# Magnetic and Ultrasonic Flowmeters: Selection and Application > Between the extremes of Coriolis accuracy and orifice simplicity sits the workhorse pair of modern flow measurement: magnetic flowmeters (magmeters) for… - Canonical URL: https://wiki.aspotomasyon.com/article/magnetic-ultrasonic-flowmeters - Publisher: ASP OTOMASYON A.Ş. (ASP OTOMASYON Wiki) - Category: Field Devices & Instrumentation - Published: 2026-08-11 · Updated: 2026-09-13 - Reading time: 4 min · Language: English Between the extremes of Coriolis accuracy and orifice simplicity sits the workhorse pair of modern flow measurement: **magnetic flowmeters (magmeters)** for conductive liquids and **ultrasonic flowmeters** for clean liquids, gases, and the applications where the pipe cannot be cut. Together they cover the majority of industrial flow applications — and the selection between them (and against the alternatives) is decided by the fluid's conductivity, cleanliness, and the installation constraints. ![A measurement chain from the process to an application. Calibration, loop checks, and process validation answer different questions; retain evidence for each applicable check.](https://wiki.aspotomasyon.com/assets/diagrams/field-devices-instrumentation-context.svg) *A measurement chain from the process to an application. Calibration, loop checks, and process validation answer different questions; retain evidence for each applicable check.* ## Magnetic Flowmeters A magmeter measures the voltage induced when a conductive liquid flows through a magnetic field (Faraday's law): the voltage is proportional to the average velocity. The measurement is: - **Independent of density, viscosity, and temperature** — no Reynolds or property corrections; the meter is essentially linear. - **Unobstructed** — no moving parts, no pressure drop beyond the pipe itself; the full-bore design handles slurries and solids (with the right liner). - **Limited to conductive liquids** — the fluid needs a minimum conductivity (typically ≥ 5 µS/cm, lower for modern meters); hydrocarbons, pure water (borderline), and gases are out. - **Liner and electrode dependent** — the liner (rubber, PTFE, PFA) matches the chemical and abrasive service; the electrodes match the fluid's chemistry; grounding requirements are strict (the meter needs the electrical potential reference). The magmeter is the default for water, wastewater, pulp slurries, acids, and most process liquids — accuracy typically 0.2–0.5% of rate with the right sizing. ## Ultrasonic Flowmeters Ultrasonic meters come in two families: - **Transit-time** — the difference in travel time of sound pulses with and against the flow gives the velocity; for clean liquids and gases, with clamp-on (no pipe cutting!) and inline versions. Accuracy depends on the flow profile and the installation (straight runs), typically 0.5–2% for good installations. - **Doppler** — the frequency shift of sound reflected off particles/bubbles gives the velocity; for dirty liquids with entrained solids or gas (wastewater, slurries); lower accuracy, the right tool for trending, not custody. The ultrasonic strengths: no pressure drop, no moving parts, clamp-on installation on existing pipes (the meter that does not require a shutdown), and bidirectional measurement. The weaknesses: sensitivity to the flow profile (straight-run requirements, disturbed flows), to pipe condition (scale, lining, poor acoustics), and the measurement is velocity-based — the volume needs the internal pipe area (and its uncertainty). ## The Selection Decision | Service | First choice | Notes | | --- | --- | --- | | Conductive liquids, clean or dirty (water, acids, slurries) | Magnetic | Accuracy, no obstruction, slurry-capable with liner selection. | | Non-conductive clean liquids (hydrocarbons, pure products) | Ultrasonic (transit-time) or Coriolis | Magmeter not applicable; transit-time for economics, Coriolis for accuracy. | | Existing pipe, no shutdown, no line cut | Clamp-on ultrasonic | The only non-invasive option; accept the accuracy limits. | | Gases | Ultrasonic (transit-time), thermal, or DP | Ultrasonic for clean gases at moderate conditions; the flow comparison article covers the full field. | | Dirty liquids with solids/gas | Magnetic (if conductive) or Doppler ultrasonic | Magmeter first; Doppler for non-conductive dirty streams. | | Custody transfer | Coriolis or certified turbine/ultrasonic | The accuracy and approvals decide; see the Coriolis article. | ## Installation Engineering - **Magmeter** — the pipe must run full (avoid partially filled pipes — mounting in a rising line or at a low point), straight runs upstream/downstream per the manufacturer (5D/2D typical, more for disturbed flows), proper grounding (the electrodes need the fluid's electrical reference — the grounding rings and the earth connection are mandatory, not optional), and the liner/electrode selection documented against the fluid. - **Ultrasonic (transit-time)** — straight runs matter most (10D upstream / 5D downstream typical, more where pumps or valves disturb the profile), the transducer placement at the correct axial position and pipe diameter measurement, coupling compound quality, and — for clamp-on — the pipe material and wall condition (a scale-lined pipe measures the scale, not the flow). ## Diagnostics and Maintenance Both technologies offer strong self-diagnostics: the magmeter reports electrode fouling, empty-pipe condition, and coil/electrode faults; the ultrasonic meter reports signal strength, signal-to-noise, and profile quality — each flagging the installation drift (electrode coating, transducer degradation, flow profile change) before the reading goes wrong. The trend of the diagnostic values is the maintenance plan: rising electrode resistance means cleaning; falling ultrasonic signal strength means re-coupling or pipe scale. The meters are low-maintenance; their diagnostics are the maintenance. ## Summary Magmeters serve the conductive-liquid world with unobstructed accuracy; ultrasonic meters serve the clean-fluid and non-invasive world with clamp-on flexibility. Select by conductivity, cleanliness, and installation constraints; engineer the installation (full pipe, straight runs, grounding, acoustics); and operate on the diagnostics. For the majority of plant liquids, one of these two is the right answer — the comparison article covers the rest of the field. ## Apply this to your project - Confirm the instrument tag, range, engineering unit, process connection, and environmental suitability against approved documents. - Check the complete measurement chain, including sensor configuration, channel scaling, quality indication, and display units. - Record reference equipment, uncertainty where relevant, test points, deviations, and the final configuration. [Follow an ASP project guide](https://wiki.aspotomasyon.com/guides) to connect the fundamentals with implementation and acceptance. ![Engineering review sequence: Specify → Inspect → Compare → Record. Keep the recorded evidence with the accepted configuration.](https://wiki.aspotomasyon.com/assets/diagrams/field-devices-instrumentation-review.svg) *Engineering review sequence: Specify → Inspect → Compare → Record. Keep the recorded evidence with the accepted configuration.* ## Primary references and further reading Use the original specifications and product documentation for implementation details. The examples in this guide are illustrative engineering scenarios, not published project results. - [FieldComm Group — HART technology](https://www.fieldcommgroup.org/technologies/hart) - [NIST — Policy on metrological traceability](https://www.nist.gov/calibrations/traceability) --- Cite the canonical article: https://wiki.aspotomasyon.com/article/magnetic-ultrasonic-flowmeters (Magnetic and Ultrasonic Flowmeters: Selection and Application, ASP OTOMASYON, updated 2026-09-13).