Asbjørn Vonsild, Owner of Denmark-based Vonsild Consulting, has announced the development ‒ in conjunction with Daniel Colbourne ‒ of a “cost-effective” and “robust” ultrasonic leak detection system for smaller equipment using flammable refrigerants.

“We tested it with R290 [propane] leaks and published our findings in a free-to-download paper available until November 14,” Vonsild said in a LinkedIn post. He indicated that the system would apply to residential units, including heat pumps, air conditioners and similar applications.

According to the published paper, conventional leak detectors using infra-red gas concentration sensors cost “tens to hundreds of euros” and “are susceptible to poisoning and other contamination that can negatively affect their performance.”

The ultrasonic leak detector (ULD) detects leaks based on airborne high-frequency sound waves generated from a leak exiting an orifice and hitting a transducer that functions as a receiver and sensor and transposes the waves into electrical signals for communication. “Since the noise from a leak travels at sonic velocity, acknowledgment [by ULD] is effectively instantaneous,” the paper noted, citing that gas sensors rely on “significantly” slower mass transfer to reach the sampling point.

The researchers built the ULD using off-the-shelf components, including low-cost (less than €1/US$1.05) transducers. “Transducer response does not drift appreciably with aging and is [not] vulnerable to contamination,” the paper said, with research that found transducers have a mean 27-year life span. As such, the ULD could outlast the 15-year life span of most small- and medium-sized refrigeration, air-conditioning and heat pump equipment.

A further ULD advantage over gas sensors is that a second transducer can regularly prove proper detection system functioning via an ultrasonic signal.


The researchers tested eight ultrasonic transducers, commonly employed for “production line controls, liquid level sensors and personnel detection.” Rated for -40 to 80°C (-40 to 176°F) operating temperatures, seven transducers had a peak frequency of around 40kHz and one around 58kHz.

Besides the transducers, the ULD system circuit used pre-amplification to remove electrical noise outside of the targeted frequency range, two-stage amplification inside of the targeted frequency range, peak detection after approximately 0.1 seconds of ultrasound noise, a comparator to turn on a leak detection light and a 5V voltage regulator for stable power supply, with a power-on indicator light.

To determine the mass flow of a release necessary to activate the leak detection light, researchers used a 5kW (1.4TR) capacity split air conditioner wall-mounted indoor unit. Tests focused on the right end of the air conditioner, where experience shows larger leaks arise. However, testing was also conducted on the left end, the center and the rear of the unit. Simulated leaks ran from 0.1 to 2.5mm (0.004 to 0.1in) in diameter using 0.1 to 0.5mm (.004 to 0.02in) increments.

The paper noted determining the appropriate “maximum safe leak rate” (MSLR) keeps costs down. “Only detecting leaks larger than MSLR is desired to mitigate the hazard.” 

The International Electrotechnical Commission specifies MSLR based on the lower flammability limit, which for R290 corresponds to 9 grams per minute (0.3 ounces per minute). The paper notes that room size and release height influence potentially flammable mixtures, too. Taking this into account, “the MSLR ranges from 6 to 23 grams [0.2 to 0.8 ounces] per minute for a unit placed at 0.5 to 2.0m [1.6 to 6.6ft] height in a 15m2 [161.5ft2] room or 9 to 38 grams [0.3 to 1.3 ounces] per minute in a 25m2 [269.1ft2] room.”


“Results were broadly positive, with the ULD being able to detect simulated leaks in the range of 4 to 13 [0.5 ounces] grams per minute,” the paper said. “For all cases where the simulated leak and sensor were within the right end section of the IDU, an active response of the ULD occurred as low as 2 grams (0.1 ounces) per minute, depending upon the chosen sensor.”

The paper noted that the sensor should be within 0.4mm (0.02in) of a potential leak point to meet MSLR. “[However,] a typical wall-type air conditioner would unlikely require more than two sensors within the right end section.” Due to sound dampening, an additional sensor may be required under pipe insulation around connections or sharp bends, where potential indoor leaks could occur.

According to the paper, false positives from other noise sources are highly unlikely. “The noise from the [air conditioner] fan/motor is relatively quiet within the ultrasonic range, and the casing appears to provide significant attenuation of external [noise] sources.”

The researchers only tested R290 refrigerant. The approach may work for other flammable refrigerants based on the working pressures. “For instance, it is likely to be more effective with R1270 [propylene] and less effective with R600a [isobutane],” the paper said.

“Overall, ULD can be an appealing technology due to the substantially lower cost than that of gas detection, better reliability, much faster response times and greater longevity,” the paper noted. “It may be regarded as an effective means of significantly extending the safe application of hydrocarbon refrigerants.”

The authors acknowledged Dr Ivan Vince of ASK Consultants for the original idea and project Proklima of the Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ), HEAT and the EU Life Front Project for supporting the study.

“We hope our work will benefit the industry, and we have made no patents on the ideas presented,” Vonsild said in his LinkedIn post.

Last May, researchers at Oak Ridge National Laboratory also released a study on a “low-cost” propane leak detector that is ready for commercialization, using off-the-shelf electrochemical sensors and long-range (LoRa) transmitters.

“Ultrasonic leak detection can be an appealing technology due to the substantially lower cost than that of gas detection, better reliability, much faster response times and greater longevity.”

Asbjørn Vonsild in International Journal of Refrigeration Paper