What does a crackle test of transformer oil detect?

The crackle test detects the presence of free or excessive moisture in transformer oil. When a small oil sample is heated, audible crackling or bubbling indicates water boiling off. This quick field test is qualitative and does not measure moisture concentration, but it helps identify urgent contamination risks. It is commonly used as a screening tool after leaks, breather failures, or sudden condition changes. Results must be confirmed with laboratory moisture testing such as Karl Fischer analysis.