【2025 Flow Meters】Stable measurement of low flow rates under unstable pressure conditions
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In the field of industrial measurement, pressure/differential pressure measurement under low flow rates and with dirty media indeed represents two major challenges. To address issues such as pressure fluctuations and low flow rates, the key is to enhance signal stability, eliminate interference, and optimize the measurement principle ； As for dirty media, issues such as clogging, adhesion, and corrosion must be addressed to ensure reliable pressure measurement. I. Stable measurement of low flow rates under unstable pressure conditions: Low flow rates produce weak signals, and pressure fluctuations can amplify measurement errors. 1. Select the appropriate type of flow meter (fundamental solution): Thermal mass flow meter: It directly measures the mass flow rate of gases, is insensitive to changes in pressure/temperature, and performs excellently at low pressures and low flow rates (as low as SCCM).
Coriolis mass flow meter: It measures mass flow directly, is unaffected by the properties of the fluid (density, viscosity) or pressure fluctuations, offers high precision, and is suitable for low-flow rates of liquids/gases.
Laminar flow differential pressure flow meter: It utilizes the principle that in a laminar flow state, the pressure difference is proportional to the flow rate; it is insensitive to changes in Reynolds number, exhibits good linearity, and is suitable for low-flow rates of clean gases.
Ultrasonic flow meter (time difference method): no pressure loss, relatively insensitive to pressure fluctuations, performs well for medium and large diameter pipes with low flow rates.
Avoid using: traditional differential pressure types (orifice plates, V-cones, etc., produce too weak differential pressure signals at very low flow rates), as well as turbines (which have difficulty starting at low flow speeds).
2. Optimize differential pressure flow meters (if necessary)
3. System design optimization – Flow stabilization: Install a flow stabilizing valve or a buffer tank with sufficient capacity upstream of the flow meter. Reduce pipe diameter: Narrow the pipe locally in the flow meter section to increase flow velocity and boost signal strength.
II. Continuous and reliable measurement of the pressure/differential pressure of dirty media (including differential pressure flow meters) relies on preventing blockage of the pressure sampling holes, isolating corrosive media, and ensuring the cleanliness of the diaphragm. 1. Physical isolation – diaphragm sealing system (most common and effective)
2. Continuous purging/flushing (applicable to gases/Some liquids): Flow control: Use a constant-flow valve or rotameter to precisely control the flow rate at a low level (to avoid affecting measurements). Purging medium: It must be clean, dry, chemically compatible, and have a higher pressure than the medium being tested. Applicable scenarios: flue gas, dust-containing gases, easily crystallizing slurries, etc. Differential pressure flowmeters require dual-channel synchronous purging. Principle: Continuously inject a small amount of clean purge gas (such as instrument air or nitrogen) or flushing fluid into the pressure tapping line to prevent dirty substances from entering it.
3. Chemical isolation – chemical sealing (for highly corrosive or prone-to-polymerization media): A compatible inert isolating fluid (such as a high-purity glycerin aqueous solution or perfluoropolyether oil) is injected before the diaphragm sealing system to create a secondary isolation barrier.
This post was last edited by The one on 2025-11-14 09:25. 4. Choose a differential pressure flow meter that is not prone to clogging. Wedge flow meter: Its V-shaped channel design requires minimal straight pipe section upstream, it has self-cleaning properties, is wear-resistant, and is suitable for media containing solids or with high viscosity. Elbow flow meter: no inserts required; utilizes the centrifugal force generated by the pipe bend, with no risk of clogging. V-cone flow meter: The cone tail stream has a cleaning effect, making it less prone to fouling than orifice plates, but caution is still required. Avoid using: standard orifice plates (the right-angled edges are prone to clogging due to residue accumulation) and Venturi tubes (the throat section is prone to blockage).