How to Measure Powder and Dry Solids Flow: Methods, Limits and Meter Selection

To measure the “powder” with a powder flow meter, you measure its flow rate, which is the amount of powder flowing through the flow meter per unit time, usually expressed in grams per minute (g/min) or kilograms per hour (kg/h) and is determined by the specific design of the flow meter.

Methods such as measuring the change in weight at set time intervals or utilizing sensors to detect the mass flow of the powder as it passes through the meter are often used.

Flow measurement methods:

• Electrostatic powder flow meter: Powder picks up electric charge from contact and friction during pneumatic conveying or free fall. Our meter uses an annular sensor to pick up the signal from each charged particle passing through it. The signal is amplified, shaped and filtered, then processed by a high-speed processor into a standard current output that is linear with powder flow rate.
• Microwave solid flow meter: The sensor emits a 24 GHz, 0.01 W microwave signal into the pipe. Moving particles reflect it, and the Doppler shift between the transmitted and reflected frequencies gives the material flow and its velocity. It is designed for metal pipes.
• Impact plate flow meter: Works on the momentum principle. Free-falling material hits an inclined detection plate, and a differential transformer or strain gauge converts the horizontal impact force into an electrical signal. The meter shows instantaneous flow and the running total. Main parts are the detection plate, a pivoted beam and the signal converter, in a fully sealed housing that keeps dust in.
• Belt scale and loss-in-weight feeder: A belt scale weighs the material on a section of belt with load cells while a speed sensor reads belt speed; mass flow = load per metre × belt speed. A loss-in-weight feeder weighs the whole hopper and calculates flow from the weight drop over time.

Factors affecting powder flow measurement:

• Particle size and distribution: Particle size changes how much charge each particle carries and how evenly it spreads across the pipe, so the meter must be set up for the actual material.
• Moisture content: Moisture changes flowability and the charge level. Condensation inside the line must be avoided.
• Powder density: For the same volume, higher density powders will have a greater mass flow rate.

Key points about measuring solid flow rate:

• Conveying velocity:

Electrostatic and microwave meters respond to moving particles, so a steady air velocity gives a steady reading. Our meter works at 1–30 m/s.

• Particle size and shape:

The size and shape of the particles can affect the accuracy of the measurement, especially in velocity or impact based methods.

• Material density:

Knowing the density of the material is important for converting flow rate measurements to mass flow.

• Conveying regime:

Dilute-phase conveying, with particles suspended in air, suits electrostatic and microwave meters. Dense-phase plugs and free fall in chutes do not give a stable reading; use an impact plate or a weighing method there.

Field Case: Wood Flour in a 6-inch Pneumatic Conveying Pipeline

A Brazilian system integrator required in-line mass flow measurement for a DN150 pneumatic conveying pipeline transporting wood flour. The material had a particle size of less than 2 mm, a moisture content of 3.2%, and a bulk density of approximately 180 kg/m³. The conveying air velocity was about 5 m/s, with a flow rate range of 1,250–7,550 kg/h. We supplied a factory-calibrated DN150 flanged electrostatic solids flow meter with an accuracy of ±5% of full scale. During the selection process, we verified that the material moisture was sufficiently low to prevent condensation within the pipe and cross-checked the process pressure against the instrument’s rated pressure. Once commissioned, routine maintenance required only wiping the ring sensor during scheduled shutdowns.