During hydrostatic testing, TPOL

(a liquid industrial detergent/surfactant composed mainly of sodium dodecylbenzene sulfonate) serves as a wetting agent and leak indicator

.

It is mixed into the test water in low concentrations for two primary reasons:

1. Lowering Surface Tension for Leak Detection

Water has a high natural surface tension, which can prevent it from passing through extremely tight, microscopic weld cracks or pinholes during a test. TPOL acts as a surfactant

, lowering the interfacial tension of the water. This allows the pressurized water to penetrate the smallest gaps, making microscopic leaks visible that might otherwise remain hidden.

2. Foaming Action to Spot Escaping Fluid

When water containing TPOL escapes through a hairline fracture or faulty seal under pressure, it shears and mixes with the outside air, creating a highly visible foam or bubble trail

at the source of the leak. This makes it significantly easier for inspectors to visually pinpoint the exact location of a structural defect.

3. Surface Cleaning and Degreasing

Prior to the final test inspection, mixing a mild detergent like TPOL helps lift light oils, manufacturing grease, or dirt from the internal surfaces. Ensuring these surfaces are clean prevents debris from temporarily "clogging" a small hole and masking a failure.