ISO 6358 Pneumatic Valve Flow Calculator — Sonic Conductance, Cv & Kv
ISO 6358 — Pneumatic fluid power — Determination of flow-rate characteristics of components using compressible fluids
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ISO 6358 is the international standard that defines how the flow-rate characteristics of pneumatic components — directional control valves, flow-control valves, fittings, and other restrictive elements — are measured and expressed. It characterises each component by its sonic conductance C [dm³/(s·bar)], which quantifies how much free air the component passes per unit of upstream absolute pressure under fully choked (sonic) conditions, and its critical back-pressure ratio b, which marks the threshold below which choked flow is established. b is a property of the component, measured by the standard's test method, and real valves are commonly in the 0.2–0.5 range; 0.528 is the ideal-nozzle value for air, widely used when no measured figure is available. Once the downstream-to-upstream absolute pressure ratio P2/P1 drops below b, the flow velocity at the restriction reaches the local speed of sound and the mass flow rate is governed entirely by upstream pressure, regardless of further decreases in downstream pressure.
ISO 6358 gives pneumatic engineers a single, consistent basis for comparing valve flow capacity across manufacturers, replacing the ad-hoc use of Cv or Kv figures that were defined under different test conditions. Cv and Kv can be related to C by fixed factors derived from the coefficient definitions — Cv ≈ 0.2955 · C and Kv ≈ 0.2556 · C, giving the exact Cv/Kv ratio of 1.156 — which allows Cv- and Kv-rated components to be lined up against ISO-characterised ones. Note that ISO 6358 itself defines no Cv or Kv conversion: these are industry conversion factors, and different suppliers publish them on different bases, so check which coefficient and which basis a data sheet quotes. MechanixCalc implements the ISO 6358 sonic-conductance method directly in its pneumatics calculator, computing C, Cv, and Kv from entered flow and pressure conditions, detecting choked flow automatically, and selecting the next standard valve size from the ISO 6358 Cv series.
Calculators that implement ISO 6358
What ISO 6358 covers
- Sonic conductance C [dm³/(s·bar)] — the primary flow-rate characteristic under choked (sonic) conditions, determined from the measured volumetric flow rate, upstream absolute pressure, and gas temperature at standard reference conditions (T₀ = 293.15 K, P₀ = 1.013 bar abs)
- Critical back-pressure ratio b — the pressure ratio P2/P1 at which the flow transitions from subsonic to choked; b ≈ 0.528 for air and nitrogen, typically measured by test per the standard's ramp-pressure test method
- Choked-flow regime — when P2/P1 < b, mass flow rate Q = C · P1 · √(T₀/T) is fixed by upstream conditions; lowering downstream pressure further cannot increase flow, a limit the standard makes explicit for system designers
- Subsonic-flow correction — when P2/P1 ≥ b, the standard applies a parabolic correction factor that reduces C towards zero as P2 approaches P1, smoothly joining the choked limit at the critical ratio
- Cv and Kv equivalence — ISO 6358 defines the conversion between its sonic conductance C and the ANSI/ISA flow coefficient Cv (SCFM, US customary) and the metric coefficient Kv (m³/h at 1 bar ΔP), enabling cross-comparison of components rated under different conventions
- Scope of components — the standard applies to directional control valves, flow-control valves, quick-exhaust valves, check valves, silencers, fittings, and any other component in a pneumatic circuit whose flow restriction affects system performance
Parts of the standard
- ISO 6358-1Determination of flow-rate characteristics of components using compressible fluids — Part 1: General rules and test methods for steady-state flow
- ISO 6358-2Determination of flow-rate characteristics of components using compressible fluids — Part 2: Alternative test methods
- ISO 6358-3Determination of flow-rate characteristics of components using compressible fluids — Part 3: Method for calculating steady-state flow-rate characteristics of assemblies
Governing formulas
