Owing to concerns about climate change, many jurisdictions are phasing out high global warming potential refrigerants in HVAC&R systems. Their near-term replacements are class A2L (mildly-flammable) refrigerants. Area monitoring detectors will be required for most future residential, commercial, and industrial HVAC systems that use these refrigerants. UL Standard 60335-2-40 requires these detectors to have a set-point of 25% of the lower flammability limit (LFL) and to detect the set-point within 10 s when exposed to a gas mixture at the LFL. Inexpensive detectors that meet these requirements do not exist, which has delayed the adoption of A2L refrigerants. A technology with good potential is based on metal-oxide semiconductors (MOS). MOS detectors are tested here, considering their response to leaks of R-32 and R-454B. They are characterized here for their sensitivity, response time, false alarms from contaminants, and poisoning. The sensors have good sensitivity with a steady-state output that is linear with respect to the logarithm of concentration. The sensors fail narrowly to meet the 10 s response time requirement for both R-32 and R-454B. The sensors do not alarm when exposed to the contaminants in the standard. However, several of the contaminants do poison the sensors, at least temporarily

Birnkrant, Michael J.

Leahy, James R.

Piech, Marcin

Sunderland, Peter B.

Wack, Garrett J.

English

Purdue E-Pubs

Purdue University Purdue e-Pubs International Refrigeration and Air Conditioning Conference School of Mechanical Engineering 2021 Characterization of MOS Sensors for R-32 and R-454B Leaks James R. Leahy University of Maryland Garrett J. Wack University of Maryland Marcin Piech Carrier Corporation Michael J. Birnkrant Carrier Corporation Peter B. Sunderland University of Maryland, pbs@umd.edu Follow this and additional works at: https://docs.lib.purdue.edu/iracc Leahy, James R.; Wack, Garrett J.; Piech, Marcin; Birnkrant, Michael J.; and Sunderland, Peter B., "Characterization of MOS Sensors for R-32 and R-454B Leaks" (2021). International Refrigeration and Air Conditioning Conference. Paper 2146. https://docs.lib.purdue.edu/iracc/2146 This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.html                                                                                                                                                                                                                                                                                                                                                                                          2424, Page 1Characterization of MOS Sensors for R-32 and R-454B LeaksJames R. LEAHY1, Garrett J. WACK1, Marcin PIECH2, Michael J. BIRNKRANT2, Peter B. SUNDERLAND1*1University of Maryland, Department of Fire Protection EngineeringCollege Park, Maryland, USApbs@umd.edu2Carrier Global Corp.DeWitt, New York, USAmarcin.piech@carrier.com*Corresponding AuthorABSTRACTOwing to concerns about climate change, many jurisdictions are phasing out high global warming potentialrefrigerants in HVAC&R systems. Their near-term replacements are class A2L (mildly-flammable) refrigerants. Areamonitoring detectors will be required for most future residential, commercial, and industrial HVAC systems that use these refrigerants. UL Standard 60335-2-40 requires these detectors to have a set-point of 25% of the lowerflammability limit (LFL) and to detect the set-point within 10 s when exposed to a gas mixture at the LFL. Inexpensivedetectors that meet these requirements do not exist, which has delayed the adoption of A2L refrigerants. A technologywith good potential is based on metal-oxide semiconductors (MOS). MOS detectors are tested here, considering theirresponse to leaks of R-32 and R-454B. They are characterized here for their sensitivity, response time, false alarmsfrom contaminants, and poisoning. The sensors have good sensitivity with a steady-state output that is linear withrespect to the logarithm of concentration. The sensors fail narrowly to meet the 10 s response time requirement