Viscosity is a measure of a fluid's resistance to flow which is one of the key properties of a lubricant. Oils are generally referred to by their viscosity grade. (For example, ISO 32 and 10w30 refer to viscosity ranges.) With the exception of engine oils, most viscosities are measured at 40°C, with results expressed as Centistokes (cSt), or at 100°F, with results expressed as Saybolt Universal Seconds (SUS). Engine oils are generally run at 100°C or 210°F. A change in the viscosity of an oil usually indicates contamination and/or degradation of the oil. Viscosity is also a good test to see if the correct oil is in a given piece of equipment.
Viscosity tests are run in a number of different types of measuring tubes in a constant temperature bath. We use a Zeitfuchs Crossarm Tube. A small amount of sample is introduced into the crossarm and the temperature is allowed to reach equilibrium with the bath. The crossarm gives the oil a constant head pressure when it is being measured. After the oil reaches the equilibrium temperature (about 5 minutes), the oil is pushed over the crossarm and flows down the tube to fill a measured volume chamber. The time it takes to fill the volume chamber is measured down to a tenth of a second. This time is then multiplied by the tube factor, and the results are in units of cSt. Centistokes can be converted to SUS when the viscosities are being run at 100°F or 210°F.