I recently learned that there is such a thing as "gas leak detector". Bah, looks like fancy soapy water to me. Looked up such things, and it turns out that soapy water can cause corrosion and other bad things. "Gas leak detector" also works at sub-zero Fahrenheit temperatures if one needs to check refrigerant lines. On top of that, I bought some because I need to somewhat regularly crack a propane line on the RV/camper, and it was all of $5 for what is probably a year's supply.