forboth R-32 and R-454B. The sensors do not alarm when exposed to the contaminants in the standard. However, several of the contaminants do poison the sensors, at least temporarily.1. INTRODUCTIONTo reduce global warming, several states and countries have passed laws requiring refrigerants with low globalwarming potential (GWP) in heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems. Forexample, California will ban the use of refrigerants with GWP 750 and higher in new residential HVAC&R systemsin 2025, while commercial and industrial systems will be limited to GWP 150 [1]. Currently hydrofluorocarbons(HFCs) are the most common refrigerants used in these systems and these typically have GWPs above 1000 [2]. Thelikely replacements are generally mildly flammable HFCs or hydrofluoroolefins (HFOs), both of which are class A2Lrefrigerants. These refrigerants have GWP values between 1 – 675 [2], but are mildly flammable [3-5].For residential HVAC systems the adoption of A2L refrigerants will require area monitoring sensors that are able toquickly detect leaks without false alarms. Many sensors exist, but most are too slow or costly for residentialapplications [6,7]. Furthermore, it is unclear if existing sensors can meet the desired service interval of 5 years(15 years preferred). UL standard 60335-2-40 [8] requires these sensors to alarm at a set-point of 25% of the lowerflammability limit (LFL) and to alarm within 10 s when exposed to a gas mixture at the LFL. The standard alsorequires sensors not to alarm when exposed to a prescribed list of possible contaminants, and that the sensor state-of-health be maintained.A leading sensor technology for this application involves metal-oxide semiconductors (MOS). These sensors are widely used for other applications, have long lifetimes, and are relatively inexpensive [6,7]. However, they have notbeen fully characterized for A2L refrigerants.The objective of this research is to evaluate the suitability of MOS sensors for R-32 and R-454B in residential HVACsystems.18th International Refrigeration and Air Conditioning Conference at Purdue, May 24-27, 2021                                                                                                                                                                                                                                                                                                                                                                                   ulm: mm Pin No. Name I Ve 2 Vout 3 VREF 4 5 G D 25.0 ll,j 23.0 _ i i ... ..::i ,:,l topv·BW sld11vi w Description Circui l voltage Output voltage Reference voltage Common ground Power Supply 2424, Page 22. EXPERIMENTAL2.1 SensorsThe Figaro FCM2630-C00 detector [9,10] was selected for thisstudy because it has the widest detection range of any MOS sensor for refrigerants and a price of approximately US$ 50. Itsdimensions are 25 × 25 × 17 mm and its mass is 4 g. Figure 1shows its schematic and pinout. For this study the excitation, Vc, was maintained at 5.0 ± 0.5 VDC. The sensor output is Vout . Thereference voltage, 𝑉REF, is 3.8 VDC and is the default alarm set-point with the intention that an alarm is indicated when Vout > VREF., i.e., when exposed to 5000 ppm or higher of R-32 [9]. Thisis lower than 25% of the LFL, which has become the standardalarm set-point. In steady state the refrigerant concentrationcorrelates with the difference V = VREF – Vout.The sensors were connected to a power supply (for excitationand ground) and to the analog inputs of a data acquisition (DAQ)system (for Vout and VREF). A manual switch connected a 5 VDC terminal on the power supply to an analog input on the DAQ andwas toggled at the instant when the sensor was exposed torefrigerant or contaminant. Figure 2 shows the wiring schematic.2.2 ApparatusAn apparatus was constructed to expose the sensors to knowngas compositions. Many of these tests involved a steady flowrate of a mixture of refrigerant and air at a known composition. The air and refrigerant flow rates were controlled with needlevalves and measured