QUICK ANSWER
What to Know First
Start with the lowest, normal and highest absolute pressures, including pump-down and venting. Record the unit and gas. Use a pressure-regime name as context, then check the actual measuring range, gas dependence and operating limits of each gauge, pump and connection.
OVERVIEW
The Technical Context
A lower absolute pressure means a deeper vacuum. The pressure remaining in a chamber affects gas flow, measurement principles, pumping arrangements and surface effects. The useful instrument and system choices can therefore change as the pressure falls.
Pressure labels are conventions rather than precise boundaries shared by every reference. A requirement stated only as “high vacuum” is incomplete: it does not say what value must be reached, where it is measured, how quickly it must be reached or what gas load must be handled.
- Use absolute pressure for a vacuum operating range.
- Always retain the unit beside each value.
- Include normal operation, pump-down and venting.
- Check equipment performance beyond its display range.
ENGINEERING GUIDANCE
Absolute, Gauge and Differential Pressure
Absolute pressure is referenced to an ideal zero-pressure state. Gauge pressure describes the difference from a reference, commonly the surrounding atmospheric pressure. Differential pressure compares two locations. A negative gauge reading is not a negative absolute pressure.
When translating a requirement, confirm which reference the drawing, sensor output or control system uses. The surrounding atmospheric pressure varies, so a gauge-pressure value cannot be turned into an exact absolute value without the relevant reference pressure.
ENGINEERING GUIDANCE
Common Vacuum Pressure Regimes
The following convention is a broad orientation aid adapted from Leybold. Boundaries vary between references and applications. They are not operating guarantees for any Aerocure product.
| Regime | Typical convention in mbar | Planning implication |
|---|---|---|
| Rough vacuum | Near atmosphere to 1 mbar | Review evacuation duty and direct or thermal measurement options. |
| Medium vacuum | 1 to 10⁻³ mbar | Check gauge principle, gas dependence and the system's changing flow conditions. |
| High vacuum | 10⁻³ to 10⁻⁷ mbar | Review suitable high-vacuum pumping, gauges, gas load and installation. |
| Ultra-high vacuum | Below 10⁻⁷ mbar | Surface condition, outgassing, seals and preparation become particularly important. |
ENGINEERING GUIDANCE
Keep Units and Scientific Notation Clear
Common vacuum units include pascal (Pa), millibar (mbar) and Torr. One millibar equals 100 Pa. One Torr is approximately 133.322 Pa. Unit conversion changes the number, not the physical pressure.
Scientific notation expresses small values compactly: 10⁻³ mbar is 0.001 mbar. Keep the exponent and unit together in specifications, screenshots, records and alarm settings. Never compare a pressure value with a leak-rate value; they describe different quantities.
ENGINEERING GUIDANCE
Specify the Complete Pressure Envelope
- Minimum target pressure and normal operating band, with units and reference
- Start-up, pump-down, venting, cleaning and abnormal pressures
- Location at which pressure is measured or controlled
- Gas composition, vapour, temperature and contamination
- Pump-down time, gas load and control or interlock purpose
ENGINEERING GUIDANCE
Why the Measurement Location Matters
A gauge indicates pressure at its port. A narrow or long connection, nearby pump, gas inlet or changing flow can make that local condition differ from another point in the system. Specify which location represents the requirement and how the sensor is connected.
A wide display interval does not establish useful accuracy across that interval. Check the exact measuring region, permitted exposure, gas correction, response and uncertainty needed for the decision. A combination gauge can widen coverage, but its transition behaviour still matters.
ENGINEERING GUIDANCE
Pressure Alone Does Not Select a Pump
Pump selection also needs the gas load, chamber volume, pumping-speed requirement, process compatibility, backing arrangement, utilities and exhaust conditions. A headline ultimate pressure is measured under specified conditions and is not a promise that a complete process system will reach the same value.
Use the full requirement to review gauges, pumps, valves and connections together. That review is more informative than choosing equipment from the regime name alone.
PRACTICAL WORKSHEET
Pressure Requirement Worksheet
Use these details to make a pressure-measurement or pump-selection enquiry clear.
Pressure definition
- Absolute, gauge or differential reference
- Minimum, normal and maximum values
- Unit and required measurement location
Process conditions
- Gas, vapour and composition changes
- Temperature, contamination and gas load
- Pump-down and venting sequence
Measurement
- Accuracy, stability and response needs
- Control, alarm or indication purpose
- Gauge, controller, connection and interface
System duty
- Volume and evacuation-time target
- Existing pumps and backing arrangement
- Utilities, exhaust and safe limits
PRIMARY REFERENCES
Technical Sources and Further Reading
These references support the general technical explanation. Always use the current instructions and approved procedure for the exact equipment and task.
