Electrochemical Sensor
1.0 Introduction
Gas detectors separate the plant operators and assets from catastrophes. They are used extensively in industries to protect personnel and equipments from hazardous gases. The asset owners are always striving to establish a balance between the maintenance cost and optimum levels of process safety. Each gas detector comes with a gas sensor which is very specific for toxic gases or combustible gases. All gas sensors comes with a service life span and shelf life which requires periodic calibration and sensor replacement. The sensor replacement interval varies among different manufacturers and it is important for the asset owner to be aware and plan for the preventive maintenance. Shorter sensor replacement interval means higher rate of sensor degradation.
Electrochemical sensors are primarily designed for detecting toxic gases. It is important to know that a degraded electrochemical sensor will not give any indication or fault warning and the performance of the gas sensor could be questionable if the users choose to use the sensor for an extended period beyond the recommended sensor replacement cycle. The cost saving initiatives could mean a compromise to process safety if toxic gas leaks undetected.
2.0 How Electrochemical sensors works?
Electrochemical sensor is a small cell that contain electrodes submerged in an aqueous or gel electrolyte (acidic base). The sensor is structured with an active electrode with a gas permeable membrane placed over it. Between the active electrode, there is another thin layer of electrolytic solution. The targeted gas molecule passes through the gas permeable membrane to react with the electrolytic solution. A small electric current is generated and measured from the chemical reaction process.
Source: Riken Sensor Technical Overview C9E-0020-170200S
3.0 Important Characteristic of Electrochemical Sensor
The gas concentration should be proportional to the current value.
Extreme temperature may affect the sensor life. The manufacturer will state an operating temperature range which is typically -30°C to +50°C. High temperatures will result in evaporation of the electrolyte and possible shifts in the Zero baseline reading. (Riken Keiki gas sensor has the zero baseline drift detection and adjustment).
The most significant influence on sensor life is humidity. The ideal environmental condition for electrochemical sensors is 20°Celsius and 60%~80% RH (relative humidity). When the ambient humidity increases beyond the rated humidity range water will be absorbed into the electrolyte causing dilution.
It should be noted that sensor sensitivity changes as the gas permeable membrane degraded through its service life. The only way to check the condition is through bump test or proper calibration.
Cross sensitivity or interference gases
The gas sensor will not be able to differentiate the gas molecules from the same family or category of gases. It is important to anticipate what are the interference gases present in the same location so that false alarms from the cross sensitivity can be minimised. Every gas detector manufacturer should be able to produce a gas interference table for a given model of the electrochemical sensor.
The table below indicate the cross sensitivity data for a Riken sensor for detecting AsH3 Arsine gas. This is only one of the models from Riken’s sensor family.
4.0 Service Life of Electrochemical Sensor
Different gas detector manufacturers publishes different recommended sensor replacement intervals which are based on the years of customer installed base experiences and continuous research and development. Electrochemical sensors for common gases such as carbon monoxide or hydrogen sulphide may have an operational life of 3 to 4 years. Some specialty gas sensors such as arsine, phosphine, chlorine and etc may have a life of only 2 to 3 years. Some ideal installed condition such as stable temperature and humidity in the region of 20°C and 60%RH with no incidence of contaminants might even prolong the service sensor life beyond 3 years.
Manufacturer would always recommend planning the purchase of the replacement sensor because every gas sensor comes with shelf life. It is recommended not to keep the sensor for more than six (6) months. The storage condition could be quite different from the installed condition that might cause the sensor signal output to drift.
5.0 How to identify a failed electrochemical sensor.
Some manufacturer claims there are technologies which could determine when an electrochemical sensor has failed. Most of these technologies are only inferring the sensor is operating through some form of electrode stimulation. There is no manufacturer at this moment of writing could defy the basic operating principle of the electrochemical sensor which are:
The targeted gas molecule needs to permeate through the gas membrane in the sensor.
The targeted gas molecule will need to react with the electrolytic to generate a small current.
The intensity of the flow of current between the electrodes through the electrolytic is proportional to the targeted gas concentration.
The only proven method of demonstrating that a sensor is working according to the manufacturer design standard is to apply calibration gas and measure the response time (T90). A bump test follow by a proper calibration are normally conducted. Some end users waited for the gas detector to trigger false alarm with abnormal output signal trending for faulty gas sensor indication. False alarm with abnormal signal output trending is an extreme stage of sensor failure. The main concern arises from the silent failed sensor which no longer trigger alarm or no longer has the sensitivity to trigger the alarm timely.
There should be no compromises on the response time of the gas sensor because if the sensor cannot perform according to the manufacturer design specification (response time <60 seconds), the gas detector cannot provide any safety assurance. Any deviation from the design specification might create more uncertainties on the gas detector such as no detection in the event of gas leakage.
Subsequent to bump test and calibration, the trained professional would normally suggest to change or maintain the existing gas sensor. Although gas sensor service life prediction is very much required by end users, there is no proven technology to predict when the sensor will fail. The only reliable indication for sensor replacement is any electrochemical sensor showing response time of more than 60 Seconds.
The recommended sensor replacement interval by manufacturer may seem conservative by some of the end user, the sensor performance can drop drastically if the sensor usage has been over-extended or the gas sensor has been occasionally exposed to targeted gas or interference gases.
It could be an expensive lesson to the plant owners if the toxic gases leak undetected. There have been many industrial accidents that claimed lives due to toxic gases leaks. A fully functional gas detector can trigger timely hazard alarm to prompt the users in the surrounding to evacuate and safe their lives.
6.0 Summary
It is important to adhere to the gas detector manufacturer’s recommendation for sensor replacement interval. If the users choose to extend the sensor’s service life, it is important to engage trained professional or certified personnel to conduct calibration and bump test for verification of fit for use. It is also important to conduct periodic bump tests to verify the functionality of the gas detectors as every installed location is unique. The response time result from a bump test is a reliable indication on the gas sensor performance and it should not deviate from the manufacturer design specification. Any prolong or extended use of the failed sensor could risk life and cause catastrophe. There have been many industrial accidents involving toxic gases leak and nobody wants it from happening. The negative impacts from the toxic gas leaks could have been minimised if the fully functional gas detectors triggered timely alarms within seconds to give ample time for safe evacuation.
