WHAT CAUSES SULPHUR DIOXIDE SENSORS TO FAIL EARLY?

What Causes Sulphur Dioxide Sensors to Fail Early?

Some customers have experienced issues with SO2 (Sulphur Dioxide) sensors failing. The typical SO2 sensor should last between two and five years. However, in some scenarios, these sensors may fail and only last a short lifespan. So, why is this and what can you do to increase the lifespan of your SO2 sensors?

Measuring Sensor Life

During sensor calibration, the raw sensitivity of the sensor is measured and then a calibration is used to translate that sensitivity into a number. That number is known as the “span reserve’ and provides an easy way to know whether a sensor is in need of replacement. If a sensor has a span reserve over 100% it’s in new condition. If the span value is between 70% and 99%, the sensor is in good condition. When the value is between 50% and 69%, it’s recommended that you start to plan for a replacement. Once the span value falls below 50%, the sensor will fail calibration and therefore replacement is required.

In most situations, the degradation of a sensor is linear and occurs gradually over time. However, if the degradation is sporadic, it is generally due to an external factor that has impacted on the life of the sensor.
 

Environmental Impacts

The life of an SO2 sensor can dramatically decline in climates where bitter cold and low relative humidity prevail for several consecutive weeks. Low humidity has a significant effect on SO2 sensors.  For example, the chart below shows the results of continuously exposing an SO2 sensor to11%RH. In just one month, the span drops by over 50% to reach a new equilibrium at that low humidity level. The sensor has 50% span remaining after about two months.

Why are SO2 Sensors Sensitive to Environmental Factors?

SO2 sensors are sensitive to the environment as they contain a water-based electrolyte. This electrolyte can easily lose its water contents when exposed to low humidity conditions, decreasing its mobility and hampering its ability to actively partake in the chemical reaction cycle inside the electrochemical sensor. Its freezing point also varies with the concentration changes due to environmental changes. In cold weather, the concentration change makes it easier for the electrolyte to freeze, resulting in a loss of mobility. This reduces the sensor sensitivity significantly and can cause the sensor to fail calibration.

Why do SO2 Sensors Recover?

While the electrolyte in SO2 sensors can easily lose its water under low humidity conditions, it can also absorb moisture from the air. When the sensor is exposed to warmer temperatures and high humidity it causes an increase in the ion mobility and results in the sensor sensitivity to recover. For example, in the image below, the wave pattern indicates that the sensor span is low in winter (January-February for the northern hemisphere) and high once it recovers under warmer temperatures and higher humidity.

What Can You Do?

The easiest way to prolong the life of an SO2 sensor is through the proper care and storage of the sensor. To avoid sensor dry-up, store gas detectors and spare sensors in a cabinet or room which holds relatively high humidity during winter. For example, placing a humidifier in an equipment storage room can prevent SOsensor problems. Another option is to use a Cigar Cooler Humidor which holds the humidity at 60%-70%RH.
 
A dried SO2 can generally be recovered by placing the sensor in a plastic zippered storage bag with a damp paper towel and storing the bag in a warm location. This restores moisture to the electrolyte and will typically recover overnight.
 

Conclusion

It’s important to recognise that SO2 sensors are susceptible to climate conditions. However, through proper care and storage, the lifespan of SO2 sensors can be dramatically improved.
 

For more information, contact your local Air-Met office

 

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