When a compressor data sheet says 8 bar, it means gauge pressure: 8 bar above the pressure of the surrounding air. In absolute pressure, measured from a perfect vacuum, that is about 9 bar. You set a compressor in gauge pressure. To calculate an air volume, a consumption figure or a compression ratio, you switch to absolute.
This guide sets out the definitions. To work out the pressure your tools need, read how to calculate compressor pressure next.
Two ways of counting the same pressure
Absolute pressure is counted from a perfect vacuum, where there are no air molecules left. A vacuum is 0 bar absolute.
Gauge pressure is counted from atmospheric pressure, the pressure of the air around us. This is what a pressure gauge shows: open to the air, it reads 0, even though the air around it weighs about 1 bar.
The link between the two fits on one line:
absolute pressure = gauge pressure + atmospheric pressure
At sea level, standard atmospheric pressure is 1.013 bar. In a workshop, rounding to 1 bar is accurate enough for everyday calculations.
The notations to recognise
| Notation | Meaning | Example |
|---|---|---|
| bar, bar g, barg | Gauge pressure, as read on the gauge | 8 barg |
| bar a, bara, bar(abs) | Absolute pressure, counted from vacuum | 9 bara |
| psig | Gauge pressure in pounds per square inch | 116 psig ≈ 8 barg |
| psia | Absolute pressure in pounds per square inch | 131 psia ≈ 9 bara |
| MPa | Megapascal, 1 MPa = 10 bar, usually gauge | 0.8 MPa = 8 bar |
When a data sheet, a tool or a compressor simply says "bar", it is almost always gauge pressure. The conversion values in this table are standard physical values: 1 bar equals 100 kPa and about 14.5 psi. Our flow and pressure converter does the arithmetic.
When do you need absolute pressure?
Whenever you calculate a quantity of air. A gas compresses in proportion to its absolute pressure: one litre of air at 8 barg holds roughly 9 litres of air taken at ambient pressure. Three cases come up often.
1. Air stored in a receiver
A 270 litre receiver filled to 10 barg holds about 270 × (10 + 1) = 2,970 litres of free air. Only part of it is usable: the air released between the compressor's cut-out and cut-in pressures, because below that the tools stop working properly. That usable reserve is calculated from the pressure difference, as explained in our guide on compressor control.
2. Consumption of a cylinder or a nozzle
A pneumatic cylinder uses its swept volume, filled to working pressure, on every stroke. At 6 bar on the gauge, you multiply that volume by 7, not by 6. The same reasoning explains why a blow-off nozzle uses much less air when you lower the pressure: its flow roughly follows the absolute upstream pressure.
3. Compression ratio
The work a compressor does depends on the ratio between absolute outlet pressure and absolute inlet pressure. At 8 barg at sea level, the ratio is about 9 ÷ 1 = 9. At 13 bar, it rises to about 14. That is why the same machine delivers less air at higher pressure: the data sheet of the S 110 gives 1.7 m³/min at 8 bar, 1.4 m³/min at 10 bar and 1.1 m³/min at 13 bar.
A worked example: tool pressure in psi
Many tools state their working pressure as 90 psi. Step by step:
- 90 psi ÷ 14.5 ≈ 6.2 bar. With no suffix, it is gauge pressure: 6.2 barg.
- In absolute terms: 6.2 + 1 ≈ 7.2 bara.
- For the compressor setting, stay in gauge pressure: 6.2 bar at the tool, plus network pressure drop and a control margin. An 8 bar compressor suits most cases.
The classic mistake is to mix gauge and absolute values in the same setting, adding or subtracting 1 bar in the wrong place. When you set a pressure, never mix the two.
What about altitude?
Atmospheric pressure falls with altitude: in the standard atmosphere it drops to about 0.9 bar at around 1,000 m. The gauge still shows gauge pressure. A compressor set to 8 bar in the mountains therefore delivers the same useful pressure to the tools, but it draws in thinner air: its compression ratio is higher and it delivers slightly less free air for the same power. If your site is at altitude, mention it in your request so that sizing takes it into account.
What it means when choosing an AIRSTAHL compressor
All the pressures on our data sheets are gauge pressures, like those of your tools:
- The S series is available in 8, 10 or 13 bar, except the S 220 PM, rated at 10 bar.
- The E series includes the E 75 HP, a two-stage piston compressor at 12.5 bar.
- The L series covers 8 to 16 bar for laser cutting.
You can compare the pressure on an AIRSTAHL data sheet directly with the one in your tool manual, without converting. Absolute pressure is only needed for volume and consumption calculations. To see when high pressure is justified, read 10, 13 or 16 bar compressor.
Key points
- "bar" on a compressor or a tool means bar gauge.
- Absolute = gauge + about 1 bar at sea level.
- Set in gauge, calculate air volumes in absolute.
The configurator starts from your tools' working pressure to suggest a model. For a special case (altitude, high pressure, process air), describe it in a quote request: our technicians will check the calculation with you.