How Electrochemical Sensors Work

How Electrochemical Sensors Work.

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Most electrochemical toxic gas sensors are based on the same principles. However, key differences exist between various manufacturers’ makes and models. Given the importance of gas detection systems to your facility, it is a good idea to understand these differences, as well as the general limitations of the technology.
How Sensors Work

An electrochemical sensor generally has three main components: electrodes (one or more of which is coated with a catalyst), electrolyte, and a membrane. Gas diffuses through the membrane and reacts at the electrolyte-catalyst interface, which creates a current. For example, the reaction for HCl is:

2HCl + H2O --> Cl2 + 4H+ + 4e-

4H+ + O2 --> 2H2O

The instrument measures the current and translates it into gas concentration. Since the number of electrons given off is proportional to gas concentration, sensor output is linear.

Lifetime

Sensor lifetime is determined by many factors, including heat, humidity, dirt, and cumulative gas exposure. Under normal operating conditions, sensors should last for at least one year, and in many cases two years. Brief exposures to high concentrations of gas can reduce sensor performance in the near term — gas that has worked its way into the sensor (response time) needs time to work its way out (recovery time).

Response

Although sensor output is linear, response time is a logarithmic function. The first molecules of gas to diffuse through the membrane cause a very rapid change in response; as the sensor’s output approaches the actual ambient gas concentration, the rate of change of response slowly decreases. The response curve of the best sensors looks very much like a step function; that of the worst looks more like a straight line. The inverse of the response curve determines a sensor’s recovery time after being exposed to gas.

Manufacturers typically define some length of time, such as 10 minutes, as the point at which a sensor reaches its maximum output. A sensor’s T90 is the time in which it reaches 90% of this value; T50 is the time to reach 50%. The lower these two numbers, the better the sensor’s performance.

Warm-Up Period

Most electrochemical sensors require a fixed bias to be maintained across the sensor electrodes. This bias is one of the key determinants of sensor performance; after it is applied, the electrolyte typically needs time to reach equilibrium. For most sensors this warm-up period is 4-8 hours. However, some manufacturers provide sensors with a built-in battery that maintains bias, thus eliminating warm-up time.

A New Breed of Gas Sensor.

Detecting the toxic and combustible gas compounds used within semiconductor facilities can pose a special challenge. The absolute need for a detection method that produces fast, reliable response to the target gas is critical for a safe and profitable operation.

For this reason a line of high performance sensors have been developed that are fundamentally different than any typical electrochemical sensor found on the market today. These high performance sensors use an entirely different physical and chemical make up. The result - an extraordinary "high performance sensor that provides extremely low zero drift, faster speed of response and recovery time, greater specificity to the target gas, and the lowest level of reliable gas detection available from any electrochemical sensor available.

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