with calibrated rotameters. They were fullymixed before entering the test vessel. The temperature of the refrigerant was monitored to ensure it remained at laboratorytemperature. Downstream of the rotameters the two gas streamspinout [9].were merged before they flowed into the test vessel. Thedischarge from the test vessel was directed into a fume hood. Theimage and schematic of this apparatus is shown in Fig. 3.The test vessel used was a 500 mL Nalgene bottle made of tritan copolyester.Holes drilled in the top and bottom and sealed to tubes allowed gas inflow andoutflow. A 20 mm plunge hole was drilled into the side of the vessel to admit thesensors. Duct tape covered the plunge hole prior to each test. Additional detailsare provided in Wack [11].2.3 Effects of HumidityThese sensors respond to changes in humidity. When a sensor was plunged fromlaboratory air (with a measured relative humidity of 50% RH) into dry air flowingfrom a compressor (with a relative humidity of 0% RH), Vout increased by 0.3 Vas seen in Fig. 4a. When it was plunged from laboratory air into moist air(previously bubbled through water to obtain 95% RH), there was no suchincrease, see Fig. 4b. To avoid this spurious signal in subsequent tests, air at 95%RH was used throughout. Figure 2: Wiring schematic.Figure 1: Figaro FCM2630-C00 schematic and18th International Refrigeration and Air Conditioning Conference at Purdue, May 24-27, 2021                                                                                                                     6 Ve 5 >4 v ... r aS t,f) ' ,:9 ~ ~ 2 v out 0 -10 0 10 20 Time, s (a) 0% RH {!:) Pressure Gauge jli Nooclle Valve 0 Pressure Regulator 0 Thennomclcr Probe 6 Ve 5 > 4 v,,r of t,f) ' E ~ !5 2 v out 0 30 -1 0 X 0 10 20 30 Time, s (b) 95% RH 2424, Page 3Figure 3: Test apparatus image and schematic.Figure 4 (a): 0% RH sensor response and (b): 95% RH sensor response.3. RESULTS3.1 Refrigerant Plunge TestsFor refrigerant plunge tests the vessel was filledwith mixtures of refrigerant and moist air. Thesewere performed at various fractions of the LFLs, which are 14.4 vol % for R-32 [12] and 11.25 vol % for R-454B [13]. A sensor was plunged into thevessel and its output was recorded with the DAQuntil Vout became steady. The sensor was thenremoved to laboratory air for at least 30 minutes forrecovery.Figure 5 shows the steady-state sensor output for R-32 and R-454B plunge tests at various refrigerantconcentrations. The correlations are nearly identicalfor the two refrigerants. The measurements arereasonably fit with the logarithmic functions shown.The output sensitivity decreases with increasingy = 0.3676ln(x) - 0.6371y = 0.3614ln(x) - 0.6417-3.5-3-2.5-2-1.5-1-0.500.511.50 20 40 60 80 100Voltage Difference, VLFL %Air onlyR-32R-454B R2 = 0.9947R2 = 0.9935Figure 5: Voltage difference V (i.e., VREF – Vout) forvarious concentrations of R-32 and R-454B.18th International Refrigeration and Air Conditioning Conference at Purdue, May 24-27, 2021                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     I I ~---.,v 2424, Page 4refrigerant concentration but is reasonable for the entire range of 0 – 100% LFL.Figure 6 shows the sensor outputs versus time forrepresentative plunge tests into mixtures at their LFLs.Horizontal lines show the steady-state outputs at 10% and25% of LFL from Fig. 5.The tests of Fig. 6 allow the sensor time constant, τ, to be found from fits to the measurements in the form ofexp ( −𝑡 / 𝜏 ) = ( 𝑉 − 𝑉∞ ) / ( 𝑉0 − 𝑉∞ ) , (1)where t is time and subscripts 0 and ∞ denote times before theplunge and in steady state. These time constants, with their95% confidence intervals (CI95), are shown in Table 2.Although these time constants are short, as desired, quantityτ is not used by standards to qualify sensors for areamonitoring.The leading standard for such qualificationsis UL 60335-2-40 [8]. Annex LL of this Table 2: Summary of plunge tests at 100% LFL.standard requires that, when plunged into a -3.5-3-2.5-2-1.5-1-0.500.511.5-10 0 10 20 30Voltage Difference, VTime, sR-32R-454B25% LFL10% LFLFigure 6: Voltage difference (VREF – Vout) for R-32 and R-454B plunge tests at 100% LFL.mixture at the LFL, the sensor alarm within10 s. This alarm is defined as producing anoutput equivalent to its steady output at 25%of the LFL, thus it is denoted t25 here. Asshown in Table 2, these sensors fail to meetthe t25 requirement of 10 s or less.Owing to this, a different sensor responsewhen plunged into a mixture at the LFL wasconsidered. Denoted t10 here, this is the timefor the sensor to produce an output equivalent to its steady output at 10% of the LFL. Asshown in Table 2, the sensors do have t10 timesof 10 s or less. This implies that setting theiralarm threshold at 10% of the LFL wouldproduce an alarm within 10 s when plunged atthe LFL. Unfortunately this modification willnot satisfy UL 60335-2-40, and it could leadto more common false alarms.3.2 Contaminant TestsUL 60335-2-40 Annex LL [8] contains a listof contaminants and their mole fractions inair, X, to which a sensor must be exposed. Thislist is reproduced in Table 3. The last row inthe standard is Silicone at 100 ppm, for whichthe last two compounds in Table 3 wereselected.UL 60335-2-40 requires that upon exposure to the contaminants in Table 3 a sensor “shall not indicate presence ofrefrigerant concentration above the set-point.” This set-point is 25% of the LFL.For each test a sensor was powered up for at least 1 hour and then installed in the vessel with moist air flowing for at least 1 min. For each gas in Table 1 (methane, butane, carbon dioxide, and ammonia), the contaminant flow rate was measured with a rotameter and added to the air stream such that the gases were fully mixed before entering the testRefrigerant Number of τ ± CI95 t25 ± CI95 t10 ± CI95 trials s s sR-32 3 4 ± 2 19 ± 10 8 ± 2R-454B 4 5 ± 0.5 17 ± 2 10 ± 2Table 3: Summary of the contaminants and their test results.X te DeterioratedContaminant Formulappm min Health?Methane CH4 500 17 Yn-Butane C4H10 300 10 Yn-Heptane C7H16 500 10 YEthyl acetate C4H8O2 200 10 NIsopropyl alcohol C3H8O 200 10 YCarbon dioxide CO2 5000 10 YAmmonia NH3 100 10 NEthanol C2H5OH 200 10 YToluene C7H8 200 55 YTrichloroethane C2H3Cl3 200 120 NAcetone C3H6O 200 10 YOctamethylcyclo-tetrasiloxaneC8H24O4Si4 100 10 NDecamethylcyclo-pentasiloxaneC10H30O5Si5 100 17 Y18th International Refrigeration and Air Conditioning Conference at Purdue, May 24-27, 2021                                                                                                                                                                                                                                                                                                                                                • • • • • • • • .................................................................................... • • • • • • • • • • • • ................... ~ ........................ ~ ............ ~ ..... , ........... . 2424, Page 5vessel. The other contaminants were liquids. For these vapor was obtained by bubbling nitrogen through a 500 mLfiltering flask on a load cell. The evaporation rate was measured with the load cell. The vapors were then fully mixedwith air before entering the test vessel. The flow rate of the mixture of air and contaminant was 1.1 – 2.6 LPM [11].For most tests V became constantand the test was stopped after anexposure time, te, of 10 minutes.For some tests V continued to rise after 10 min so the exposure timecontinued until a plateau wasreached. The exposure times are shown in Table 3.Figure 7 summarizes thecontamination tests. The sensorhad the strongest response to n-butane and the weakest response toisopropyl alcohol. Although Vincreased upon exposure to eachcontaminant, it never approachedthe set-point of 25% of the LFL,i.e., 0.55 V. Therefore thesesensors satisfy the requirements ofUL 60335-2-40 for false alarmsfrom contaminants.A state-of-health test wasperformed on each sensor soonafter each contaminant testaccording to UL 60335-2-40 [8]. This involved a test like that ofFig. 