Using electrochemical sensors and long-range (LoRa) transmitters, researchers have developed a low-cost sensor for detecting flammable refrigerant leaks at less than 5% lower flammability limit (LFL), according to Moonis Ally, Senior Research Staff at Oak Ridge National Laboratory (ORNL).

He described the leak detector in a presentation at the IEA Heat Pump Conference, held in Chicago, Illinois, May 15–18.

The system detects and communicates the presence of combustible gasses – propane (R290), butane and hydrogen – at programmed intervals, which can be as frequent as every second. Transmitters send data using unlicensed (no-fee) LoRa radio frequencies at 902–928MHz with a 5 to 10km (3.1 to 6.2mi) coverage range to a single monitoring station, allowing data from several sensors to be centralized and recorded, Ally said.

“We built an inexpensive propane detector with off-the-shelf components soldered together into a circuit,” said Ally, showing the platform next to a U.S. quarter for size comparison. “If propane is detected, a corresponding voltage is generated, and a signal sent out.” 

“The platform provides fast response rates and requires no field calibration,” he said. In addition, the sensor self-checks. If a self-check signal is not received at the programmed time interval, the sensor has failed and needs to be replaced.

“This technology is now available for [commercial] licensing,” said Ally, adding that the lab currently builds the platform for US$25.00 (€23.21). Ally believes the cost could be less than half of that if it is mass-produced.

Those interested in commercializing the detection system should contact Andreana Leskovjan, Commercial Manager at the ORNL Licensing Office, at


With the LFL of propane at 2.1% by volume of propane in air, the platform testing used standardized propane concentration levels of 47.6% LFL (1% propane in air), 28.8% LFL (0.6% propane in air) and 4.8% LFL (0.1% propane in air).

The detections of propane at the three concentration levels are similar, with a voltage response of around 2.25V for 47.6% LFL, 2.0V for 28.8% LFL and 1.25V for 4.8% LFL. “As soon as the sensor is exposed to propane, it generates a peak,” Ally said. Removal of the concentration “quickly” returns the voltage to the baseline.

“The platform verifies the presence of combustible gases with testing performed for sensing propane and hydrogen,” said Ally, with a voltage response for hydrogen similar to those seen for propane. The sensor has not been tested for gases that may raise a false alarm: paint, nitrous oxide, nitrogen oxide or nitrogen dioxide.

Refrigerant leak data

European data concerning propane refrigerant systems show leaks most often occur at the condenser – the system’s high-pressure site, Ally said, with small leaks happening in other parts of the system. 

Surprisingly, data indicates that if leaks are going to occur, they “normally occur between two and three years of installation.” Ally summarizes the main reason leaks occur in propane refrigeration systems is due to the “jolting, dropping and jerking” of the equipment during installation instead of the expected deterioration.

Propane is an efficient refrigerant, but safety standards are in place due to its flammability. “This platform will assist OEMs looking for long-term solutions with equipment life [greater than] 15 years and in accordance with regulations and safety standards in the United States and the European Union,” said Ally in an ORNL-submitted paper for the IEA conference.

“We’ve operated this one sensor for over a year now with no issues,” Ally said when asked about the life expectancy of the platform.

“The target reduction of greenhouse gas emissions was the main driver of this project with funding from the U.S. Department of Energy,” said Ally, citing propane’s low GWP and the rapid growth of the market.

“We built an inexpensive propane detector with off-the-shelf components soldered together into a circuit.”

Moonis Ally, Senior Research Staff at Oak Ridge National Laboratory