Success in a laboratory relies in large part on precision. Whether setting a temperature, mixing a solution or taking a measurement, control of the process determines the consistency of results. Even so, lab operators who require vacuum for their applications often default to simply “turning on” the vacuum, allowing the pump to operate at maximum capacity. The reality is that many lab technicians don’t realize there are alternatives to this binary approach.
“Many lab technicians don’t realize there are alternatives to using uncontrolled vacuum.”
With uncontrolled vacuum, lab operators may encounter difficulties they believe are inherent to using vacuum. Filtrate you are trying to collect may evaporate. Sensitive evaporations may bump or foam, leading to loss of sample material. Processes may take much longer than expected. In this blog entry, we’ll examine manual vacuum control options, which can provide a modest level of control while the pump operates at a fixed speed. In our next entry, we’ll address electronic options.
Manually-operated bleeder valves
One of the simplest ways to control vacuum depth is by using an air-admittance valve. Also called bleeder valves, these manually-operated devices allow air to leak into the vacuum line. Because the pump has to handle this additional air, it produces less vacuum than the pump is designed to provide. The control is quite rough and influenced by the rate of evaporation of the application. This approach also leads to considerable pump noise and, when applied to oil-sealed (rotary vane) pumps, produces smelly oil mist. At its worst, mixing air with organic solvent vapors can create flammable exhaust mixtures.
Manually-operated needle or diaphragm valves
An alternative to an air bleed is a needle or diaphragm valve between the pump and the application. The needle or diaphragm valve limits the rate at which the pump can pull air or vapors from the application. This “flow control” process is an indirect way to achieve rough vacuum control; it balances the rate of evaporation from the application with the pumping capacity. Needle and diaphragm valves eliminate the noise, carburetion and oil-mist disadvantages of bleeder valves. However, they require constant oversight for applications with solvent mixtures, since the changing mix of solvents change the balance point between evaporation and pumping, and thus the vacuum level.
Each of these manual control options is better than uncontrolled vacuum for many applications. The continuous oversight makes them prone to user error, however, and each is a time-consuming and tedious approach to control. The direct cost of manual control – the cost of the valve – is much lower than electronic control, but the productivity and sample protection advantages of electronic control make them well worth consideration.
In our next blog entry, we’ll take a look at electronic vacuum control methods.