Have you ever encountered this situation? After replacing the oil in a vacuum system, the ultimate vacuum still cannot reach the required level. You inspect the seals and check the cold trap, only to discover that the wrong oil was added to the pump. Mechanical pump oil was added to a diffusion pump, diffusion pump oil was used in a booster pump, or even the same oil was used in all three pumps. What happens next? Mechanical pump oil cracks and smokes under the 200°C operating temperature of a diffusion pump, while high-viscosity silicone diffusion pump oil cannot provide proper sealing when added to a mechanical pump. A high-vacuum system is not a single pump. It is a pumping chain consisting of a backing mechanical pump, booster pump, and diffusion pump connected in series. Each stage has completely different requirements for its working fluid. Choosing the wrong oil may result in failure to achieve the target vacuum or, in severe cases, complete pump damage. Today, IOTA explains all three types of vacuum pump oil in one article.
Three-Stage Pumping Chain and Pump Oil Functions in a High-Vacuum System
A typical high-vacuum system consists of three stages connected in series. The first stage is the backing mechanical pump, such as a rotary vane pump or rotary piston pump. It evacuates the system from atmospheric pressure to a rough vacuum of 10⁻¹–10⁻² Pa. At this stage, the pressure difference is large and friction is significant. The primary functions of the pump oil are sealing, lubrication, and cooling. Mineral oil with a viscosity of ISO VG 68–100 is normally used. It should have moderate viscosity, good demulsibility, and a flash point above 160°C.
The second stage is the booster pump, such as an oil-sealed Roots pump or diffusion ejector pump. It operates within the range of 10⁻¹–10⁻² Pa and acts as a bridge between the mechanical pump and the diffusion pump. The working fluid used in an oil-sealed booster pump is usually mineral oil or synthetic hydrocarbon oil. It requires a lower vapor pressure and better thermal stability than mechanical pump oil, although its performance requirements are still below those of diffusion pump oil.
The third stage is the diffusion pump, which operates in the high-vacuum or ultra-high-vacuum range of 10⁻¹ to 10⁻⁷ Pa or lower. It heats the working fluid to generate a high-speed vapor jet that carries gas molecules toward the pump outlet. The operating temperature of diffusion pump oil can reach 200–250°C. Therefore, the fluid must have an extremely low saturated vapor pressure of less than 10⁻⁶ Pa and excellent thermal oxidation stability. This is the fundamental reason silicone oils such as DC 704/IOTA 704 and DC 705/IOTA 705 are preferred as diffusion pump fluids. Mineral oil rapidly cracks at these temperatures, causing its vapor pressure to rise sharply and destroying the ultimate vacuum performance.
Comparison and Key Parameters of Three Types of Vacuum Pump Oil
The core selection principle is to identify the pump type and target vacuum first, and then match the oil according to vapor pressure, thermal stability, and viscosity. Mechanical pumps normally use mineral oil with a viscosity of VG 68–VG 100 and a flash point above 160°C, with particular attention paid to demulsibility. Booster pump oil should be selected according to the pump type. Oil-sealed booster pumps generally use low-vapor-pressure mineral oil or synthetic hydrocarbon oil. Diffusion pumps must use silicone oil or polyphenyl ether with a required vapor pressure below 10⁻⁶ Pa. The following table compares the key parameters of the three types of pump oil:
Parameter Mechanical Pump Oil Booster Pump Oil Diffusion Pump Oil (IOTA 704/705)
Pump Type Rotary vane pump/rotary piston pump Oil-sealed Roots pump/ejector pump Oil diffusion pump
Target Vacuum 10⁻¹–10⁻² Pa 10⁻¹–10⁻² Pa (transition stage) 10⁻⁴–10⁻⁸ Pa
Typical Oil Mineral oil VG 68–100 Mineral oil/synthetic hydrocarbon Silicone oil (IOTA 704/705)
Vapor Pressure at 25°C Approximately 10⁻²–10⁻³ Pa Approximately 10⁻³–10⁻⁴ Pa 2 × 10⁻⁸ torr (IOTA 704)/3 × 10⁻¹⁰ torr (IOTA 705)
Operating Temperature 60–80°C 80–120°C 200–250°C
IOTA Diffusion Pump Silicone Oil: How to Choose Between 704 and 705
IOTA’s vacuum product line focuses on silicone diffusion pump fluids and includes two products designed as alternatives to the Dow DC series.
IOTA 704 is tetramethyltetraphenyltrisiloxane with CAS No. 3982-82-9 and a molecular weight of 484. It has a vapor pressure of 2 × 10⁻⁸ torr at 25°C, a viscosity of 38 ± 3 cSt, a flash point of at least 210°C, and an ultimate vacuum of 10⁻⁷–10⁻⁸ torr. It is suitable for conventional high-vacuum coating, vacuum metallurgy, optical coating, and similar applications.
IOTA 705 is pentaphenyltrimethyltrisiloxane with CAS No. 3390-61-2 and a molecular weight of 546. Its vapor pressure is as low as 3 × 10⁻¹⁰ torr. It has a viscosity of 170–175 cSt, a flash point of at least 243°C, a refractive index of 1.5765–1.5785, and an ultimate vacuum of 10⁻⁹–10⁻¹⁰ torr. It is designed for ultra-high-vacuum applications such as mass spectrometers, particle accelerators, and semiconductor processes.
The selection logic is straightforward: choose IOTA 704 when the target vacuum is around 10⁻⁷ torr, and choose IOTA 705 when the system must reach 10⁻⁹ torr or lower. Their chemical structures are identical to those of DC 704 and DC 705, with the same CAS numbers. Their prices are approximately 50–60% of Dow’s products, and domestic stock can be delivered within 3–5 days.
Failure Data from Mixing Three Types of Pump Oil and the Correct System Configuration
According to vacuum equipment maintenance industry data, using the wrong pump oil is the leading cause of diffusion pump failure, accounting for more than 40% of incidents.
In one typical case, a vacuum coating factory mistakenly added mineral mechanical pump oil to a diffusion pump. After heating to 200°C, the mineral oil cracked and produced large quantities of light components with high vapor pressure. The ultimate vacuum deteriorated from 10⁻⁶ Pa to 10⁻² Pa, while severe carbon deposits formed on the inner walls of the pump. The entire diffusion pump had to be disassembled and cleaned, resulting in more than two weeks of production downtime.
Another common mistake is adding silicone diffusion pump oil to a mechanical pump. Silicone oils have relatively high viscosities: IOTA 704 has a viscosity of 38 cSt, while IOTA 705 reaches 170 cSt. These values are significantly different from the VG 68–100 range used for mechanical pump oils and may lead to poor rotary vane sealing, reduced pumping speed, or even pump seizure.
The correct configuration for a high-vacuum system is as follows: use VG 100 mineral vacuum pump oil in the backing mechanical pump, low-vapor-pressure synthetic hydrocarbon oil in the booster pump stage, and select IOTA 704 for a target vacuum around 10⁻⁷ torr or IOTA 705 for a target vacuum around 10⁻⁹ torr in the diffusion pump stage. Never mix the oils used in these three stages, and thoroughly clean the pump chamber whenever changing the oil.
Selecting a silicone diffusion pump fluid requires matching multiple parameters, including vapor pressure, ultimate vacuum, and thermal stability. IOTA 704 and IOTA 705 have achieved chemical equivalence with DC 704 and DC 705. Contact us by direct message to obtain the complete TDS specifications. If you have questions about the target vacuum or pump compatibility of your high-vacuum system, please contact the technical team at IOTA Silicone Oil Anhui Co., Ltd. through the technical consultation channel on our official website.