Q = C · P1 · √(T₀ / T) [Nl/s]where C = sonic conductance [dm³/(s·bar)]; P1 = upstream absolute pressure [bar abs]; T₀ = 293.15 K (standard reference temperature); T = actual upstream temperature [K]; mass flow rate is independent of P2 once choked — lowering P2 further has no effect
Q = C · P1 · √(T₀ / T) · √( 1 − ((P2/P1 − b) / (1 − b))² ) [Nl/s]where b = critical back-pressure ratio (≈ 0.528 for the ideal air nozzle; measured per component under the standard, and typically 0.2–0.5 for a real valve); P2 = downstream absolute pressure [bar abs]; the correction term → 1 as P2/P1 → b (approaching the choked limit) and → 0 as P2 → P1 (no pressure difference, no flow)
Cv ≲ 0.2955 · C Kv ≲ 0.2556 · C (ideal-nozzle ceiling)where Cv = flow coefficient [US gpm of water at 1 psi ΔP]; Kv = metric flow coefficient [m³/h of water at 1 bar ΔP]; C = ISO 6358 sonic conductance [dm³/(s·bar)]. The Cv/Kv ratio of 1.1561 IS fixed by the unit definitions, but the ratio to C is NOT: it depends on the component, and the figures above are the ideal-nozzle upper bound. A real valve chokes at a higher terminal pressure-drop ratio and loses pressure downstream of its throat, so its own ratio is lower — roughly 0.23 to 0.28 — which is why catalogues commonly quote nearer 0.25 · C.
Frequently asked questions
What is ISO 6358 used for?
ISO 6358 is the international test and characterisation standard for the flow-rate properties of pneumatic components — valves, fittings, silencers, and other restrictive elements. It replaces the inconsistent use of Cv and Kv ratings (which were defined under different test conditions by different bodies) with a single, physically grounded parameter: sonic conductance C [dm³/(s·bar)], measured under choked conditions. Engineers use it to size directional control valves, check for choked flow in a circuit, compare components from different suppliers on a common basis, and verify that a valve will not be the bottleneck that limits cylinder speed or actuator force.
What is sonic conductance and what does it mean physically?
Sonic conductance C is the proportionality constant between the choked volumetric flow rate (in normalised litres per second) and the upstream absolute pressure (in bar). Physically, it is a measure of how 'open' the flow path through a valve is under the speed-of-sound limit — a larger C means more flow for the same upstream pressure. A valve with C = 2 dm³/(s·bar) passes twice as much air as one with C = 1 dm³/(s·bar) at the same supply pressure, regardless of what the downstream pressure is (provided choked conditions hold, i.e. P2/P1 < 0.528 for air).
When does choked (sonic) flow occur in a pneumatic valve?
Choked flow occurs when the downstream-to-upstream absolute pressure ratio P2/P1 falls below the critical back-pressure ratio b. For air and nitrogen b ≈ 0.528 — so at a typical 6 bar g supply (7.013 bar abs), choked flow is established any time the downstream absolute pressure is below 7.013 × 0.528 ≈ 3.70 bar abs (2.69 bar g). In a well-designed pneumatic circuit driving a cylinder at low back-pressure, choked flow is the normal operating condition at the directional-control valve, which is why the choked formula is the primary sizing equation. The MechanixCalc valve tab automatically detects the flow regime and flags when the circuit is subsonic.
How does ISO 6358 relate to Cv and Kv flow coefficients?
Cv (ANSI/ISA, used in North America) and Kv (metric, used in Europe) are older flow coefficient conventions defined under different test conditions. ISO 6358 provides a harmonised basis for C and b themselves; the conversion to Cv and Kv is not part of the standard. Derived from the coefficient definitions it is Cv ≈ 0.2955 · C and Kv ≈ 0.2556 · C, where C is the ISO 6358 sonic conductance in dm³/(s·bar), so a valve rated at C = 3 dm³/(s·bar) is equivalent to approximately Cv = 0.89 or Kv = 0.77. Those figures are the ideal-nozzle ceiling of the ratio; a real valve chokes at a higher terminal pressure-drop ratio and loses pressure downstream of its throat, so manufacturers commonly publish a smaller factor of around 0.25 · C. Always check which coefficient, and on which basis, a data sheet quotes. The MechanixCalc pneumatics calculator computes all three from the entered flow condition so you can match the valve datasheet notation your supplier uses.
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