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Exploring Measurement Principles in Refrigerant Gas Detection

When 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 specific 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 specific categories of refrigerant sensor technology, significant variation exists, necessitating a thorough understanding of the options and balance of benefits 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 flammable gases. Typically composed of metallic oxides deposited on a silicon wafer, the sensor’s surface is heated to temperatures ranging from 300 to 800ºF (149 to 426ºC), depending on the targeted gases. The composition of the mixed oxides and the operational temperature dictate the sensor’s response to various toxic gases, vapors, and refrigerants.

During normal operation, oxygen molecules from the atmosphere 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 affected by factors such as water vapor, high humidity, temperature fluctuations, and low oxygen levels, further increasing the risk of false readings.

In practical terms, false alarms can stem from exposure to various materials, including solvents, cleaning products, vehicle exhaust emissions, and hydrogen from electrical charging stations (e.g. from forklift trucks). To mitigate this issue, utilizing an alarm delay function can be effective. 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-effective and remain valuable tools in refrigerant gas detection applications, including HFC and HFO refrigerant leak detection. Understanding these limitations and employing appropriate 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 finds application across various gases, including HFCs and HFOs, and CO2, whose chemical bonds absorb infrared energy at specific wavelengths within the infrared spectrum. Notably, most refrigerants, including HFCs and HFOs, absorb light around the 9 μm wavelength, owing to hydrogen-fluorine bonds.

In practice, measurement takes place as air from the sample location enters an optical bench, either through diffusion 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 reflectivity 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 concentration. 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 influencing 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 applications, 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 configurations.

Smaller-format infrared refrigerant sensors are more commonly used in diffusion-based gas detectors, being more cost-effective and therefore more readily deployed in a distributed detection system. Whilst not offering the same level of precision or lower detectable limit for HFCs and HFOs, they provide the same advantages 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.

Infrared sensors enjoy immunity to cross-gas effects or interferences in refrigerant applications, coupled with good levels of resolution and accuracy. Shifts in temperature are compensated for effectively within the sensor software, and the specificity of measurement targets only the refrigerant. It is therefore unaffected by the type of transient cross-gas interference that can affect semiconductor sensors.

A well-designed infrared sensor is very stable, cannot be poisoned, and is not prone to drift over time. This further reduces the risk of false alarms and ensures a long-sensor lifetime of typically ~10 years. This long lifetime and stability can make infrared sensors particularly suitable for applications where sensors are integrated directly into appliances such as heat pumps or refrigerated display cases.

The attributes of infrared refrigerant sensors render them an excellent choice for HFC and HFO leak detection applications where precise measurement is paramount or where ambient conditions and interfering gases pose potential challenges. Although carrying a higher price-point, infrared sensing technology exhibits superior performance in achieving lower minimum detectable levels compared to semiconductor sensors when applied to HFCs and HFOs, further bolstering its appeal in gas detection scenarios where there is a benefit to be gained from detecting at a lower level. For CO2, a refrigerant gas detector with an infrared sensor is the only realistic option, making the choice of detector importance regarding its suitability for the application and the environment in which it will be installed.

Emerging sensor technologies

New sensor technologies for the detection of refrigerants have begun to emerge over recent years. For the most part, these are limited to applications detecting in the range of flammability, giving out put in percentage of Lower Flammability Limit (%LFL) rather than in lower ppm levels.

Acoustic measurement technology functions in a way that can equated with infrared sensors, only in this case there is no absorption of a light source but rather the reduction in speed of a soundwave as it passes through the measurement chamber. The speed at which the soundwave traverses the distance from emitter to detector is equated to the gas concentration. Whilst claiming a reduction in the effect of environmental factors in comparison to more traditional refrigerant detection technologies, the range of detectable gases appears to be smaller, parts per million level measurement is not currently available for refrigerants, and data appears limited in order to make a meaningful comparison with the selectivity of infrared detection. Nevertheless, it is an interesting development in refrigerant gas detection options.

