A common area of confusion when specifying pressure switches arises between absolute pressure and vacuum switches. Our goal is to ensure the most appropriate switch is chosen to provide accurate, repeatable, and most importantly expected performance every time.

Most engineers will agree there is no shortage for units of pressure. It is easy to become confused when converting between bar, psi, Pa, atm, mmHg, or any of the dozens of units of pressure. For simplicity, Precision Sensors has developed an internal standard of pressure units used to distinguish between absolute pressure and vacuum pressure.

Units of Absolute and Vacuum Pressure

Absolute pressure equals zero at full vacuum and references relative pressure above this level. When discussing absolute pressure, Torr is used only to represent units of absolute pressure. Zero Torr is thus equivalent to zero absolute pressure and atmospheric pressure is approximately 760 Torr.

Conversely, vacuum pressure equals zero when at the local atmospheric pressure and references relative vacuum below this level. Units of mmHg are only used to reference the level of vacuum below atmospheric pressure. Therefore, zero mmHg is equivalent to atmospheric pressure and 760 mmHg below atmospheric pressure (or negative 760 mmHg) is nearly full vacuum.

While the magnitude of one Torr is equal to the magnitude of one mmHg, they should not be used interchangeably in the context of absolute and vacuum pressure switches.

So which type of pressure switch is right for my application?

It is not uncommon for local atmospheric pressure to vary by 15 mmHg in just 24 hours. With this much variation, it is possible that an incorrectly specified absolute pressure switch will fail to trip due to the offset on the process pressure due to atmospheric pressure variations. In this context, a vacuum switch would be the correct choice as it references the local ambient pressure.

The critical question to ask when specifying these switches is what should my reference pressure be? If your process requires a system operate at the same absolute pressure every time, an absolute pressure switch is required. If your process involves a system that operates at a level of vacuum below atmospheric pressure, a vacuum switch is required.

Examples

Example 1: I need to know when my process is near atmosphere to allow load lock door operation, do I need an absolute or vacuum switch?

For this application a PV48W series vacuum switch is recommended. The unit can be calibrated for near atmospheric pressure. With a PV48W vacuum switch calibrated for 10±5 mmHg below atmospheric pressure, the switch contacts will change state as the system pressure approaches atmospheric

conditions indicating near equilibrium of the ambient pressure relative to the system pressure allowing for load lock door operation.

Example 2: I need to know when my process is near full vacuum so that I can perform the required chemical process, do I need an absolute or vacuum switch?

For this application an E48W absolute pressure switch is recommended. The unit can be calibrated to 10±5 Torr to indicate the system pressure is near full vacuum (zero absolute pressure). The change in state of the switch contacts at 10±5 Torr indicates the system is near full vacuum and the process can now be performed.

Precision Sensors is available to assist in your decision if you are still unsure what type of switch to use. Submit a question through our Ask An Engineer form and one of our engineers will follow up with more information.

Ask an Engineer