ASTM D2161

cSt to SUS

Saybolt Universal Seconds (SUS, also written SSU) are the old efflux time for 60 mL of oil through a Saybolt viscometer. ASTM D2161 is the conversion to and from mm²/s. The petroleum industry reports kinematic viscosity in mm²/s now. The conversion is still how you read a pump curve, a 1960s manual, or a bearing catalog that never got updated.

Use the converter for a number. This page is the method behind it.

The equation

For a kinematic viscosity ν in cSt, ASTM D2161 gives Saybolt Universal seconds at 100°F:

SUS(100°F) = 4.6324 ν + (1 + 0.03264 ν) / [(3930.2 + 262.7 ν + 23.97 ν² + 1.646 ν³) × 10⁻⁵]

At another Fahrenheit temperature t, the same centistoke value converts as:

SUS(t) = SUS(100°F) × [1 + 0.000061 (t − 100)]

The centistoke value and the Saybolt result are at the same temperature. The factor is a property of the Saybolt instrument, not a viscosity-temperature correction. Twenty centistokes at 40°C is not the Saybolt viscosity of that oil at 100°F unless you have already moved the viscosity to 100°F.

A published check

20.00 cSt at 100°F is 97.8 SUS in the ASTM D2161 table. The same equation gives 32.6 SUS for 2 cSt, 39.2 for 4 cSt, 48.8 for 7 cSt, and 58.8 for 10 cSt, all at 100°F. At 210°F those four become 32.8, 39.5, 49.1, and 59.2. Above roughly 75 cSt the correction term is small and SUS is close to 4.632 × cSt, still at 100°F before the temperature factor.

Below about 32 SUS (near 2 cSt at 100°F) the Universal scale is outside its usual range. Heavy oils were often reported as Saybolt Furol seconds instead. This site converts Universal seconds only.

What people mix up

• SUS and centipoise. SUS is a kinematic unit. Centipoise is dynamic and needs a density. See cSt to cP.
• A 100°F Saybolt number and an ISO VG. ISO VG is mm²/s at 40°C (104°F). The temperatures are close. The units are not, and 40°C is not 100°F exactly.
• A single factor of 4.6 applied to a cold, thick reading when you needed the viscosity at running temperature. Change the temperature with two data-sheet points first, then convert the result.

Questions

Are SUS and SSU the same?

Yes. Saybolt Universal Seconds and Saybolt Seconds Universal are two names for the ASTM D2161 unit.

Can I convert SUS back to cSt?

Yes. ASTM D2161 also publishes the reverse equations. This converter runs cSt to SUS, which is the direction of a modern data sheet into an old spec.