochemical sensors are a better match for the demands of the process sector.

You can find more information on gas detection and meet with other people facing similar situations in the Emerson Exchange365 community. It is a forum for users in all types of industries around the world. Look for the Plant Safety and IIoT Groups, and other specialty areas for suggestions and answers.

2 Replies

  • Thanks for sharing. Great to learn something new. So is 628 an amperometric sensor?

    H2S is just another example of plants moving from periodic manual check with portable testers, to permanent sensing with wireless sensors; this automated data collection is the foundation of digital transformation. Plants become more predictive, more productive, and keep folks out of harm's way. Other good examples include corrosion, vibration, acoustic noise (valve leaks and steam trap failure), and temperature etc. Learn what other plants are doing to automate data collection as part of their digitization initiatives as part of their operational excellence programmes: www.linkedin.com/.../
  • In reply to Jonas Berge:

    Yes, Jonas, the 628 H2S sensor is an amperometric sensor. In amperometric sensors, a fixed potential is applied to the electrochemical cell, generating an electrical current, due to a reduction or oxidation reaction. Under this condition, the cell current can be correlated with the bulk concentration of the detecting species or solute in the cell. The linear dependence of electrical current on gas concentration, real zero, good specificity to target gas, low power regime, and high sensitivity to gas concentrations make amperometric sensors ideal for use in gas detectors. The key consideration of amperometric sensors is that they operate at a fixed potential and usually under limiting current.