SOS sensor detects hidden leaks

Argonne National Laboratory has developed a leak detection technology, known as the speed of sound or ‘SOS’ detector, which employs an ultrasonic pulse train to measure the concentration of trace gasses.

The system measures the speed of sound and attenuation in the leaking gas-air mixture, from which the concentration and type of the trace gas are determined.

The prototype device has been tested on US Navy jet planes and the results are said to be encouraging. The Navy plans additional testing of the SOS technology and anticipates that it will eventually be adopted as a standard means of checking the planes for fuel leaks.

The F-14 has five internal fuel tanks that must hold up to 9,000 litres of jet fuel under extreme temperature, vibration, high G forces and other adverse conditions. If a sufficiently large leak in the fuel system should occur near a heat source, there could be disastrous consequences.

To detect fuel leaks in an F-14 using the SOS detector, the plane’s fuel system is first pressurised to approximately 90 psi using an 80 percent helium in oxygen mixture, which is still breathable for safety considerations.

Maintenance technicians then pass the wand attached to the handheld SOS detector along the fuel system components to ‘sniff’ for any traces of the helium gas leaking from the fuel tanks or lines. The ultrasonic technology can detect leaks as small as 10-4 Atm-cc/s, with a response time of less than one second.

A leak this small would take more than a minute to form even a small bubble using the ‘soapy water’ technique employed by maintenance technicians to look for smaller leaks in an F-14’s fuel system.

The ‘soapy water’ test involves pressurising the plane’s fuel system up to 400 psi with a nitrogen gas mixture and observing if the pressure decays over time.

If pressure decay occurs, technicians employ a ‘soapy water’ check, which involves squirting water with a soap additive onto fuel system components to look for bubbles caused by the escaping gas.

‘Using the Argonne SOS detector we were able to locate a small fuel leak in an F-14 engine bay area that we would not have found without an extended pressure decay test or after disassembling the plane,’ said Don Waller, F-14 maintenance supervisor at the Naval Aviation Depot in Florida.

Researchers at Argonne are investigating other applications for the SOS ultrasonic sensor technology, including testing helicopter blades for stress cracks. The detector can also be used to detect heavier gases, such as freon during inspections of cooling and air conditioning systems.