Precision Sensors
Vacuum vs. Absolute


A question frequently asked in the semiconductor process industry is whether to specify a vacuum switch or an absolute pressure switch. The following information should assist you in making the proper selection and specifying the pressure setting.

Absolute Switches   (E Type Switches)

Absolute pressure switches are what their name implies “absolute”. They operate at the same settings above absolute zero pressure despite atmospheric pressure changes. See Figure_1.

Vacuum Switches  (PV Type Switches)

Vacuum switches are negative gauge pressure switches that operate at a predetermined constant pressure below atmospheric. This means that the absolute pressure of the trip point will vary. See Figure_1.

Using a vacuum switch to indicate a specific absolute pressure could result in frequent, unnecessary and costly adjustments to offset atmospheric variations. An absolute pressure switch would be the proper choice and would eliminate the need for adjustment. Conversely, using an absolute switch to measure the pressure differential across a load lock door can result in popping because the switch does not compensate for atmospheric pressure variations. A vacuum switch would ensure that the pressure difference, relative to atmospheric, remains the same.

Specifying Vacuum and Absolute Switches

Much of the confusion results from the incorrect use of “Torr” and “mm Hg” in specifying a pressure switch set point. Although these units are equal in magnitude, they should not be used interchangeably. Torr is an absolute pressure unit used to express the amount of pressure above absolute zero. Millimeters of Hg should be used to express an amount of pressure below atmospheric i.e. a vacuum.

As producers of pressure switches, we often encounter the following scenarios. A customer or distributor calls and specifies a pressure switch to “trip on increasing vacuum at 150 Torr”. Would this mean that the switch:

A) trips at 150 mm Hg below atmospheric [610 Torr above absolute on a standard day (760)]

B) trips at 150 Torr (610 mm Hg below atmospheric on a standard day)

The difference in these interpretations is 460 mm Hg which could cause a semiconductor process tool to malfunction. This ambiguity can be avoided by specifying settings using “Torr” for absolute pressure switches and “mm Hg” below atmospheric for vacuum switches.

Specifying pressure switches correctly will help us all. It will eliminate unnecessary confusion and cost, and increase the performance of your equipment, resulting in better products for your customer.

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