Electromagnetic flowmeters (mag meters) are the workhorse of liquid flow measurement in water, wastewater, chemical, food and mining applications. They are also one of the easiest instrument families to misselect: a liner or electrode that is wrong for the medium, an oversized meter or a missing approval can turn a routine purchase into plant downtime. This guide walks through the selection decisions in the order we recommend making them – medium, conductivity, liner, electrode, sizing, output and approvals – and ends with a Promag series comparison for quick shortlisting. It complements our radar level transmitter selection guide and differential pressure transmitter selection & replacement guide.
Where Electromagnetic Flowmeters Work – and Where They Do Not
An electromagnetic flowmeter measures flow using Faraday’s law of electromagnetic induction: as a conductive liquid moves through the meter’s magnetic field, a voltage proportional to the flow velocity is induced and picked up by two electrodes. The method has no moving parts and nothing obstructing the pipe, so it causes virtually no pressure loss and tolerates dirty, slurry-laden media that would jam mechanical meters. A solid technical overview of the principle is available in Wikipedia’s magnetic flow meter article.
The same principle defines the hard limit: the medium must be electrically conductive. Water, wastewater, brines, acids, alkalis, fruit juice, milk and pulp slurries are all fine. Hydrocarbons, gases, steam and most solvents are not measurable, because they do not conduct. Ultrapure water sits at the borderline and needs a meter explicitly rated for very low conductivity.
Step 1 – Confirm Media Conductivity
Every Promag meter has a minimum conductivity requirement, typically around 5 µS/cm for standard versions. Three practical rules:
- Ask for the conductivity of the medium as it actually runs in the pipe – not just the name of the chemical. A “chemical” that is 95 % water is usually a non-issue; a concentrated acid can be less conductive than expected.
- Watch for multiphase media. A slurry that temporarily separates (settling sand, entrained air) can drop below the measurable threshold and cause signal spikes.
- If conductivity is unknown, send us the process description – we check it against the manufacturer data before quoting.
Step 2 – Select the Liner Material
The liner insulates the pipe wall and faces the medium continuously; it is usually the first part to age. Choose by corrosivity, abrasiveness and temperature:
| Liner | Best for | Watch out for |
|---|---|---|
| Hard rubber | Water, wastewater, dilute chemicals; lowest cost | Limited temperature range; not for solvents |
| Polyurethane | Abrasive slurries, sand-laden water, mining duty | Not for strong acids/alkalis or high temperature |
| PTFE | Chemicals, high temperature, sticky media | Permeation with special media; careful flange torquing |
| PFA | Chemicals plus higher temperature and vacuum ratings | Higher cost |
For replacement projects the fastest route is the original order code – it names the liner and electrode exactly as built. If the nameplate is unreadable, our E+H order code guide explains where to look and how to decode it.
Step 3 – Select the Electrode Material
The electrodes sit in the medium and must resist corrosion and abrasion for years:
- 316L stainless steel – the default for water, wastewater and many neutral media.
- Hastelloy C – acids, chlorides and oxidizing media; the common upgrade for chemical duty.
- Tantalum – strong acids such as hydrochloric, where Hastelloy still corrodes.
- Platinum-iridium – special and hygienic duties; the highest-cost option.
Manufacturer corrosion tables are the reference. When two materials both work, prefer the one with a proven service history in the same industry.
Step 4 – Size the Meter, Not Just the Pipe
Mag meters tolerate a wide velocity range, but accuracy and wear both depend on velocity. The practical band is roughly 1-3 m/s: below about 0.3 m/s signal quality drops, and sustained high velocities accelerate liner and electrode wear in abrasive media.
The classic mistake is ordering DN = pipe size even when the flow range would suit a smaller meter. Reducing the meter size raises velocity into the accurate band and often lowers cost – a DN100 line carrying 15 m³/h is measurably better served by a DN80 meter. Installation is forgiving by mag-meter standards: only short straight runs are needed (typically 3 DN upstream, 2 DN downstream), but the pipe must stay full. For gravity or part-full lines, choose a water series with empty-pipe detection such as the Promag W 300.
