How does the SSU (Saybolt Seconds Universal) number relate to the viscosity of a refrigerant oil?

The SSU (Saybolt Seconds Universal) number is a measurement used to characterize the viscosity of fluid substances, including refrigerant oils. In this context, a higher SSU number indicates thicker oil. This happens because the SSU test measures the time it takes for a specific volume of oil to flow through a standard orifice at a specified temperature. Thicker oils have greater resistance to flow, leading to longer flow times and, consequently, higher SSU readings.

In contrast, lower SSU numbers correspond to thinner oils that flow more readily, thus taking less time to pass through the same orifice. Therefore, when interpreting SSU numbers for refrigerant oils, a higher SSU number directly correlates with increased viscosity, confirming that the oil is thicker. This understanding is essential for selecting the appropriate oil for various refrigeration applications, ensuring system efficiency and performance stability.