Flow Meters Turndown Ratio

Overview

Engineering reference data for Flow Meters Turndown Ratio in fluid mechanics.

Key Formulas

Reynolds Number

$Re = \frac{\rho v D}{\mu}$

Ratio of inertial to viscous forces — determines flow regime.

Bernoulli's Equation

$P + \frac{1}{2}\rho v^2 + \rho g h = \text{const}$

Conservation of energy for steady, inviscid, incompressible flow.

Continuity Equation

$A_1 v_1 = A_2 v_2$

Conservation of mass for incompressible flow.

Darcy-Weisbach

$\Delta P = f \frac{L}{D} \frac{\rho v^2}{2}$

Pressure drop due to friction in a pipe.

Variables

Symbol	Description	Unit
$Re$	Reynolds number	—
$\rho$	Fluid density	kg/m³
$v$	Flow velocity	m/s
$D$	Characteristic dimension	m
$\mu$	Dynamic viscosity	Pa·s
$P$	Pressure	Pa
$f$	Darcy friction factor	—

Turndown Ratio - Rangeability

The turndown ratio (TR), or rangeability, compares the span of a flow measurement device. It is expressed as:

$TR = \frac{q_{max}}{q_{min}}$

Where:

• $TR$ = Turndown Ratio
• $q_{max}$ = Maximum flow (within specified accuracy and repeatability)
• $q_{min}$ = Minimum flow (within specified accuracy and repeatability)

Turndown Ratio Example: Orifice Meter

For an orifice meter with a maximum flow of 12 kg/s and a minimum flow of 3 kg/s, the turndown ratio is:

$TR = \frac{12 \text{ kg/s}}{3 \text{ kg/s}} = 4$

This is typically expressed as a turndown ratio of 4:1. In general, orifice plates have a turndown ratio between *3:1 and 5:1.

Turndown Ratio and Measurement Signal

Flow meters based on differential pressure principles (e.g., orifice or venturi) use the pressure drop across an obstruction to indicate flow rate. According to the Bernoulli equation, the differential pressure increases with the square of the flow velocity.

This relationship means that a large turndown ratio can cramp the measurement signal at low flow rates, potentially reducing accuracy and sensitivity in the lower end of the measurement range.

Variables

Symbol	Description	Unit (Example)
TR	Turndown Ratio	-
q_max	Maximum Flow Rate	kg/s, m³/h
q_min	Minimum Flow Rate	kg/s, m³/h

References

• Original Source