Step 5 – Output, Communication and Diagnostics
Standard output is 4-20 mA with HART, plus pulse/frequency and status contacts; Modbus and PROFIBUS are available for PLC-centric plants. HART remains the default for handheld configuration and asset documentation – the protocol is maintained by the FieldComm Group. In-situ diagnostics are worth paying for in modern plants: the Promag 400 and Promag 500 lines include Heartbeat Technology, which verifies meter health without interrupting the process or pulling the meter for calibration.
Step 6 – Housing, Installation and Approvals
- Compact vs remote: compact is simpler; remote (separate sensor and transmitter) is right for hot pipes, vibration, flooded chambers or hard-to-reach locations.
- Protection class: IP67 covers outdoor and washdown duty; IP68 options exist for chambers with flood risk.
- Hazardous areas: ATEX/IECEx certificates must match the zone. Never swap an Ex-certified device for a general-purpose unit – the same warning we give in the differential pressure transmitter guide.
- Grounding: grounding rings or lining protectors are mandatory with unlined or cathodically protected pipes – the most common installation omission we see.
Five Selection Mistakes That Cost Money Later
- Selecting by pipe size instead of flow range.
- Ignoring the minimum conductivity of borderline media such as demineralized water or nearly-pure products.
- Choosing the liner by price on abrasive slurries – polyurethane outlives rubber several times over there.
- Skipping grounding rings, then chasing noise-related accuracy complaints.
- Ordering by model name instead of order code – two meters with the same short name can differ in every wetted part.
Promag Series Comparison for Quick Shortlisting
The table maps the current Promag families to their typical use. All links go to the product pages on this site; the complete range is in our electromagnetic flowmeter category, and official configurations and datasheets are published by Endress+Hauser on endress.com.
| Series | Positioning | Typical use |
|---|---|---|
| Promag 10 W | Entry level, water version | Water supply, irrigation, utilities |
| Promag W 300 | Water & wastewater standard | Municipal water, sewage, part-full detection |
| Promag W 500 | High-end water version | Demanding water duty, custody-relevant points |
| Promag P 300 | Chemical process standard | Acids, alkalis, process chemicals (PTFE/PFA liners) |
| Promag H 100 | Hygienic, compact | Food, beverage, pharma; hygienic approvals |
| Promag D 400 | Wafer version, advanced | Space-saving installations with Heartbeat verification |
| Promag 400 | Advanced, Heartbeat Technology | Process industries with in-situ verification needs |
| Promag 500 | High-end | Broadest diagnostics, difficult applications |
Legacy meters (Promag 50, 53, E 100 and earlier generations) remain common in plants; we supply current equivalents matched via order code – see our model number guide for how the codes map.
How We Help With Selection and Replacement
We specialize in Endress+Hauser instrument supply for international buyers. Send us the order code or a nameplate photo and we confirm the configuration, check availability and return a quotation with lead time – typically within one working day. Start at the Request Quotation page, or use Technical Support if you first need help identifying an installed meter. For other measurement points, our radar level transmitter guide and differential pressure transmitter guide follow the same selection logic, and the full range is browsable under flow measurement.
Frequently Asked Questions
What is the minimum conductivity for an electromagnetic flowmeter?
Most Promag meters measure media from about 5 µS/cm upward, and selected versions go lower. Below the threshold the induced signal becomes unreliable and a different measuring principle should be chosen for that line.
Can a mag meter measure distilled or demineralized water?
Usually not reliably – the conductivity is too low. Choose a meter explicitly rated for very low conductivity, or a different principle for that application.
How much straight pipe does a mag meter need?
Very little compared with mechanical or differential-pressure meters: typically 3 DN upstream and 2 DN downstream. Keep elbows and valves further away where the layout allows.
Why does my installed mag meter show zero or a noisy signal?
The usual causes are an empty pipe, air entrainment, conductivity below the meter’s limit, or missing grounding rings that let noise onto the electrodes. Check fullness and grounding first, then verify the configuration against the order code.
Endress+Hauser, Promag and Heartbeat Technology are trademarks of their respective owner. Minimum conductivity limits, material compatibility and approvals vary by exact order code and must be confirmed against the latest official documentation. For manufacturer data, visit endress.com.