Which statement describes how heat affects viscosity and penetration rate?

• A. Heat decreases viscosity and increases rate
• B. Heat increases viscosity and can reduce penetration rate
• C. Heat does not affect viscosity or rate
• D. Heat decreases viscosity but reduces rate inconsistently

Answer: (B)

Heat lowers the viscosity of most penetrants, which lets the liquid flow into surface-breaking flaws more easily. When the penetrant is warmer, its molecules move faster and overcome resistance to flow, so capillary action and wicking into tiny cracks occur more readily. That faster flow translates into a higher penetration rate, assuming the temperature stays within the recommended range so the penetrant doesn’t dry out too quickly or evaporate essential components.

So the statement that heat decreases viscosity and increases the penetration rate matches what actually happens in typical LPI practice. The other ideas—no effect, viscosity increasing with heat, or viscosity decreasing but rate dropping in an inconsistent way—don’t reflect how penetrants behave under heat in standard procedures.