Ultimate vacuum is one of the important parameters of Roots vacuum pump. Because it is commonly expressed as pressure, it is also called ultimate pressure. It can be seen from the characteristic curve that when the Roots vacuum pump reaches the ultimate vacuum, the forward air flow through the pump inlet is zero, that is, the actual pumping speed of the pump is zero. If working in series with the foreline vacuum, the foreline pump also reaches the ultimate vacuum at the same time due to the continuity of the airflow.

When the Roots vacuum pump is used as a mechanical booster pump, its ultimate vacuum is related to the backing vacuum pump, and different ultimate vacuums can be obtained by selecting different backing pumps. When the water ring vacuum pump is used as the backing pump, the ultimate vacuum of the single-stage Roots vacuum pump is about 1 Torr, and the two-stage Roots vacuum pump can reach 2*10-2 Torr. When the slide valve vacuum pump is used as the backing pump, the single-stage Roots vacuum pump has a The ultimate vacuum is 5*10-4Torr, and the two-stage Roots vacuum pump can reach 1*10-5Torr.

The ultimate vacuum of the wet Roots vacuum pump that discharges the atmosphere directly is not high, generally about 60~40 Torr, and the degree of vacuum is related to the quality of the pump. However, this Roots vacuum pump is often used in the occasion of rough vacuum, and the required vacuum is much lower than the ultimate vacuum, so the ultimate vacuum of this pump is of little significance for use.

When testing the ultimate vacuum of the Roots vacuum pump, it is generally required to start the Roots vacuum pump for 10~20 minutes to achieve the ultimate vacuum according to the regulations of the ministry. The new Roots vacuum pump may take longer to reach the ultimate pressure, because there may be volatile cleaning fluid on the parts, or because the lubricating oil of the pump absorbs water and gas, it takes a certain time to fully volatilize. It is best to try two or three times for comparison.