Every oil-sealed vacuum pump relies on its oil to do three jobs at once: seal the fine clearances inside the pumping chamber, lubricate the vanes and bearings, and carry heat away from the compression zone. When the oil is the wrong grade — or simply worn out — ultimate pressure rises, internal parts wear faster, and the pump starts running hot. Reading a vacuum pump oil datasheet is the first step toward avoiding all three problems. This guide walks through the key specifications of vacuum pump oil, shows what typical values look like, and explains how to match the oil to your pump and process.
The Main Specifications of Vacuum Pump Oil
1. Kinematic Viscosity (ISO VG Grade)
Viscosity is the single most important number on the datasheet. It is measured at 40°C in mm²/s (cSt) and grouped into ISO VG grades. Most vacuum pump oils fall into ISO VG 46, 68, or 100. VG 68 is a common choice for rotary vane pumps, larger pumps and higher operating temperatures often call for VG 100, and VG 46 suits cooler installations or some smaller pumps. Always start from the pump manufacturer's recommendation: oil that is too thin cannot seal the pumping chamber properly, while oil that is too thick wastes power and makes cold starts difficult.
2. Viscosity Index (VI)
The viscosity index tells you how stable the viscosity stays as temperature changes. A VI of 90 or higher means the oil thins less at running temperature and thickens less during a cold start. This matters for pumps that begin the day at ambient temperature and then settle into steady operation, because the oil keeps an effective lubricating film across the whole temperature swing.
3. Vapor Pressure
Vapor pressure is what separates a genuine vacuum pump oil from ordinary machine oil. The oil sits directly inside the vacuum, so any fraction that evaporates limits the ultimate pressure the pump can reach and can contaminate the process chamber. Dedicated vacuum pump oils are deeply refined to keep vapor pressure extremely low — quality products typically specify a vapor pressure below 0.01 Torr at 100°F (about 38°C), and premium grades go orders of magnitude lower. If a datasheet does not state vapor pressure at all, treat that as a warning sign.
4. Flash Point
The flash point is the temperature at which oil vapors briefly ignite in air. It is not a direct measure of oil quality, but it is a practical indicator of volatility and safety margin. Vacuum pump oils generally specify a minimum open-cup flash point between 215°C and 240°C depending on the viscosity grade — far above normal pump operating temperatures.
5. Pour Point
The pour point is the lowest temperature at which the oil still flows. For pumps installed in unheated workshops, or equipment that sits idle overnight in winter, a pour point of −9°C or lower ensures the oil circulates from the first seconds of operation instead of leaving vanes and bearings dry at start-up.
6. Total Acid Number (TAN)
TAN measures the acidic compounds in the oil, expressed in mg KOH/g. Fresh vacuum pump oil should start at 0.1 mg KOH/g or below. In service, the number climbs as the oil oxidizes, which makes TAN one of the most useful values to track when judging whether a charge of oil is still healthy or due for replacement.
7. Demulsibility (Water Separation)
Vacuum processes regularly pull water vapor into the pump, where it condenses into the oil. Good demulsibility means oil and water separate quickly instead of forming a stable milky emulsion that destroys lubricity and corrodes internal surfaces. Datasheets express it as the separated volumes of oil, water, and emulsion after a standard settling test — for example 40/38/2 ml within 30 minutes, which indicates fast, clean separation.
8. Carbon Residue and Color
A low carbon residue (typically 0.02–0.05% maximum) means the oil leaves fewer deposits on hot internal surfaces, keeping the pumping chamber clean over long service intervals. The color number is a quick visual indicator of refinement level: lighter oil generally reflects deeper refining, though color alone says nothing about in-service performance once the oil darkens with use.
Typical Vacuum Pump Oil Specification Values
The table below shows typical specification limits for the three most common viscosity grades, based on the widely used SH0528-92 standard for vacuum pump oil. Individual branded products often exceed these minimums, so always confirm against the actual product datasheet.
| Property | ISO VG 46 | ISO VG 68 | ISO VG 100 |
|---|---|---|---|
| Kinematic viscosity at 40°C, mm²/s | 41.4–50.6 | 61.2–74.8 | 90–110 |
| Viscosity index, min | 90 | 90 | 90 |
| Density at 20°C, g/cm³, max | 0.880 | 0.882 | 0.884 |
| Pour point, °C, max | −9 | −9 | −9 |
| Flash point (open cup), °C, min | 215 | 225 | 240 |
| Total acid number, mg KOH/g, max | 0.1 | 0.1 | 0.1 |
| Color, max | 0.5 | 1.0 | 2.0 |
| Carbon residue, %, max | 0.02 | 0.03 | 0.05 |
Mineral vs. Synthetic Vacuum Pump Oil
Beyond the numbers, the base oil type shapes how the oil behaves in service:
- Deeply refined mineral oil is the standard choice for most rotary vane and oil-sealed pumps. It offers the best cost-performance balance and meets the full specification range shown above.
- Synthetic oils (PAO or ester based) deliver better oxidation stability, a wider temperature window, and longer service life. They earn their higher price on hot-running pumps, continuous-duty installations, and equipment that is difficult to access for maintenance.
- PFPE (perfluoropolyether) oil is reserved for oxygen-rich, corrosive, or chemically aggressive service such as semiconductor and chemical processing. It is chemically inert but far more expensive, and it must never be mixed with mineral or hydrocarbon oils.
How to Match the Oil to Your Pump and Process
Start with the pump manual — the recommended viscosity grade is matched to the pump's internal clearances and operating temperature. From there, let the process conditions refine the choice. Heavy water vapor loads call for strong demulsibility. Corrosive or reactive gases push you toward PFPE or a pump built with chemical-resistant materials. Food and pharmaceutical lines may require registered food-grade oils. And whatever type you choose, do not mix different oil types in the same pump: drain completely before switching, because incompatible additives and base stocks can form sludge.
Sourcing matters too. Oil bought on specification alone, without traceable refining quality, is a common cause of disappointing ultimate pressure. When you buy vacuum pump oil from a pump manufacturer, the grade has already been validated against the pumps it will serve.
Signs the Oil Needs Changing
Even the right oil does not last forever. Watch for these practical indicators between scheduled changes:
- Oil that has turned dark brown or black, or looks milky — the milky appearance means condensed water has emulsified into the oil.
- Ultimate pressure that no longer reaches the value the pump used to hold, with no other obvious leak.
- Rising running temperature, louder operation, or a burnt smell from the exhaust.
- A measured TAN well above the fresh-oil value, if you include oil analysis in your maintenance routine.
Two accessories pay for themselves quickly here: an oil mist filter on the exhaust keeps oil loss and workplace emissions down, and an external oil filtration system removes contaminants continuously, extending each charge of oil well beyond its unfiltered life.
Conclusion
The specifications that matter most on a vacuum pump oil datasheet are viscosity grade, vapor pressure, flash point, pour point, acid number, and demulsibility — together they determine how well the oil seals, lubricates, and survives in your process. Choose the grade your pump builder recommends, match the base oil type to your process conditions, and change the oil on evidence rather than guesswork.
As a manufacturer of the oil sealed rotary vane vacuum pump and a full range of vacuum equipment, InPowerVac supplies vacuum pump oil formulated and tested for its own pumps, together with oil mist filters, oil filtration systems, vanes, and other spare parts. If you are unsure which grade fits your pump or process, contact the InPowerVac team for a recommendation based on your actual operating conditions.
