Big Blu is the bubble leak detector from Refrigeration Technologies, the same outfit behind Viper cleaners and Nylog sealants. The formula clings to a surface instead of running off it, so the bubbles hold on long enough for you to see where gas is actually escaping. It is non-corrosive on metal and rated safe on any gas, including oxygen. In the bay that covers AC line fittings, tire bead leaks, and fuel vapor lines, when you would rather not drag out the sniffer.