In the literature, pressure values of 7 bar and 6 bar are frequently mentioned, often without specifying whether they refer to absolute values (bar(a)) or gauge values (bar(g)).
The pressure value of 6 bar(g) is of particular importance in pneumatics, as many manufacturers determine the nominal flow rate using 6 bar(g) inlet pressure and 5 bar(g) outlet pressure. Similar to Kv-value determination with water, the resulting pressure drop is therefore 1 bar.
This condition has become a quasi-standard for comparative measurements. Neither ISO 6358 nor VDMA 24568 contains a clause explicitly prescribing 6 bar(g) / 5 bar(g). Instead, it is a widely adopted convention used for practical, technical, and historical reasons, but not formally codified.
The Purpose of These Pressure Values
Using 6 bar(g) inlet pressure and 5 bar(g) outlet pressure results in a ratio of:
This value is well above the critical flow threshold of approx. 0.528, which means that the flow remains subsonic. As a result, “choked flow” effects are avoided, where further reduction of the outlet pressure would not increase the flow. This ensures that the flow is more linear and therefore more comparable. Measurement results are stable, reproducible, and transferable across different valves.
In addition, 6 bar(g) is a pressure level typically delivered by most standard compressors and is therefore widely used.
Other Frequently Used Pressure Values
In the literature and in many data sheets, a pressure value of 7 bar(g) is also frequently found. This does not necessarily refer to a test flow at 7 bar(g) → 6 bar(g), but often to the specification of the maximum supply pressure or another reference condition. Particularly with values given as “7 bar,” there is a risk of confusion between absolute pressure values (bar(a)) and relative pressure values (bar(g)).
Conclusion
An inlet pressure of 6 bar(g) and an outlet pressure of 5 bar(g) are the established standard for flow measurements, as these values ensure a sufficient margin from critical flow conditions and thus provide linear comparability.
