FAQ: Partially Filled Pipe Flow Meter – Selection Guide | RB Flowmeter

FAQ: Partially Filled Pipe Flow Meter – Selection Guide | RB Flowmeter

A Complete Technical Guide for Engineers & Plant Managers

Based on RBEF‑P Series – Dedicated Partially Filled Pipe Electromagnetic Flow Meter

Technical Application Guide – 2025 Edition

1. Do You Really Need a “Partially Filled” Flow Meter?

In many industrial applications — wastewater treatment, stormwater drainage, irrigation, industrial discharge — pipelines often run partially full. Conventional electromagnetic flow meters require full pipe conditions to provide stable, accurate readings. When the pipe is not full, they may fail completely or produce erratic data.

✅ If your pipe operates with a free liquid surface, gravity flow or intermittent discharge, a standard magmeter will not work reliably. You need a dedicated partially filled pipe flow meter.

2. How Does a Partially Filled Pipe Flow Meter Work?

It combines two independent measurements in one instrument:

  • Flow velocity – using Faraday’s Law of electromagnetic induction (same as a standard magmeter).
  • Liquid level – typically an ultrasonic level sensor that measures the height (h) of the fluid in the pipe.

The actual flow rate is then calculated as:

Q = Av × Vh

Where: Av = actual cross‑sectional area of the fluid (from h), and Vh = average flow velocity.

✅ A partially filled pipe flow meter = Electromagnetic velocity sensor + Ultrasonic level sensor + Smart converter

3. Key Technical Specifications to Check

When evaluating a model, always verify these critical parameters (based on RBEF‑P series):

ParameterTypical Range / RequirementSelection Tip
Pipe diameter (DN) DN100 – DN1200 (4″–48″) Smaller diameters (
Liquid level range 10% – 100% of pipe cross‑section Below 10% accuracy may decrease
Flow accuracy (partially full) ±2.5% FS Full pipe mode can reach ±0.5%
Fluid conductivity ≥50 μS/cm (standard) Low conductivity → use special signal cables
Ambient temperature -10°C to +55°C High temperature media require special liners
Straight pipe run Upstream ≥10D, Downstream ≥5D Critical for stable velocity profile
Level input signal 4–20 mA (two‑wire) Meter provides 24V power for level sensor
Outputs 4‑20mA, pulse, RS485 (MODBUS), alarms Match with your control system
Grounding resistance ≤10Ω Essential to avoid EMI

4. Installation Rules That Make or Break Performance

Poor installation is the #1 cause of measurement errors. Follow these five rules:

  • 1. Always keep the metering section wetted – use a downward slope or a U‑shaped wet trap. Avoid high points or free vertical discharge.
  • 2. Mount electrodes horizontally – electrodes must be on the horizontal axis to avoid bubbles or sediment interfering with the signal.
  • 3. Prevent negative pressure – if the pipe drops more than 5 metres after the meter, install a vacuum breaker.
  • 4. Never install on the pump suction side – always install downstream of the pump.
  • 5. Separate the converter in high‑vibration areas – use a remote‑mount converter and flexible pipe connections.
⚠️ Ignoring these rules leads to unstable readings, empty pipe alarms, and premature liner damage.

5. Typical Applications & Selection Examples

ApplicationRecommended DNKey Concern
Wastewater inlet DN300–DN1000 Fast level changes, debris
Stormwater drainage DN400–DN1200 Low conductivity, risk of submersion
Agricultural irrigation DN150–DN500 Battery option often preferred
Industrial discharge DN100–DN600 Corrosive media → ceramic liner

6. Common Mistakes to Avoid

❌ Mistake✅ Correct approach
Using any magmeter in partially filled pipes Choose a dedicated partially filled pipe flow meter
Ignoring the 10% minimum level requirement Keep level >10% of diameter
Placing the ultrasonic level sensor arbitrarily Aim perpendicular to liquid surface; avoid blind zone (3 cm)
Skipping grounding Always ground to ≤10Ω
Setting parameters once and forgetting Re‑calibrate zero, damping, and empty‑pipe alarm on site

7. Final Selection Checklist (3 Steps)

  • Step 1 – Define your conditions
    Is the pipe partially full for most of the time? Expected level range (10%–100%)? Conductivity, temperature, abrasiveness?
  • Step 2 – Match the meter
    DN100–DN1200 → Suitable partially filled magmeter (e.g. RBEF‑P series). Smaller or special materials → consult factory.
  • Step 3 – Plan the installation
    Straight pipe runs, horizontal electrodes, no negative pressure, proper grounding. Remote mount if vibration is present.
✅ A correctly selected and installed partially filled pipe flow meter eliminates “no signal” or “unstable reading” problems and significantly reduces maintenance costs.

8. Why RBEF‑P Series?

  • Dedicated for partially filled circular pipes (DN100–DN1200)
  • Ultrasonic level sensor with 3 cm blind zone, 4‑20mA output
  • Electromagnetic velocity measurement unaffected by debris, sediment or bubbles
  • RS485 MODBUS, pulse, alarm outputs
  • Empty pipe detection & self‑diagnostics
  • IP65 converter, remote mount available for harsh environments

Need more help?

Every application is different. For custom pipe sizes, special liners, or open‑channel requirements, contact our technical team.

RB Flowmeter
Website: www.rbflowmeters.com
Email: info@rbflowmeters.com rbflowmeters@gmail.com

© RB Flowmeter – Technical Application Guide. Information based on RBEF‑P Series operating manual.
All specifications subject to change without notice. For the latest version, visit www.rbflowmeters.com