Buying guide
Types of Flow Meters: Electromagnetic vs Vortex vs Ultrasonic (and How to Size One)
Flow meters are often bought by pipe size alone — and then read wrongly for years. This guide compares the main technologies used in Pakistani textile, sugar, food, pharma and water plants, and shows how to size a meter so it reads accurately across your real operating range.
Quick comparison
| Type | Works with | Typical accuracy | Strengths | Watch out for |
|---|---|---|---|---|
| Electromagnetic (mag) | Conductive liquids (water, effluent, slurries, chemicals) | ±0.2–0.5% of reading | No moving parts, no pressure drop, handles solids | Will not measure oil, gas, steam or demineralised water |
| Vortex | Steam, gases and clean liquids | ±0.75–1.5% | Excellent for saturated/superheated steam | Needs minimum velocity and long straight pipe runs |
| Ultrasonic (clamp-on) | Clean liquids | ±1–2% (installation dependent) | Fits outside the pipe, no shutdown | Sensitive to pipe condition and bubbles |
| Turbine | Clean, low-viscosity liquids and gases | ±0.5–1% | Low cost, good for fuel and batching | Moving parts wear; dirt damages bearings |
| Coriolis | Liquids and gases (mass flow) | ±0.1–0.2% | Direct mass flow and density | Higher cost and pressure drop |
Electromagnetic flow meters
A mag meter applies a magnetic field across the pipe; a conductive liquid flowing through it generates a voltage proportional to velocity (Faraday’s law). Because nothing protrudes into the flow, they’re the standard choice for water, wastewater, effluent and slurries. The liquid must be conductive — typically above 5 µS/cm — so they don’t work on oils, solvents or high-purity RO water.
- Liner – PTFE for chemicals and hot liquids, rubber (hard/soft) for water and abrasive slurries.
- Electrodes – SS316L for water; Hastelloy, titanium or tantalum for corrosive media.
- Grounding – use grounding rings or electrodes on plastic or lined pipes, or readings will drift.
- Full pipe – install where the pipe is always full (vertical upward flow is ideal).
Vortex flow meters
A bluff body in the pipe sheds vortices at a frequency proportional to velocity. Vortex meters are the workhorse for steam metering in boiler houses and process plants, and they also handle compressed air and gases. They need a minimum flow velocity to produce a stable signal, so oversizing is the classic mistake — and they generally need around 10–20 pipe diameters of straight run upstream.
Ultrasonic flow meters
Transit-time ultrasonic meters time sound pulses travelling with and against the flow. Clamp-on versions are ideal for surveys, energy audits and retrofits where you can’t cut the pipe. Accuracy depends heavily on knowing the pipe’s wall thickness and material and on good coupling, so treat them as a measurement tool rather than custody transfer unless inline.
How to size a flow meter (the step most people skip)
Size by flow velocity, not by pipe diameter. Most electromagnetic meters are most accurate at roughly 1–3 m/s and usable from about 0.3 to 10 m/s. If your pipe is oversized for the flow, choose a smaller meter with reducers so the velocity stays in range.
| Meter size | Flow at 1 m/s | Flow at 3 m/s |
|---|---|---|
| DN25 (1") | 1.8 m³/h | 5.3 m³/h |
| DN50 (2") | 7.1 m³/h | 21 m³/h |
| DN80 (3") | 18 m³/h | 54 m³/h |
| DN100 (4") | 28 m³/h | 85 m³/h |
| DN150 (6") | 64 m³/h | 191 m³/h |
| DN200 (8") | 113 m³/h | 339 m³/h |
Outputs and integration
- 4–20 mA (often with HART) for PLC/SCADA analogue input.
- Pulse/frequency output for totalising and batching.
- Modbus RTU (RS-485) for reading flow, totals and diagnostics over one cable.
- Display – local LCD for operators; remote-mount converters where the pipe is hot or hard to reach.
Selection checklist
- Fluid and its conductivity, temperature, pressure and solids content.
- Minimum, normal and maximum flow rate.
- Pipe size, material and available straight run.
- Required accuracy (process control vs billing).
- Wetted materials: liner and electrodes or body material.
- Output signal and power supply (24 V DC or 220 V AC).
- Ingress protection (IP67/IP68 for pits and outdoor lines).
Browse flow meters including Endress+Hauser, Yokogawa and Emerson, or send us your fluid, pipe size and flow range and we’ll recommend the right meter and size.
Frequently Asked Questions
Can an electromagnetic flow meter measure RO or demineralised water?
Usually not. Mag meters need a minimum conductivity (commonly 5 µS/cm, some 20 µS/cm). For RO or DM water, use an ultrasonic, vortex or Coriolis meter instead.
Which flow meter is best for steam?
Vortex meters are the most common choice for saturated and superheated steam. For accurate energy measurement, combine them with temperature and pressure compensation.
Should the flow meter be the same size as the pipe?
Not necessarily. Size the meter so the normal flow velocity sits in its accurate range (often 1–3 m/s for mag meters). If the pipe is oversized, a smaller meter with reducers gives better accuracy.
How much straight pipe does a flow meter need?
It depends on the technology and the upstream fittings. Mag meters typically need about 5 pipe diameters upstream and 2 downstream; vortex and ultrasonic meters need considerably more. Always follow the manufacturer’s installation guide.