How to understand the ultimate pressure while choosing a vacuum pump

vacuum-select ultimate pressure

How to choose a vacuum pump that can achieve required vacuum for the customers? It is necessary to understand its application and the required vacuum level to be maintained in the working process. Generally speaking, the selected vacuum pump shall have an ultimate pressure with one order of magnitude (or more) higher than the required ultimate pressure for the process. But why?

What is ultimate pressure

The pressure data indicated on the pump nameplate is mostly the ultimate pressure. It means the lowest stable pressure that the equipment can eventually achieve after a vacuum pump operates for a long time till no more air can be pumped out. However, it’s not a good option to keep the pump running at the ultimate vacuum. The working efficiency is very low, and the pumping speed is very small even zero at that time. If the required working vacuum of a process is close to the pressure limit of a vacuum pump, the flow rate of the pump will be so low that the vacuum level cannot be maintained, and the working pressure will rise consequently.

Vacuum range of common mechanical vacuum pumps

vacuum pumps ultimate pressure

Optimal operating pressure of vacuum pumps

For all kinds of vacuum pumps, there is an optimal operating pressure range before it reaches its ultimate vacuum. This range varies depending on different construction of different vacuum pump type. In this operating range, the pump operates more efficiently, and this pressure is definitely higher than the pump’s ultimate pressure. Therefore, the ultimate pressure makes no big sense for vacuum pump selection.

In fact, it’s more important to ensure that the working pressure of the pump in the process shall be in the range of the best pumping speed. Conversely, if the vacuum pump works efficiently in a pressure range, it can be determined that its ultimate vacuum must be lower than this working pressure. Let’s take an example: a vacuum pump operates well at a working pressure of 1 mbar. It means the ultimate pressure of this pump must be lower than 1 mbar. That could be 0.1 mbar (by one orders of magnitude), or possibly more, about 0.01 mbar.

Now back to the point. It’s recommended to check the performance curve of the vacuum pumps while choosing them. In case you find the working pressure is included in the optimal pumping speed range, then that’s the vacuum pump which can meet your demand. The nominated ultimate vacuum can be a reference but again, it’s not important.

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