Both electromagnetic and ultrasonic flow meters are popular choices in industrial applications — and for good reason. Neither has moving parts. Both deliver high accuracy. And both are well suited to a range of water and process fluid applications.

But they work very differently, and the right choice depends on factors that aren’t always obvious from a spec sheet. This guide explains how each technology works, where each excels, and how to make the right call for your process.

How Electromagnetic Flow Meters Work

An electromagnetic flow meter — also called a magmeter or mag flow meter — uses Faraday’s law of electromagnetic induction. Two electrodes sit inside the pipe, and a magnetic field is applied across the fluid. As the conductive fluid flows through the field, it generates a small voltage proportional to the flow velocity. The meter measures that voltage and converts it to a flow rate.

The key requirement: the fluid must be electrically conductive. This makes electromagnetic meters ideal for water, wastewater, slurries and most aqueous solutions — but they cannot measure non-conductive fluids like hydrocarbons or pure water.

Electromagnetic flow meters are available in full-bore designs for permanent pipe installation, with liner materials (rubber, PTFE, ceramic) suited to different fluid chemistries.

How Ultrasonic Flow Meters Work

Ultrasonic flow meters use sound waves to measure flow velocity. Two transducers send ultrasonic pulses through the fluid — one upstream, one downstream. The difference in transit time between the pulses is proportional to flow velocity.

The key advantage: clamp-on ultrasonic meters attach to the outside of the pipe. No pipe cutting. No process shutdown. No wetted parts. This makes them ideal for retrofit installations where interrupting flow isn’t practical.

Ultrasonic flow meters work on both conductive and non-conductive fluids, making them more versatile in terms of fluid compatibility.

Side-by-Side Comparison

FeatureElectromagneticUltrasonic
Operating principleElectromagnetic inductionTransit-time ultrasonic
Fluid requirementConductive only (≥5 µS/cm)Conductive or non-conductive
Moving partsNoneNone
Accuracy (typical)±0.2–0.5%±0.5–2% clamp-on; ±0.5–1% inline
InstallationFull-bore, pipe cutting requiredClamp-on (no cutting) or inline
Pipe size rangeDN6 to DN3000+DN15 to DN3000+
Solids toleranceHandles slurries and suspended solidsAccuracy reduces with aeration or solids
MaintenanceVery lowVery low
Relative costModerate to highClamp-on: lower; Inline: comparable

Fluid Compatibility — The Most Important Difference

Electromagnetic meters require electrically conductive fluid — minimum conductivity of around 5 µS/cm. Most industrial water and wastewater applications meet this easily. However, they cannot be used with:

  • Hydrocarbons (oils, fuels)
  • Deionised or ultrapure water
  • Gases

Ultrasonic meters are far less restricted — they work on virtually any fluid including non-conductive liquids. If you’re measuring a non-conductive liquid, ultrasonic is the only non-mechanical option.

For standard industrial water, potable water, or wastewater, both technologies are compatible — and the selection decision shifts to installation, accuracy and cost.

Accuracy

  • Electromagnetic meters typically achieve ±0.2–0.5% of reading across a wide flow range
  • Ultrasonic inline meters typically achieve ±0.5–1% of reading
  • Clamp-on ultrasonic meters are typically ±1–2% — adequate for monitoring, not custody transfer

For billing, custody transfer or tight process control, electromagnetic is the preferred choice where fluid conductivity permits.

Installation — The Retrofit Advantage

Electromagnetic installation requires cutting the pipe, flanging and process shutdown — standard for a new build or planned maintenance window, but disruptive on a live system.

Clamp-on ultrasonic installation is non-invasive: attach transducers to the outside of an existing pipe and start measuring — sometimes in under an hour, with zero process interruption.

If retrofitting onto an existing system without a planned shutdown, clamp-on ultrasonic is almost always the fastest path.

Maintenance

Both are low-maintenance compared to turbine or positive displacement flow meters — no moving parts to wear or replace.

The main consideration for electromagnetic meters is electrode fouling in high-solids or scaling applications — routine cleaning resolves this in most cases. Clamp-on ultrasonic transducers require periodic checking of the coupling gel and are sensitive to pipe surface condition.

Application Scenarios — When to Choose Each

Choose electromagnetic when:

  • Your fluid is conductive (water, wastewater, slurry, aqueous solutions)
  • You need highest accuracy — custody transfer, billing, tight process control
  • You’re measuring fluid with suspended solids
  • The installation is permanent and pipe access is available
  • You’re specifying Siemens flow meters for a Siemens-integrated system

Choose ultrasonic when:

  • Your fluid is non-conductive (hydrocarbons, pure water, gases)
  • You need a retrofit solution with no process shutdown
  • Pipe is large diameter (DN200+) where clamp-on is more cost-effective
  • Portability or temporary measurement is required
  • You want a digital flow meter solution with data logging capability

BES Flowmeters Supplies Both Technologies

BES Flowmeters is an Australian specialist in industrial flow measurement. We stock electromagnetic and ultrasonic flow meters across a full range of pipe sizes and process conditions — and can assist with technology selection, specification and supply.

Our electromagnetic range includes the Siemens FMS500 and FMT020. Our ultrasonic range covers clamp-on and inline designs from monitoring through to high-accuracy measurement.

→ Get a Quote or Discuss Your Application

FAQs

Q1: What is the main difference between electromagnetic and ultrasonic flow meters?

Electromagnetic flow meters use Faraday’s law — they require an electrically conductive fluid and measure from inside the pipe. Ultrasonic flow meters use transit-time sound waves — they work on conductive and non-conductive fluids, and clamp-on designs measure from outside the pipe with no cutting required. Both have no moving parts and offer low maintenance.

Q2: Can an ultrasonic flow meter measure non-conductive liquids?

Yes — this is one of the key advantages of ultrasonic technology. Ultrasonic flow meters work on both conductive and non-conductive fluids, including hydrocarbons, oils and deionised water. Electromagnetic flow meters require a minimum conductivity of approximately 5 µS/cm and cannot measure non-conductive liquids. If your fluid is non-conductive, ultrasonic is the correct choice.

Q3: Which flow meter type requires less maintenance — electromagnetic or ultrasonic?

Both are low-maintenance with no moving parts. Electromagnetic meters occasionally require electrode cleaning in high-solids or scaling applications. Clamp-on ultrasonic meters require periodic transducer coupling checks. Inline ultrasonic meters have virtually no maintenance requirement. Both types require significantly less maintenance than mechanical flow meters.

Q4: Are clamp-on ultrasonic flow meters as accurate as electromagnetic flow meters?

Generally, no. Clamp-on ultrasonic meters typically achieve ±1–2% accuracy, adequate for monitoring but below the ±0.2–0.5% typical of electromagnetic meters. Inline ultrasonic meters achieve ±0.5–1% — closer to electromagnetic performance. For custody transfer, billing or tight process control, electromagnetic meters are the preferred choice where fluid conductivity permits.

Q5: Which type of flow meter is better for retrofitting onto existing pipework?

Clamp-on ultrasonic meters are purpose-designed for retrofit. They attach to the outside of an existing pipe — no cutting, no flanging, no process shutdown — and installation can often be completed in under an hour. Electromagnetic meters require pipe cutting and process shutdown, making them better suited to new builds or planned maintenance windows.