What does a crackle test of transformer oil detect?

The crackle test is a quick field screen for free water or high moisture contamination. A small oil sample is heated on a hot plate, and audible crackling or visible bubbling indicates water boiling off. It does not quantify moisture and it cannot replace Karl Fischer moisture testing, but it helps technicians decide whether immediate dehydration is urgent, especially after a leak, breather failure, or rapid temperature cycling. Because it is qualitative, results should be confirmed with laboratory moisture measurement and dielectric breakdown testing before making major maintenance decisions.