Pressure is the first specification people look at on a compressor, yet it is often the most misunderstood. A "10 bar" compressor is not necessarily better than an "8 bar" one: it all depends on what your tools actually need, and on how much your network loses along the way.
The bar, in a nutshell
The bar is the unit of pressure used in workshops. One bar is roughly equal to atmospheric pressure at sea level. You will also come across the MPa (1 MPa = 10 bar) on original manufacturer data sheets, and psi on American equipment (1 bar ≈ 14.5 psi).
The pressures stated on compressors and tools are gauge pressures: they are measured relative to atmospheric pressure. A pressure gauge reading 0 bar is not "empty", it is simply at ambient pressure.
Pressure and air flow - two different things
The two are often confused, even though they answer two different questions:
- Pressure (bar) tells you how forcefully the air comes out: it must be sufficient for your most demanding tool.
- Air flow (l/min or m³/min) tells you how much air the compressor delivers: it must cover the consumption of all the tools used at the same time.
A compressor can have enough pressure but not enough air flow: the tool starts fine, then loses power after a few seconds because the air receiver empties faster than it fills. For air flow, read our guide What size compressor do I need?.
The 4-step calculation method
Step 1: find the working pressure of each tool
Every air tool states its working pressure in its manual or on its data sheet. This is the pressure at which it delivers its rated performance. Note it down for each tool and keep the highest one: that is what sets your requirement.
Step 2: estimate the pressure drop in the network
Between the compressor and the tool, the air passes through the air receiver, the dryer, the filters, the piping, the fittings and the hoses. Each component causes a small loss of pressure: this is the pressure drop. It increases with the length of the network, bends, quick couplings, clogged filters and undersized pipes.
The best way to know it is to measure it: one pressure gauge at the compressor outlet, another at the furthest workstation, while the tool is running. The difference between the two is your actual pressure drop.
Step 3: add a setting margin
A compressor does not deliver a fixed pressure: it cycles between a cut-in pressure and a cut-out pressure. The lowest pressure in that cycle must stay above your requirement. So allow a small margin above the result.
Step 4: add it all up
Pressure to set on the compressor = working pressure of the most demanding tool + measured pressure drop + setting margin.
The compressor's maximum pressure must be higher than this total.
Worked example
The figures below are an example, not values to copy as they are. Take a workshop whose most demanding tool requires 6 bar in operation. The pressure gauges show a difference of 0.5 bar between the compressor outlet and the furthest workstation. With a setting margin of about 1 bar, the compressor must be able to maintain at least 7.5 bar: an 8 bar model is suitable, while a 10 bar model provides headroom if the workshop plans to add more demanding tools.
Why not just set everything to maximum?
Because every extra bar comes at a cost. The higher the pressure demanded, the harder the compressor works and the more electricity it consumes, with no benefit at all for a tool that does not need it. Excessive pressure also makes leaks worse and wears out your tools.
The right approach is the opposite: reduce pressure drop (correctly sized pipes, fittings in good condition, clean filters) rather than raising the pressure to compensate for it.
Signs of incorrectly set pressure
- Tools lose power at the end of the cycle or when several workstations are running at once.
- The compressor runs almost non-stop even though nothing in the workshop has changed.
- The difference between the two pressure gauges increases over time: look for a leak or a clogged filter.
And at AIRSTAHL?
The screw compressor 7.5 kW 4-in-1 operates at 8 bar, and the screw compressor 22 kW permanent magnet goes up to 10 bar. The pressure of each model is listed on its data sheet. Send us the list of your tools and the length of your network in a quote request: we will do the calculation with you.