If a diffusion pump reaches the expected vacuum after an oil change but loses performance after continuous operation, the problem may not be the pump alone. Yellowing, increased viscosity, nozzle deposits and backstreaming can indicate oil degradation or incorrect oil selection.
Viscosity matters, but three parameters are more important: vapor pressure, thermal stability and molecular structure.
Oil is heated to approximately 200–300°C at the bottom of the pump. Its vapor travels through the nozzles as a high-speed jet, carrying gas molecules toward the foreline pump. The vapor then condenses on the cooled pump wall and returns to the boiler.
The final vacuum depends on the complete system, including oil, pump design, forepump, cooling, cold trap, sealing and cleanliness.
Vapor pressure determines the oil-vapor background and strongly influences the achievable vacuum. Lower vapor pressure generally supports a higher vacuum.
Always confirm:
Vapor pressure at 25°C
Measured or extrapolated value
Trapped or untrapped ultimate vacuum
Test method and units
Do not compare Torr directly with Pa: 1 Torr ≈ 133.322 Pa. Ultimate vacuum is a system result; vapor pressure is an oil property.
Prolonged heating can produce light fractions that increase vapor pressure and heavy degradation products that cause yellowing, viscosity growth and nozzle deposits.
Flash point is a safety parameter, not a complete measure of thermal stability. A proper evaluation should also include TGA data, high-temperature mass loss, viscosity and acid-value changes, deposits and long-term operating results.
Higher molecular weight and additional phenyl groups generally strengthen intermolecular forces and reduce vapor pressure. However, they also increase viscosity, startup time and heating requirements.
| Parameter | 704 Type | 705 Type |
|---|---|---|
| Chemical | Tetramethyl tetraphenyl trisiloxane | Pentaphenyl trimethyl trisiloxane |
| CAS number | 3982-82-9 | 3390-61-2 |
| Molecular weight | 484 | 546 |
| Vapor pressure at 25°C | About 2×10⁻⁸ Torr | About 3×10⁻¹⁰ Torr |
| Untrapped ultimate vacuum | 10⁻⁷–10⁻⁸ Torr | 10⁻⁹–10⁻¹⁰ Torr |
| Viscosity at 25°C | About 38–39 cSt | About 170–175 cSt |
| Flash point | ≥210°C | ≥243°C |
Use 704-type oil for conventional high-vacuum coating, metallurgy and lamp-production systems. Select 705-type oil when lower vapor pressure is required for electron microscopes, mass spectrometers and other higher-vacuum equipment. Do not automatically choose 705 if the process does not require it: its higher viscosity can extend startup time and increase energy consumption.
IOTA 704 and IOTA 705 have published core specifications comparable to the corresponding DC 704/705 types. However, identical CAS numbers do not guarantee identical finished-product quality. Purity, moisture, light fractions, acidity, packaging cleanliness and batch consistency must also be verified.
For critical equipment, review the TDS and batch COA, then test pump-down time, ultimate vacuum, backstreaming, oil colour, viscosity and deposits before full replacement.
For complete specifications or samples, visit www.siliconeoil.net or contact Iota Silicone Oil Anhui Co., Ltd.
