Calculate the normalised flow of a pneumatic component from sonic conductance C, critical pressure ratio b, subsonic index m and pressure ratio.
C, b and m come from the test report or catalogue of the actual valve. The result is a steady component characteristic, not a full network or dynamic simulation.
Enter C, b and m from the component's ISO 6358 test data.
Calculate a component's mass and normalized flow from its already-known test values C, b and m, and identify choked versus subsonic operation.
Upstream p₁, downstream p₂ (each as gauge pressure), sonic conductance C, critical pressure ratio b and exponent m from the component's test report, plus air temperature.
C and b come from standardized component testing and are not universal valve constants. Pressure ratio determines whether lowering downstream pressure still increases mass flow; temperature changes density and flow.
If p₂/p₁ <= b, sonic conditions occur at the restriction and normalized flow saturates at qn = C*p1 (choked). Above b, flow falls with rising downstream pressure per the ISO 6358 approximation qn = C*p1*[1-((p₂/p₁-b)/(1-b))^2]^m.
C = 1 dm^3/(s*bar), b = 0.5, m = 0.5, p1 = 6 bar(g), p2 = 0 bar(g), T = 20 degC: absolute pressure p1 = 7.013 bar, pressure ratio p2/p1 = 0.145 is below b = 0.5 -> flow is choked. Normalized flow qn = C*p1 = 7.013 dm^3/s = 420.8 Nl/min, mass flow ~= 8.44 g/s.
Sources and limits: ISO 6358-1:2013 (steady-state characteristics and flow equation). This calculator applies already-known test values C, b and m; it does not determine them from measurement data -- that would be parameter identification via the test procedure described in ISO 6358-1/-2.
Calculate a component's mass and normalized flow from its already-known test values C, b and m, and identify choked versus subsonic operation.
ISO 6358 consists of several parts with different purposes: Part 1 defines the steady-state characteristics (sonic conductance C, critical pressure ratio b) and the flow equation, Part 2 describes alternative test methods (charge/discharge method) for determining those characteristics, Part 3 covers calculating system properties from known component characteristics of several components in series. This calculator applies only the flow equation from Part 1 to a single component with already-known C, b and m values. It does not determine those characteristics from raw measurement data -- that would be parameter identification via the test procedure described in Part 1 or Part 2, a separate calculation step.
qn = C * p1 (choked, if p2/p1 <= b)
qn = C * p1 * [1 - ((p2/p1 - b)/(1 - b))^2]^m (subsonic, if p2/p1 > b)p1 = p1,g + 1.01325 (absolute pressure from gauge pressure)| Symbol / input | Meaning |
|---|---|
| C | Sonic conductance of the component in dm^3/(s*bar), a test value per ISO 6358-1. |
| b | Critical pressure ratio below which flow is choked. |
| m | Subsonic-curve exponent, a test value (often around 0.5, but not universally). |
| p1, p2 | Absolute pressure upstream and downstream of the component. |
| qn | Normalized flow rate at reference conditions. |
Upstream p₁, downstream p₂ (each as gauge pressure), sonic conductance C, critical pressure ratio b and exponent m from the component's test report, plus air temperature. C and b come from standardized component testing and are not universal valve constants. Pressure ratio determines whether lowering downstream pressure still increases mass flow; temperature changes density and flow.
Enter upstream and downstream pressure, the component's sonic conductance C, critical pressure ratio b and exponent m (from the test report or manufacturer datasheet), and air temperature. The calculator automatically detects whether flow is subsonic or choked.
C = 1 dm^3/(s*bar), b = 0.5, m = 0.5, p1 = 6 bar(g), p2 = 0 bar(g), T = 20 degC: absolute pressure p1 = 7.013 bar, pressure ratio p2/p1 = 0.145 is below b = 0.5 -> flow is choked. Normalized flow qn = C*p1 = 7.013 dm^3/s = 420.8 Nl/min, mass flow ~= 8.44 g/s.
As long as p2/p1 stays below the critical pressure ratio b, sonic velocity occurs at the narrowest cross-section -- normalized flow is then set entirely by p1, C and temperature and no longer responds to a further reduction of p2. Above b, flow falls as p2/p1 rises per the approximation equation, until no flow occurs at p2 = p1. This distinction is not a side note: if a valve is inadvertently operated in the subsonic range in service while the design assumed choked flow, it delivers noticeably less flow than expected.
Pressures entered in bar(g), converted internally to bar(a); sonic conductance C in dm^3/(s*bar); mass flow in g/s, normalized flow in Nl/min.
Checking whether a valve with a given C value delivers the required mass flow at the intended pressure conditions, comparing several valves using their manufacturer ratings, estimating the air consumption of a known component at its operating point.
ISO 6358-1:2013 (steady-state characteristics and flow equation). This calculator applies already-known test values C, b and m; it does not determine them from measurement data -- that would be parameter identification via the test procedure described in ISO 6358-1/-2.
Common mistake: Don't carry C, b and m over from a different valve or size -- all three values come from a standardized ISO 6358-1 or -2 test and apply only to the specific component tested. C, b and m cannot be uniquely back-calculated from a single data point (only total flow and total pressure difference) -- that requires a series of measurements across several pressure ratios.
C describes the maximum possible normalized flow of a component per bar of absolute pressure under choked flow -- a characteristic of the component's geometry, determined in a standardized ISO 6358-1 test, not a universal constant.
The pressure ratio p2/p1 below which flow at the narrowest cross-section reaches sonic velocity and flow no longer depends on further lowering p2 (choked flow).
Flow at sonic velocity at the narrowest cross-section: mass flow is then set entirely by upstream pressure, and further lowering downstream pressure brings no additional flow.
A single measurement gives only one data point of pressure ratio and flow; but C, b and m describe an entire characteristic curve across the full pressure range. Only several measurements at different p2/p1 uniquely fix all three parameters -- this calculator already assumes those characteristics are known.
Part 1 defines the steady-state characteristics and the flow equation for a single component (the basis of this calculator). Part 2 describes an alternative test method (charge/discharge method) for determining the same characteristics. Part 3 covers calculating system properties from the known characteristics of several components connected in series.
Components without valid ISO 6358 test values, strongly unsteady or pulsating operating conditions, and networks of several components -- the latter is covered by ISO 6358-3 with its own combination rules, not this single-component calculator.
All main influencing quantities are shown as inputs or intermediate results. Check factors, material data and boundary conditions against the complete standard.
The local standards provide terminology and series values. This tool deliberately identifies its simplified scope and does not claim conformity.
Use the result as a starting point for selection and plausibility; then verify manufacturer data, load spectra and safety requirements.