Technical Guide
April 16, 2026
Instrument Selection Guide: Flow Meters (Part I)
A comprehensive technical review of measurement units, range selection, and differential pressure flow elements.
1. Units, Range, and Accuracy
1.1 Selection of Measurement Units
The choice of flow units must comply with the following industrial standards:
- Volume Flow: Cubic meters per hour (m³/h), Liters per hour (L/h).
- Mass Flow: Kilograms per hour (kg/h), Metric tons per hour (t/h).
- Standard Gas Volume: Nm³/h (defined at 0°C and 0.101325 MPa).
1.2 Range Selection and Application
Flowmeter scale ranges are categorized into Square Root and Linear types:
Square Root Scale (0-10)
- Full Scale Reading: 10.0
- Maximum Flow: Should not exceed 9.5
- Normal Flow: Ideally 6.0 – 8.5
- Minimum Flow: Not less than 3.0
Linear Scale (0-100%)
- Full Scale Reading: 100%
- Maximum Flow: Should not exceed 90%
- Normal Flow: Ideally 40% – 70%
- Minimum Flow: Not less than 10%
Technical Illustration: Square Root vs. Linear Scale Mapping
1.2c Selection Rules for Square Root vs. Linear
- Gas & Steam (DP Type): Use Square Root output. Square root extraction and T/P compensation should be performed in the Control System (DCS) or flow computer.
- Liquid (DP Type): Use Linear output. Square root extraction is typically performed within the transmitter; T/P compensation is usually unnecessary.
- Special Applications: Select Square Root output and implement extraction and compensation within the control system logic.
2. Differential Pressure (DP) Flow Meters
2.1 Standard Throttling Devices
| Device Type | Optimal Application |
|---|---|
| Standard Orifice Plate | General fluid measurement. Concentric, square-edged with bevel for general use; square-edged for bi-directional flow. |
| Nozzle / Venturi Nozzle | Clean fluids with lower accuracy requirements but necessitating very low permanent pressure loss. |
| Venturi Tube | Clean fluids requiring both high measurement accuracy and exceptionally low permanent pressure loss. |
2.2 Non-Standard Throttling Devices
Selection of non-standard devices (Balanced, Wedge, Pitot, etc.) must follow manufacturer standards and these specific rules:
Restriction Orifice: Only for flow limitation or pressure reduction; not for flow measurement.
Eccentric Orifice: For pipes > DN100. Best for low-viscosity media containing solid particles where sediment might accumulate.
Segmental Orifice: For pipes > DN100. Suitable for low-viscosity liquids, media containing gases, or liquids with solid particles.
Balanced Flow Meter: For clean gas, liquid, or steam (Reynolds Number 200 to 10 million). Offers high accuracy, wide range, and low straight-run requirements.
Integral Orifice DP Transmitter: For micro-flow of clean media in small pipes (DN ≤ 40mm). For steam, max temp < 120°C. Usually supplied with a calibrated straight pipe run.
Wedge Flow Meter: Specifically designed for high-viscosity fluids or low Reynolds numbers (down to 500).
Pitot / Averaging Tube: For large diameter pipes (DN100 – DN2000). Ideal when low pressure loss is required and accuracy demands are moderate.
Note: All non-standard throttling devices used for flow measurement must undergo formal flow calibration.
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