Final vacuum
The HS-Group Silicone Diffusion Pump Oil OI-808 achieves ultimate vacuums of 10-6 to 10-7 mbar thanks to its low vapor pressure—provided it complies with the regulations of UHV technology (surface-treated parts and metal or Viton seals), and provides adequate heat output and sufficiently low pre-vacuum.
The following vacuum data are based on a test bench designed according to the rules of the UHV technology. The test set-up for the determination of the final vacuum is shown systematically in the adjacent figure. After the baffles and recipient are baked out over several hours, the resulting final pressure is evaluated. The test duration is 24 hours.
When using an ionization vacuum gauge, suitable for high vacuum measurements, the vacuum measurement is carried out. Depending on the design of the pump, the heating power and the method of measurement, the values for the final vacuum range from 10-6 to 10-7 mbar, unless the equipment is baked out and baffles are not used.
If the special baffles are cooled with liquid nitrogen, significantly better values (up to 10-10 mbar) can be achieved.
Permissible vacuum pressure (counterpressure resistance)
The upper limit of the vacuum system—that is to say, the maximum permissible vacuum pressure (counter pressure resistance)—which is still permissible for the operation of a diffusion pump is critical to the requirements. The pre-vacuum of the HS-Group silicone diffusion pump oil OI-808 should be in the range of 10-1 to 10-2 mbar.
In the figure opposite, the schematic setup for measuring the maximum the permissible vacuum pressure is shown. Here, the final pressure on the intake side of the diffusion pump is first measured.
With the aid of the gas metering valve, air is supplied to the pre-vacuum pressure until the high-vacuum collapses in the course of the falling motive steam flow inside the pump. As a rule, the measurements are carried out without gas load and with different heat outputs at the diffusion pump.
Oil backflow
The diffusion pump oil backflow is that amount of oil which enters the high vacuum chamber under the same operating conditions in a certain time unit. Compared to hydrocarbons (organic oils), the HS-Group silicone diffusion pump oil OI-808 is remarkably favorable due to the very narrow molecular weight distribution.
It is remarkable that even after one week of operating the diffusion pump with diffusion pump oil OI-808 and by using a water-cooled baffle, no contaminated quantity could be detected.
The diagram on the right-hand side shows the setup for measuring oil backflow of the diffusion pump.
The diffusion pump is a propellant pump when the suction pressure in the mixing chamber is substantially lower than the static pressure in the propellant jet (vacuum pump). The name comes from the fact that, in the diffusion pump, the mixing of blowing jet and aspirated gas takes place by diffusion of the gas in the beam; while in the other jet pumps this is preferably done in a turbulent boundary layer. The diffusion pump has basically the same function as a steam-jet pump. However, diffusion pumps have a lower heat output and lower operating pressures (between 10-3 mbar and 10-11 mbar). In operation, the heated propellant enters the steam pipes upwards and passes through one or more nozzles when the steam passes the jets with multiple speed of sound and finally hit the pump body It is deflected downward and carries the suctioned gas molecules along. The gas is then pumped out by the backing pump, the condensed propellant is running on the walls of the pump body down, back into the boiling chamber. Today, mostly high molecular weight oils are used as propellant and are based on oil, silicones, or certain esters.
HZ: Heater, SR: Boiler, PG: Pump housing,
K: Water cooling, HF: high vacuum flange,
GM: gas molecules, DS: accelerated oil steam,
VV: pre-vacuum flange, A,B,C,D: jet system,
TD: oil steam from boiler