Purdue University engineers have created a system that saves energy and servicing costs by indicating when air conditioners are low on refrigerant.

Professor James Braun, who led the research, said the new "virtual refrigerant charge sensor" is particularly practical for automotive air conditioners, which tend to leak refrigerant more than other types of units, and also for household central air conditioning units.

Braun said maintaining the proper "charge," or amount of refrigerant in a system, saves energy because air conditioners low on refrigerant must operate longer to achieve the same degree of cooling as properly charged units.

"Not only does the energy efficiency go down, but you also reduce the lifetime of the unit because it has to work harder, causing parts to wear out faster," Braun said.

The new technology works by using sensors to monitor the temperature of refrigerant at various points along an air-conditioning unit's tubing. The technique is easy to use because the sensors are simply attached to the outside of the tubing, Braun said.

The research that included former doctoral student Haorong Li was presented this week in Louisville, Ky., during a meeting of the American Society of Heating, Refrigerating and Air Conditioning Engineers. The achievement was also reported in the journal HVAC&R Research.