REFRIGERANTS FANS Infrared sensors enjoy immunity to cross-gas eff ects or inter-


ferences in refrigerant applications, coupled with good levels of resolution and accuracy. Shifts in temperature are compensat- ed for eff ectively within the sensor software, and the specifi city of measurement targets only the refrigerant. It is therefore unaff ected by the type of transient cross-gas interference that can aff ect semiconductor sensors. A well-designed infrared sensor is very stable, cannot be poi- soned, 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. Al- though carrying a higher price-point, infrared sensing technolo- gy 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 benefi t 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 fl amma- bility, 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 eff ect of environmental factors in comparison to more traditional refrigerant detection technol- ogies, 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 detec- tion. Nevertheless, it is an interesting development. Molecular Property Spectrometer™ gas sensors have been making an appearance in refrigerant gas detection applica- tions, again targeted and limited to %LFL measurement of fl ammable refrigerants (and other fl ammable gases). With the single-source manufacturer boasting claims of very long sensor lifetime, immunity to poisoning, and no false alarms, for refrig- erant gas detection the benefi ts 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 diffi cult a technology comparison.


Download the ACR News app today Electrochemical sensors for NH3 leak 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. Lower-level detection is also needed due to the toxic eff ects of ammonia at low concentrations. Lower-level detection is achieved by using electrochemical sensors, which can be specifi cally tailored for diff erent ranges of measurement. In the operating principle of an electrochemical cell for


NH3, gas diff uses 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 pro- duces an electrical signal that is proportional to the ammonia gas concentration. 2 NH3


➞ N2 + 6 H+ O2 + 4 H+ + 4 e- ➞ 2 H2


+ 6 e- O


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. Neverthe- less, 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 fi eld proven NH3


sensors


with a lifetime in excess of fi ve 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 ef- fective refrigerant detection system. The limited lifetime is vital to note, and there is no getting away from the fact the electro- chemical sensors come at a relatively high cost. Ideally the sen- sor, not the whole gas detector, should be possible to replace in the fi eld. The sensors can also be poisoned by contaminants or even by over-exposure to very high levels of ammonia, and they can be aff ected 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 eff ectively.


www.acr-news.com • October 2024 21


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 – such as from forklift trucks.


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