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
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. For air, b = 0.528 — meaning that once the downstream-to-upstream absolute pressure ratio P2/P1 drops below 0.528, 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. The standard's flow coefficients are related by fixed conversion factors: Cv ≈ 1.62 · C and Kv ≈ 1.40 · C, allowing Cv- and Kv-rated components to be compared against ISO-characterised ones. 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 air); 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 ≈ 1.62 · C Kv ≈ 1.40 · Cwhere Cv = ANSI/ISA flow coefficient [SCFM · √(°R/psia)]; Kv = metric flow coefficient [m³/h at 1 bar ΔP, water]; C = ISO 6358 sonic conductance [dm³/(s·bar)]; the ratios are fixed by the unit-system definitions and the standard reference conditions (T₀ = 293.15 K, P₀ = 1.013 bar)
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 and gives fixed conversion factors: Cv ≈ 1.62 · C and Kv ≈ 1.40 · C, where C is the ISO 6358 sonic conductance in dm³/(s·bar). This means a valve rated at C = 3 dm³/(s·bar) is equivalent to approximately Cv = 4.86 or Kv = 4.20. 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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