When choosing a vacuum pump for their laboratory, many people find vacuum technology a bit confusing. Rather than wrestle with the details of vacuum pump specifications and how they relate to the application at hand, the temptation may be to choose the pump they had before (even though it may reflect aging technology), look for the least expensive option, or buy the deepest vacuum they can find for the dollar. All of these approaches may lead to disappointment.
The following tips can put you on track to find the right pump for your specific needs.
What vacuum depth does your project require?
Before you can determine which pump is right for you, identify how deep a vacuum your project requires. For most lab applications, vacuum in the “rough” vacuum range (between atmospheric pressure and 1 mbar/0.7 Torr) will be sufficient.
Operations that use vacuum to move liquids, such as vacuum filtration or liquid aspiration, rely on pressure differentials and need only modest vacuum. These activities can typically be accomplished with a vacuum no deeper than 100 mbar. Recognizing that atmospheric pressure at sea level is about 1000 mbar (760 Torr), vacuum of 100 mbar (76 Torr) represents about 90% of the total pressure differential available.
The vacuum level necessary for evaporative applications, on the other hand, will often require significantly greater vacuum depths – closer to the 2 mbar (1.5 Torr) bottom of the rough vacuum range. The pump must be capable of achieving the specific solvent’s vapor pressure. The vacuum needed will depend on the solvents involved and the conditions of the application (temperature, other solvents present, etc.) and require careful calculation to determine the specific depth (and pump) that is necessary.
Other applications, such as freeze drying and molecular distillation, require vacuum depths well below 1mbar, and therefore demand pumps designed to deliver such deep vacuum levels.
Which pump will achieve the necessary depth?
To choose the appropriate vacuum technology, look for a pump that has an ultimate vacuum (the lowest possible vacuum a pump can achieve) below – but close to – the level required for your project. Too little vacuum will slow the process; too much vacuum may evaporate materials you want to collect, or make it very difficult to control the vacuum process.
For applications requiring rough vacuum, we recommend using a diaphragm pump. Diaphragm pumps are among the easiest to maintain laboratory vacuum pumps, capable of generating up to 1mbar without using oil. The pumping action operates on principles similar to the motion of your heart. Diaphragm vacuum pumps can operate for thousands of hours with virtually no repair or upkeep costs. The infrequent service and repairs lead to significant savings in total lifetime costs of service.
In the past, water jet aspirators were an acceptable alternative to pumps to generate vacuum well into the rough vacuum range – down to as little as 15 mbar (12 Torr), depending on water pressure and temperature. In order to achieve these vacuum levels, however, they consume huge amounts of water. Because the water is in contact with the vapors coming off vacuum operations, this water gets contaminated in the process, opening the door to environmental concerns labs could have easily avoided by choosing different technology. The water waste and contamination has led some municipalities and states to prohibit use of these devices, while institutions are wary of the cost of water supply and treatment that may be overlooked by the user in the lab.
If you need to equip an entire lab with vacuum in the rough vacuum range, consider local vacuum networks. VACUU·LAN® technology from VACUUBRAND lets you use one small, quiet pump under your bench or fume hood to support numerous users at once. This saves space and energy, and provides deeper, more stable vacuum than central systems. This approach is suitable both for upgrading existing labs, or for new construction.
When the application calls for a vacuum deeper than 1mbar, a rotary vane pump is likely your best option. Rotary vane pumps rely on a rotational mechanism with very tight tolerances to generate the deeper vacuum. As a result, most require oil to operate smoothly, and therefore demand significantly more upkeep than the diaphragm pump technology. More recently, oil-less rotary vane pumps have been introduced. They offer the advantage of oilless operation, but typically use materials that may not be as corrosion resistant as diaphragm pumps. Check with the manufacturer for compatibility with the reagents in use in your lab.