6 to identify the t10 responsetime. The sensor health wasconsidered to be deteriorated ifand only if this time exceeded10 s. These tests are summarizedin Fig. 8. As shown in Fig. 8 andTable 3, only for four of thecontaminants was the state-of-health maintained.Additional state-of-health testswere performed after 24 hours with the sensor in air. These tests indicated that in most cases the sensors recovered their initial state-of-health.CONCLUSIONSMOS sensors were tested to characterize their response times to R-32 and R-454B, and their response to contaminants,according to UL 60335-2-40. The main findings are as follows.1. These sensors do not meet the requirement for alarming within 10 s that the set-point (25% of LFL) was reachedwhen plunged into a refrigerant-air mixture at the LFL. Instead, the mean t25 response time was 18 s. Howeverthe sensors did satisfy this requirement for a set-point of 10%. This set-point does not satisfy the standard andwould likely increase the incidence of false alarms.2. The sensors did not return any false alarms when exposed to the contaminants prescribed by UL 60335-2-40.3. The sensors did show deteriorated health shortly after being exposed to these contaminants. However in mostcases this improved within 24 hours of removing the sensor to air.-3.5-3-2.5-2-1.5-1Voltage Difference, VContaminantAir onlyFigure 7: Steady-state V for contami ants t the concentrations shown inTable 3 and in the order tested.051015202530Response Time, sState of Health Test10% LFL (healthy)Figure 8: Sensor t10 response times f llowing ach contaminant test in theorder tested.18th International Refrigeration and Air Conditioning Conference at Purdue, May 24-27, 2021     Based on these findings, it is recommended that small improvements in MOS sensor performance should be made such that they meet the requirements of UL 60335-2-40.  REFERENCES 1. ACHR News, California Approves Rules To Phase Down HFCs, https://www.achrnews.com/articles/144212-california-approves-rules-to-phase-down-hfcs. Accessed Feb. 3, 2021. 2. S. Kujak, K. Schultz, Insights into the next generation HVAC&R refrigerant future, Science and Technology for the Built Environment 22 (2016) 1226–1237. 3. S.G. Davis, J.L. Pagliaro, T.F. DeBold, M. van Wingerden, K. van Wingerden, Flammability and explosion characteristics of mildly flammable refrigerants, J. Loss Prevention in the Process Industries 49 (2017). 4. P. Papas, P. Verma, R. Lord, L. Burns, Turbulent deflagrations of mildly flammable refrigerant-air mixtures, J. Loss Prevention in the Process Industries, 61 (2019) 255-261. 5. D.K. Kim, P.B. Sunderland, Viability of various sources to ignite A2L refrigerants, Energies 14 (2021) 121. 6. R.E. Tapscott and C.W. Sohn, Halocarbon Refrigerant Detection Methods, USACERL Technical Report 96/22, U.S Army (1996). 7. M. Wagner and R. Ferenchiak, Leak detection of A2L refrigerants in HVACR equipment AHRTI Report No. 9009 (2017). 8. UL 60335-2-40, Standard for Household And Similar Electrical Appliances - Safety - Part 2-40: Particular Requirements for Electrical Heat Pumps, Air-Conditioners and Dehumidifiers (2020). 9. Figaro, A2L Refrigerant Gas Sensor Module FCM2630, https://www.figarosensor.com/product/feature/fcm2630.html, accessed Feb. 3, 2021. 10 K. Izawa, SnO2-based gas sensor for detection of refrigerant gases, MDPI Proceedings 14 (2019) 32. 11. Wack, Garrett, Characterization of metal-oxide semiconductor sensors for R-32 and R-454B leaks, M.S. Thesis, University of Maryland, 71, pp., Aug., 2020. 12. D.K. Kim, A.E. Klieger, P.Q. Lomax, C.G. McCoy, J.Y. Reymann, P.B. Sunderland, An improved test method for refrigerant flammability limits in a 12 L vessel, Science and Technology for the Built Environment 24 (2018) 861-866.  13. Opteon™ XL41 (R-454B) Refrigerant, https://www.opteon.com/en/products/refrigerants/xl41, accessed Feb. 3, 2021.           2424, Page 618th International Refrigeration and Air Conditioning Conference at Purdue, May 24-27, 2021

Characterization of MOS Sensors for R-32 and R-454B Leaks

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Characterization of MOS Sensors for R-32 and R-454B Leaks

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