Shortly after the pump starts working, the lowest pressure P'g occurs. The pump continues to run, and due to the increase of pump temperature, the saturated vapor pressure of the pump oil increases and the viscosity decreases, and the inlet pressure slightly rises to Pg. Pg is the stable minimum pressure obtained by the pump, and is also the ultimate pressure of the pump.

The ultimate pressure of the rotary vane pump is related to the type of vacuum gauge used in the measurement. When measured with a compression vacuum gauge, the ultimate pressure value is about an order of magnitude lower than the result measured with a thermal conductivity vacuum gauge (thermocouple gauge or Piton gauge, etc.). This is because the former can only measure the partial pressure of the permanent gas, while the latter measures the full pressure of the compressed gas. my country's current regulations use compression vacuum gauges to measure the ultimate pressure of oil-sealed mechanical pumps.

The amount of oil circulating in the pump also affects the ultimate pressure of the pump. As the amount of oil increases, the sealing effect is better, and the relationship between the ultimate pressure of the pump and the amount of oil changes, and finally reaches the saturated vapor pressure Po of the oil. In fact, the pump oil contains a lot of air and water. After the pump oil enters the pump cavity, the air and water absorbed in the pump oil are released again, and the relationship between the pump limit pressure and the oil volume develops upward. Generally, the pump oil should be kept at the specified oil level to keep the circulating oil volume of the pump constant. For a two-stage pump, the pump oil can only enter the high vacuum stage after degassing in the low vacuum stage, which can reduce the amount of air released by the pump oil in the high vacuum stage to reduce the ultimate pressure of the pump.

The ultimate pressure of the pump is related to the working temperature of the pump oil. The higher the oil temperature, the higher the saturated vapor pressure of the oil and the higher the ultimate pressure of the pump.