REFRIGERANTS Taking the right measures
Tom Burniston, Samon’s marketing director, explores measurement principles in refrigerant gas detection
Semiconductor sensors and catalytic
bead sensors, or the type
commonly used for fl ammable gas detection, can be used to detect high concentrations of ammonia approaching its LFL of 15%/vol.
W
hen it comes to safeguarding industrial environments against refrigerant leaks, the choice of detection technology plays a pivotal role. A diverse array of gas
sensor technologies stands ready to be deployed. Each technology boasts its own unique strengths and drawbacks, making the selection process a nuanced task. The journey begins with identifying the specifi c refrigerant to be detected and determining at what concentration level refrigerant gas alarms need to be activated, laying the foundation for informed decision- making. However, delving deeper reveals that even within specifi c categories of refrigerant sensor technology, signifi cant variation exists, necessitating a thorough understanding of the options and balance of benefi ts for the application against the cost of implementation. The dynamic landscape of gas sensor technologies needs some unravelling to get to the complexities of modern refrigerant detection.
Semiconductor sensors Semiconductor sensors, also known as metal oxide sensors, stand
out as versatile tools for refrigerant gas detection. These sensors have the ability to detect a wide range of gases at concentrations measured in parts per million (ppm) as well as in combustible ranges for fl ammable gases. Typically composed of metallic ox- ides deposited on a silicon wafer, the sensor’s surface is heated to temperatures ranging from 300 to 800ºF (149 to 426ºC), depend- ing on the targeted gases. The composition of the mixed oxides and the operational temperature dictate the sensor’s response to various toxic gases, vapours, and refrigerants. During normal operation, oxygen molecules from the at- mosphere adhere to the sensor’s surface, creating a resistance barrier. However, when a reducing gas contacts the sensor, such as in the case of a refrigerant leak, these oxygen molecules undergo a redox reaction, altering the resistance and increasing electrical conductivity. This change in conductivity is then measured and correlated to determine the concentration of the gas present. Despite their versatility, semiconductor sensors exhibit some
drawbacks. They lack selectivity and can respond to any reducing gas, leading to potential false alarms. Additionally, they can be af- fected by factors such as water vapor, high humidity, temperature fl uctuations, and low oxygen levels. In practical terms, false alarms can stem from exposure to various materials, including solvents, cleaning products, vehicle exhaust emissions, and hydrogen from electrical charging sta- tions – such as from forklift trucks. To mitigate this issue, utilizing an alarm delay function can be eff ective. This function ensures that the leak detector does not trigger an alarm immediately but instead activates after a set period, allowing transient gases to dissipate and reducing the likelihood of false alarms. While semiconductor sensors have their limitations, they are highly cost-eff ective and remain valuable tools in refrigerant gas
20 October 2024 •
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detection applications, including HFC and HFO refrigerant leak detection. Understanding these limitations and employing appro- priate mitigation strategies is essential for ensuring accurate and reliable gas detection in commercial industrial settings.
Infrared sensors At the heart of infrared (IR) sensor technology lies a fundamental
principle: the absorption of infrared radiation by the target gas to be measured. This principle fi nds application across various gases, including HFCs and HFOs, and CO2
, whose chemical bonds
absorb infrared energy at specifi c wavelengths within the infrared spectrum. Notably, most refrigerants, including HFCs and HFOs, absorb light around the 9 μm wavelength, owing to hydrogen-fl u- orine bonds. In practice, measurement takes place as air from the sample lo- cation enters an optical bench, either through diff usion or sample aspiration. Within this setup, light emitted by an infrared source passes through the gas in the bench, directed towards a detector element. The walls of the sensor, often micro-polished and plated with precious metal, enhance refl ectivity to ensure maximum passage of light and energy, thereby optimizing the response at the infrared detector and the resolution of measurement. The reduction in intensity of the infrared light source, attributed to the presence of the target gas, correlates directly with gas concentra- tion. Internal electronics and software process this data to produce a linearized output signal, facilitating precise measurement. For HFCs and HFOs, the size of the optical bench, or rather the pathlength through which the infrared light passes through the gas, emerges as a critical factor infl uencing resolution and accuracy. Longer path lengths are essential for achieving high resolution and accuracy. These longer pathlengths are generally restricted to aspirated systems in refrigerant detection applica- tions, due to the size and relatively high cost. This level of infrared sensor technology, while superior in resolution and accuracy, may present challenges in deploying multiple sensors across a facility due to larger sensor sizes. Economic considerations further drive system design towards centralized confi gurations. Smaller-format infrared refrigerant sensors are more common- ly used in diff usion-based gas detectors, being more cost-eff ective and therefore more readily deployed in a distributed detection system. Whilst not off ering the same level of precision or lower detectable limit for HFCs and HFOs, they provide the same advan- tages generally attributed to infrared gas sensor technology. CO2
refrigerant sensors are generally available in smaller format, as the absorption is stringer meaning a longer pathlength is less necessary. A key factor in CO2
leak detection is ensuring
that a sensor and a refrigerant gas detector with a fast enough response time is selected, both in order to meet refrigerant safety standards requirements and to ensure the safety of personnel at risk of exposure to a leak from a CO2
system. Download the ACR News app today
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