Electrochemical sensors are utilised for the detection of oxygen and toxic gases. They do this through the measurement of the concentration of a current within an external circuit. There are many types tuned to monitor both toxic and non-toxic gases of various concentration ranges, and all have capacity to withstand challenging environments sometimes with high humidity and temperature.
The sensors are used in diffusion mode where gas in the ambient environment enters through a hole in the face of the cell. Some instruments use a pump to supply air or gas samples to the sensor where diffusion takes over and transports the gas into the measurement parts of the sensor. A PTFE membrane is fitted over the hole to prevent water or oils from entering the cell.
Sensor ranges and sensitivities can be varied in design by using different size holes. Larger holes provide higher sensitivity and resolution, whereas smaller holes reduce sensitivity and resolution but increase the range.
They are popular in industrial environments where the need for the monitoring of hydrogen sulphide, carbon monoxide, and other industrial gases is a legal and safety requirement. And they have many highly beneficial characteristics including being very robust, not requiring significant power, being easy to certify intrinsically safe, and generally (though not always) having a reasonable price tag.
Hazard
Within the transport and energy sectors, wide scale use of fossil fuels results in the emittance of pollutant gases such as carbon monoxide, nitrogen oxides, hydrocarbons and volatile organic compounds (VOCs). This is hazardous for the sector’s workforce. It also poses wider environmental and societal dangers due to air pollution. Electrochemical sensors monitor these risks through leak testing, automotive and boiler control, to sense which hazardous gases are emitted in real time and their concentration or build up. They can also help to optimise combustion processes and reduce emissions by improving energy efficiency, and warn of hazardous concentrations of waste gas types.
Due to the use of carbon dioxide and ethanol in the food and beverage sector there is a need for ongoing gas detection to avoid ingestion of poisonous, harmful gases. Areas of concern include steam processes, inert gases for packaging, CO2 used for carbonation, toxic gases used for sterilisation and ammonia used in refrigeration. It is a legal and ethical requirement to monitor levels of toxic gases within these environments and choosing the best sensor for the job is vital.
Due to their capacity to detect low levels of toxic gases, like hydrogen sulphide, electrochemical sensors are particularly beneficial in helping safeguard those working within the oil and gas sectors. Their robustness also serves them well in environments on offshore platforms and drilling rigs by maintaining their availability to report gas releases.
They are found in petrochemical and chemical plants, natural gas plants and refineries, gas storage and loading facilities, and pipelines and compressor stations. Petrochemical processing plants tend to be large with compact layouts including equipment, piping and tanks. Processing areas needing gas detection can be partially or fully outdoors which requires gas monitors to withstand sunlight, heat, humidity, fog, rain, wind and sometimes sand. The versatility and robustness of electrochemical sensors suits the needs of these plants and helps to overcomes the complications posed by the differing gas risks.
Three main factors affecting the life of an electrochemical sensor are temperature, exposure to extremely high gas concentrations and humidity. Other factors include sensor electrodes and extreme vibration and mechanical shocks.
Variable life span – The life span of an electrochemical sensor is dependent upon the environment which they are operational within, including the environmental condition, humidity, heat and gas concentrations.
Sensors for common gases such as carbon monoxide or hydrogen sulphide tend to have an operational life of 2-3 years. Gas sensors used for hydrogen fluoride may have a life of only 12-18 months. It’s worth noting that with stable temperature and humidity in the region of 20˚C and 60%RH, alongside no incidence of contaminants, electrochemical sensors have lasted for as long as 11 years.
Wear – Long term exposure to low humidity causes the liquid electrolyte in an electrochemical sensor to lose water and gain water at high humidity which can cause leakage and degradation of the sensor’s functionality. Although sensors can dry out over time, it can take up to a month of being stored in ambient air, and there is the chance the sensitivity will be lowered and/or the response time increased.
