Do you need a Vacuum Switch ?

With the slow hardeners typically used in vacuum infusion, it is common practice to run the vacuum pump for 12 hours or more— often overnight or even throughout an entire weekend. This is not a problem for the pump. The oil-sealed rotary vane pumps most commonly used for infusion work are designed to run continuously between oil changes, which may be required only after 500 operating hours or more.

Apart from saving a small amount of electrical energy, I see little reason to use a vacuum switch. To be effective, a vacuum switch will always limit the ultimate vacuum that the pump can achieve. If the pump cycles on and off at all, the vacuum level must drop to activate the switch.

If the switch is set very close to maximum vacuum, it will cycle too frequently, which can severely shorten the life of the pump motor. Electric motors do not tolerate frequent start-stop cycles well, as the starting current is much higher than the running current. Repeated starts can lead to serious overheating problems.

The most important reason not to use a vacuum switch with the oil-sealed pumps commonly used for infusion work is related to water vapour. These pumps need to run hot in order to effectively expel the water vapour that is almost always present in composite materials.

When the pump is hot, water is boiled off and discharged. When the pump is cold, water vapour condenses and emulsifies in the oil, reducing lubrication quality, shortening oil life, and promoting internal corrosion. Water vapour is the invisible enemy behind many vacuum-related problems in composite construction. I explain this in more detail in the chapter on vapour pressure further on.

This is also why it is good practice to run the vacuum pump for at least half an hour before starting any infusion work, allowing it to reach proper operating temperature.