Flow Measurement · Glossary

What is Discharge Coefficient?

Symbol: CdimensionlessFlow Measurement

Discharge coefficient is the ratio of actual flow to the theoretical flow through a DP flow element.

Discharge Coefficient Explained

Ideal Bernoulli theory over predicts flow through a restriction because of friction and the jet contraction. The discharge coefficient corrects for this.

For orifice plates it is about 0.6 and is calculated with the Reader Harris Gallagher equation in ISO 5167-2. Venturis and nozzles have coefficients close to 1.

Discharge Coefficient Formula

C = Q_actual ÷ Q_theoretical

Q_actual	measured flow
Q_theoretical	flow from ideal Bernoulli equation

Discharge Coefficient: Typical Values

Orifice plate	about 0.60 (ISO 5167-2 equation)
Classical venturi, machined inlet	0.995 (ISO 5167-4)
Classical venturi, as cast inlet	0.984 (ISO 5167-4)
Cone meter	0.800 (ISO 5167-5)

Calculators for Discharge Coefficient

CalculatorOrifice Flow CalculatorFree online calculator with formula and worked example.CalculatorVenturi Flow CalculatorFree online calculator with formula and worked example.CalculatorFlow Nozzle CalculatorFree online calculator with formula and worked example.CalculatorCone Flow Meter CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is Discharge Coefficient?

Discharge coefficient is the ratio of actual flow to the theoretical flow through a DP flow element.

Why is the orifice coefficient so low?

The jet keeps contracting after the plate to the vena contracta, so the effective flow area is smaller than the bore.

Does C change with flow?

Slightly, with Reynolds number. ISO 5167 equations include this effect.

Learn more on the blog

Mistakes in DP flow measurement

A practical guide on instrumentationblog.in with more detail on discharge coefficient and how it is applied on real plants.

Read Full Article

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