Molecular Property Spectrometer™ gas sensors have been making an appearance in refrigerant gas detection applications, again targeted and limited to %LFL measurement of flammable refrigerants (and other flammable gases). With the single-source manufacturer boasting claims of very long sensor lifetime, immunity to poisoning, and no false alarms, for refrigerant gas detection the benefits appear to not be dissimilar to those of infrared refrigerant sensors, albeit for a more limited range of applications. Limited data appears to be available on the measurement principle, making difficult a technology comparison in relation to the needs of refrigerant leak detection applications.

Electrochemical Sensors for NH3 Leak Detection

Semiconductor sensors and catalytic bead sensors, or the type commonly used for flammable gas detection, can be used to detect high concentrations of ammonia approaching its LFL of 15%/vol.

Lower-level detection is also needed due to the toxic effects of ammonia at low concentrations.

Standards and regulations vary by country, but typical levels are as below.

NH3 concentration in air

Effects

25ppm

Long term exposure limit – 8 hours TWA

35-50ppm

Short-term exposure limit – 15 minutes, some physical discomfort

70-300ppm

Severe irritation of nose, throat, and airways, risk of fluid accumulation in the lungs

300ppm

IDLH limit (Immediate Danger to Life & Health)

5,000ppm

Rapid respiratory arrest

15-28%

Flammable, explosive

Lower-level detection is achieved by using electrochemical sensors, which can be specifically tailored for different ranges of measurement.

In the operating principle of an electrochemical cell for NH3, gas diffuses through a gas-permeable membrane to an electrode where it is either reduced or oxidised. In basic terms, the sensor consists of a sensing/working electrode, a counter electrode, a reference electrode, and electrolyte.

A redox reaction at the sensing and counter electrodes produces an electrical signal that is proportional to the ammonia gas concentration.

2 NH3     →    N2 + 6 H+ + 6 e

O2 + 4 H+ + 4 e–    →   2 H2O

To enhance stability, a reference electrode maintains a constant voltage on the sensing electrode to compensate for the degradation of the electrolyte due to the reaction on the electrode surface, extending the life of the sensor. Nevertheless, the typical lifetime for most electrochemical sensors for NH3 is circa two years. There are, however, some refrigerant gas detectors now on the market with field proven NH3 sensors with a lifetime in excess of five years.

Generally speaking, there are some drawbacks to the use of ammonia sensors that should be noted to ensure the proper maintenance routines and installation practices deliver an effective refrigerant detection system. The limited lifetime is vital to note, and there is no getting away from the fact the electrochemical sensors come at a relatively high cost. Ideally the sensor, not the whole gas detector, should be possible to replace in the field. The sensors can also be poisoned by contaminants or even by over-exposure to very high levels of ammonia, and they can be affected by very high or very low levels of humidity.

This is balanced out by the positives of NH3 detection with electrochemical sensors. There is a high degree of selectivity, and false alarms are not likely. Accuracy is very good, and appropriately low levels or ammonia can be detected reliably and effectively.

Selecting the Right Sensor

The choice of refrigerant gas detector and the refrigerant sensor technology used is by nature a subjective decision, depending on both the requirements of the application and the preferences of the user. What is certain is that there are choices available.

Specialist refrigerant gas detection suppliers are likely to carry a range of sensor types to meet the varied requirements of their customers. Most of the time, there is no one-size-fits-all approach to gas detection, so it is well advised to seek out a discussion with an expert to help make the decision that is right for each user or each project.

About the Author

Tom Burniston is SAMON’s Marketing Director, and Group Product Management Lead for Safe Monitoring Group. He has 20 years’ experience in gas detection, working in disciplines including marketing, international technical sales, channel and product management and strategic planning. With experiences in industries such as refrigeration, oil & gas, landfill and biogas, Tom is experienced in new product and market development, product positioning, and in aligning products with industry standards and best practices. His most recent travels have taken him to the USA, Spain, Italy, and Germany where he is presenting information on refrigeration during major conferences and trade shows. Tom is a graduate of the University of Leicester and resides in the U.K