Measurement Principles of Liquid Flow Meters

The measurement principles of liquid flow meters vary depending on the type. Here are explanations for common types:

1. Electromagnetic Flow Meter‌

Based on Faraday's Law of Electromagnetic Induction, when a conductive liquid (conductivity ≥5 μS/cm) flows through a pipe with magnetic field strength ‌B‌ at velocity ‌v‌, an induced electromotive force ‌E‌ (E = B·v·D, where ‌D‌ is pipe diameter) is generated. By measuring ‌E‌, the flow rate can be calculated. Key features include ‌no pressure loss‌, ‌suitability only for conductive liquids‌, and ‌insensitivity to temperature/pressure changes‌.

2. Ultrasonic Flow Meter‌

Uses the ‌time-difference method‌: Upstream and downstream probes alternately transmit ultrasonic signals. Fluid flow causes a time difference in signal transmission, which is used to calculate flow based on the relationship between time difference and velocity (Δt = 2Lv/c², where ‌L‌ is sound path length and ‌c‌ is sound speed). Advantages include ‌ability to measure corrosive/radioactive liquids‌, but it is ‌limited by temperature (usually ≤200°C)‌.

3. Turbine Flow Meter‌

Relies on fluid pushing turbine blades to rotate. The rotational speed is proportional to flow velocity, and a ‌Hall sensor converts mechanical rotation into electrical pulse counts‌, which are then converted into flow rate. ‌Full-pipe installation and straight pipe requirements‌ must be observed.

4. Differential Pressure Flow Meter (e.g., Orifice Plate)‌

Based on Bernoulli's principle, when fluid passes through a restriction (e.g., orifice plate), a pressure difference ‌ΔP‌ is generated. Flow rate ‌Q‌ is proportional to the square root of ‌ΔP‌. It has a ‌simple structure but causes pressure loss‌ and is suitable for ‌gases, liquids, and steam‌.