Viscosity and classification of Viscosity Grades
Viscosity is the most important characteristic of an oil. Viscosity can be described as the measure of a fluid’s resistance to flow. Typically industrial lubricants are classified according to their kinematic viscosity at 40 °C into ISO viscosity grades. The most common unit of kinematic viscosity is mm2/s or cSt. DIN 51519 defines 18 different ISO viscosity grades ranging from 2 to 1000 cSt, where every grade is described by the mean viscosity at 40 °C and an allowable deviation of +/- 10% of this value.
| ISO Viscosity Grade | Midpoint viscosity (cSt) at 40 °C | Minimum viscosity (cSt) at 40 °C | Maximum viscosity (cSt) at 40 °C |
| ISO VG 2 | 2,2 | 1,98 | 2,42 |
| ISO VG 3 | 3,2 | 2,88 | 3,52 |
| ISO VG 5 | 4,6 | 4,14 | 5,06 |
| ISO VG 7 | 6,8 | 6,12 | 7,48 |
| ISO VG 10 | 10 | 9,00 | 11,0 |
| ISO VG 15 | 15 | 13,5 | 16,5 |
| ISO VG 22 | 22 | 19,8 | 24,2 |
| ISO VG 32 | 32 | 28,8 | 35,2 |
| ISO VG 46 | 46 | 41,4 | 50,6 |
| ISO VG 68 | 68 | 61,2 | 74,8 |
| ISO VG 100 | 100 | 90 | 110 |
| ISO VG 150 | 150 | 135 | 165 |
| ISO VG 220 | 220 | 198 | 242 |
| ISO VG 320 | 320 | 288 | 352 |
| ISO VG 460 | 460 | 414 | 506 |
| ISO VG 680 | 680 | 612 | 748 |
| ISO VG 1000 | 1000 | 900 | 1100 |
| ISO VG 1500 | 1500 | 1350 | 1650 |
Effect of Temperature on Viscosity
The viscosity of an oil is highly dependent on the temperature. With increasing temperature, the viscosity will drop exponentially and with decreasing, temperature the viscosity will increase exponentially. The temperature dependency is described by the viscosity index (VI) of an oil. PAO and PAG based oils typically have a high VI and their viscosities are less dependent on temperature. Mineral oils typically have a relatively lower VI and therefore more dependent on temperature. With ASTM D341 the expected viscosity at a given temperature can be extrapolated or interpolated.
Effect of gases on viscosity
For gas compressors, refrigeration compressors, and in heat pumps, the respective gas or refrigerant will dissolve in the oil during operation. This can drastically decrease the viscosity, depending on several factors. This is dependent on the temperature and pressure, but more importantly on the affinity between the gas and oil. By consulting a pressure-viscosity-temperature graph (PVT graph, also known as Daniel plots), one can determine how much gas is dissolved in the oil and also determine the expected viscosity at a given temperature and pressure. These PVT graphs are unique for every oil and gas combination.
PVT graphs and Next Lubricants PVT Calculation Tool
PVT graphs form the basis for evaluating compressor lubrication under operating conditions, and are a critical tool for OEMs to select the right oil. For example, too low viscosity of an oil can result in poor lubrication and undesired wear, whereas too high viscosity will result in decreased efficiency and could negatively affect oil return properties.
In addition to PVT graphs Next Lubricants offers their customers and OEM partners an online-accessible PVT calculation tool. This easy to use tool allows users to determine the effect of a refrigerant/gas mixture on the properties of an oil
Upon request, Next Lubricants can supply miscibility graphs, full Daniel plots, and offer lubricant recommendations. Get in touch to find out more.
