Which instrument is commonly used to approximate the viscosity of aviation oils?

• A. Saybolt Universal Viscometer
• B. Hydrometer
• C. Thermometer
• D. Refractometer

Answer: (A)

Viscosity is how resistant a lubricant is to flow, which is crucial for film strength and pumpability in aircraft engines. The Saybolt Universal Viscometer determines viscosity by timing how long a fixed amount of oil takes to flow through a capillary between two marks at a set temperature. The measured flow time is reported as Saybolt Universal Seconds; a longer time means a thicker oil. This instrument is widely used for aviation oils because it gives a quick, practical approximation of viscosity that correlates well with performance and is convenient for field or shop checks. In contrast, a hydrometer measures density, a thermometer only the temperature, and a refractometer the refractive index, none of which directly indicate viscosity. Remember that viscosity varies with temperature, so the test must be performed at the specified temperature to be meaningful.