Comparing vacuum pumps – how to tell which provides a stronger vacuum?

When choosing a vacuum pump for any lab or industrial process such as vacuum filtration, rotary evaporators or for a vacuum oven, you may want to compare different models.

Since vacuum can be expressed in so many different units (mbar, bar, Torr, mm Hg, Pa, psi…) this can be difficult to do!

Looking at the ultimate vacuum in mbar is usually the fastest way to see which provides the strongest vacuum – and remember that a lower number means a stronger vacuum.

We’ve taken a look at why this is below.

What is mbar?

Millibars (written mbar for short) is the commonly used form of the Bar, the S I unit for measuring pressure (and so also vacuums – areas of lowered pressure).

1 Bar is equal to 100,000 Pa (Pascals), 29.53 mm Hg (mm of mercury), or 0.987 atm (atmospheres)

What is ultimate vacuum?

When a vacuum pump states it’s “ultimate vacuum” this is the lowest pressure that pump can achieve, i.e. the strongest vacuum.

Why is a lower number better in ultimate vacuum?

As we’re measuring in mbar of pressure, and a vacuum is an area of lowered pressure, a lower value equates to a stronger vacuum.

• 0 mbar is a perfect vacuum – i.e. no molecules existing in the vacuum evacuated chamber (impossible in real laboratory situations!)
• 1000 mbar is atmospheric pressure – i.e. the normal pressure in a room or outside, with a normal number of molecules in the air. (NB the atmospheric pressure varies with altitude and even weather, so this is a guide value)

So the stronger the vacuum, the more molecules of gas evacuated, the closer to 0.

For example;

• Pump A has an ultimate vacuum of 292 mbar
• Pump B has an ultimate vacuum of 100 mbar

Therefore pump B can achieve a lower pressure, a stronger vacuum.

What else needs to be considered?

The ultimate vacuum is useful as a comparison tool to see which pump can ultimately achieve a stronger vacuum, but you should also think about;

• The application (do you need chemical duty? Oil free? Is it for with a particular piece of equipment like an oven or rotary evaporator? Use our filters on this page to help narrow down the choice)
• The flow rate – in L/min (if two pumps both have the same ultimate vacuum, but one has a larger flow rate in L/min, that pump will reach the ultimate vacuum faster)

For more information about choosing vacuum pumps see our previous posts, follow the links above or contact us;