Cv Flow Coefficient in Diaphragm Valves

Published 2026-11-05 · Updated 2026-11-05 · Valonix Engineering

Cv expresses how much flow a valve passes at a given pressure drop. In a specialty gas system it is the fastest way to compare a 1/4 in point-of-use valve with a 1 in header valve, and to avoid the two classic errors: starving a tool or oversizing the line.

What Cv means

By definition, a valve with Cv 1.0 passes one US gallon per minute of water at 60 F with a 1 PSI pressure drop. For gases the calculation also accounts for compressibility, upstream pressure and temperature, but Cv remains the comparable sizing figure.

Typical values in a UHP valve family

A 1/4 in seat gives Cv around 0.20-0.27; 1/2 in gives 0.70-0.80; 3/4 in gives 2.8; and 1 in gives 3.5. High pressure versions trade some flow capacity for a reinforced seat.

• 1/4 in: Cv 0.20-0.27
• 1/2 in: Cv 0.70-0.80
• 3/4 in: Cv 2.8
• 1 in: Cv 3.5

The cost of oversizing

A larger valve has more internal volume. That volume must be purged between gas changes, so an oversized valve lengthens cycle time and increases carry-over risk, especially on switching panels.

The cost of undersizing

Too small a valve forces a high pressure drop at peak flow. On a tool with tight delivery pressure, that shows up as recipe drift or a low-pressure interlock.

A quick sizing workflow

Start with required mass or volumetric flow, set the allowable pressure drop, convert to the required Cv for the gas conditions, then pick the smallest valve that clears it with a modest margin and confirm the end connection.

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