Vacuum Pressure

Vacuum pressure is often a source of ambiguity because engineers use two conventions. Absolute pressure is measured relative to a perfect vacuum, so it is always positive. Gauge vacuum is measured relative to local atmospheric pressure, so it is reported as a negative gauge pressure or as an amount below atmosphere.

For a local atmospheric pressure p_\text{atm}:

If a gauge reads -80 \, \text{kPa}, the absolute pressure is not negative; it is approximately 21 \, \text{kPa} if atmospheric pressure is 101 \, \text{kPa}.

Engineering use

Vacuum systems are used for degassing, drying, coating, semiconductor processing, leak testing, material handling, braking, laboratory chambers, and altitude simulation. The relevant pressure range determines the sensor and pump technology. Rough vacuum, medium vacuum, high vacuum, and ultra-high vacuum behave differently because gas density, mean free path, outgassing, and leakage become important.

Liquids add another constraint: if local absolute pressure falls near or below vapor pressure, boiling or cavitation can occur. In suction piping and pumps, vacuum pressure must therefore be evaluated together with vapor pressure, elevation, friction losses, temperature, and available inlet head.

Common mistakes

A common mistake is writing “vacuum pressure = -90 kPa” without stating gauge reference and local atmospheric pressure. Another is assuming a vacuum pump rating applies at the chamber when conductance losses, leaks, vapor load, and outgassing dominate. A strong vacuum-pressure review states absolute or gauge basis, gas or vapor species, temperature, pressure range, sensor type, leak rate, pump location, line conductance, and whether vapor pressure or cavitation limits apply.