Flow Measurement

Accurate flow measurement using weirs, flumes, or electromagnetic meters

Overview

Flow measurement is critical for process control, hydraulic design, and regulatory compliance. Common devices include Parshall flumes, V-notch weirs, rectangular weirs, venturi meters, and magnetic flow meters. Accuracy requirements: ±5% for daily, ±10% for instantaneous.

Theory - V-Notch Weir

$$Q = C \times \tan\left(\frac{\theta}{2}\right) \times \sqrt{2g} \times H^{2.5}$$

For 90° V-notch: $Q = 1.38 \times H^{2.5}$ (SI units: m³/s, m)

Theory - Rectangular Weir

$$Q = C \times L \times H^{1.5}$$

Where C = 1.84 for suppressed weir

Theory - Parshall Flume

$$Q = C \times H_a^n$$

Constants C and n depend on throat width

Parameters

Device	Range	Accuracy	Headloss	Application
90° V-Notch	0.001-0.1 m³/s	±2-5%	High	Clean water, low flow
Rectangular	0.01-10 m³/s	±3-5%	Moderate	General purpose
Parshall	0.001-100 m³/s	±3-5%	Low	Wastewater, wide range
Mag Meter	Any	±0.5-2%	None	Pressurized pipes

Worked Example

Given: 90° V-notch weir, H=0.25m Calculate: Flow

Q = 1.38 × 0.25^2.5 = 1.38 × 0.0884 = 0.122 m³/s = 122 L/s

References

1. ISO 1438 (2017). Water flow measurement in open channels using weirs
2. ASTM D5390. Parshall Flumes