Comparing flow metres shouldn't require a spreadsheet for each technology. This guide presents a comprehensive, side-by-side comparison of the eight primary flow metre technologies used in UK industry.
Use this chart to identify which technology fits your application. Then dive into the detailed discussion of strengths, limitations, and failure modes.
Technology Overview
The flow meter market includes eight primary technologies, each with distinct operating principles, accuracy ranges, and applications. Understanding these differences is critical to selecting the right metre for your specific process conditions.
Coriolis Mass Flow Metres
Accuracy: ±0.2%–0.5% across the operating range. This is the highest accuracy available in flow measurement.
Why it excels:
- Direct mass measurement – No temperature or pressure compensation required
- Multi-phase capable – Can measure gas-liquid mixtures directly
- Viscosity-independent – Works equally well with 1 cP water or 1,000 cP crude
- Density measurement – Simultaneously outputs fluid density for quality checks
- Repeatability – ±0.1% repeatability for regulatory custody transfer
When it fails:
- Large diameter (>6 inches) – Cost becomes prohibitive (£20k+)
- Pressure loss intolerance – 0.5–2.0 bar introduces significant back-pressure
- Pulsating flow – Air pockets or severe pulsation can cause spikes
- Thermal shock – Rapid temperature swings affect tube vibration frequency
Electromagnetic Flow Metres
Accuracy: ±0.5%–2.0%, depending on manufacturer and installation quality.
Why it excels:
- Zero pressure loss – <0.1 bar pressure differential
- Large-diameter cost-effective – Scales linearly with size
- Wide turndown – 20:1–40:1 enables seasonal load variation
- No moving parts – No erosion risk, minimal maintenance
- Bi-directional – Measures forward and reverse flow equally
When it fails:
- Non-conductive fluid – Cannot measure pure water, mineral oil, or solvents
- Electrode coating – In slurries, electrodes can corrode or build up deposits
- Flow profile disturbance – Excessive swirl or turbulence upstream degrades accuracy
- Reynolds number minimum – Below Re ~4,000, accuracy declines significantly
Vortex Shedding Flow Metres
Accuracy: ±1.0%–2.0% under ideal conditions.
Why it excels:
- Steam measurement – One of few suitable for saturated steam
- Cost-effective – £1.5k–£4k for most applications
- No moving parts – Obstruction is stationary; no wear
- Gas measurement – Works across wide density range
When it fails:
- Low Reynolds number (<4,000) – Vortex formation becomes unstable
- Viscous fluids – Viscosity > 5 cP suppresses vortex shedding
- Excessive upstream disturbance – Swirl or recent elbows cause instability
- Low-frequency vibration – Pipeline vibration can lock metre to vibration frequency
Ultrasonic Flow Metres
Accuracy: ±0.5%–2.0% (transit time); ±2%–5% (Doppler)
Why it excels:
- Exceptional turndown – 50:1–100:1, the best of any technology
- Clamp-on option – Non-invasive installation on external pipe
- Zero pressure loss – No obstruction; no back-pressure
- Gas measurement – Excellent for low-density fluids
When it fails:
- Opacity or particle loading – Doppler requires suspended particles
- Air bubbles – Even small bubbles scatter ultrasound, causing signal loss
- Refractory linings – Ceramic/composite linings attenuate ultrasound
- Temperature variation – Ultrasonic speed varies with temperature
Turbine Flow Metres
Accuracy: ±0.5%–1.0%, respectable for cost.
Why it excels:
- Cost-effective high accuracy – ±0.5% Coriolis costs £8k; ±0.8% turbine costs £2.5k
- Custody transfer approved – MID and SIL 2/3 certified
- Mechanical simplicity – Few moving parts, long service life (15+ years)
- Compact size – Fits in tight spaces
When it fails:
- Viscosity >5 cP – Rotor drag increases; accuracy drops significantly
- Slurries and abrasives – Rotor erosion changes blade geometry
- Low flow – Below ~10% of maximum range, rotor drag dominates
- Cavitation or water hammer – Shock waves damage rotor bearings
Positive Displacement Metres
Accuracy: ±0.2%–0.5% across entire range. Constant accuracy is its defining feature.
Why it excels:
- Billing and custody transfer – Constant accuracy across 100:1+ turndown
- Viscosity-friendly – Gear metres work with oils up to 500+ cP
- High-pressure capable – Many rated to 250+ bar
- Turndown excellence – 100:1 standard; 1,000:1 achievable
When it fails:
- Large diameter – Cost becomes prohibitive at 6+ inches
- Pressure loss – 1.0–5.0 bar typical, highest of all technologies
- Maintenance complexity – More moving parts than other technologies
- Compressible fluids – Sensitive to free gas; causes erratic readings
Differential Pressure Metres
Accuracy: ±1.0%–2.0% (Venturi); ±3%–5% (orifice plate)
Why it excels:
- Lowest capital cost – Orifice plate: £500–£2k
- Retrofit-friendly – Can be installed in existing pipelines
- Regulatory history – Centuries of use; regulatory bodies trust DP
- No moving parts – Zero mechanical complexity
When it fails:
- Poor turndown – 3:1–4:1 maximum
- Accuracy degradation at low flow – DP proportional to velocity²
- Pressure loss unrecoverable – 0.5–3.0 bar permanently lost
- Orifice degradation – Sharp edge erodes in slurries
Variable Area Metres
Accuracy: ±2%–5% depending on construction.
Why it excels:
- Cost – £200–£2k. Lowest capital cost of any technology
- Visual indication – Flow visible directly on tube; no electronics required
- Simple operation – No calibration, no setup
When it fails:
- Non-linear output – Float position not proportional to flow
- Viscosity-sensitive – Fluid viscosity changes shift float height
- Pressure-sensitive – Downstream pressure changes affect buoyancy
- Size-limited – Effective only up to ~10 litres/minute
When Each Technology Fails
Understanding failure modes is critical. These are not theoretical edge cases; these are real failures encountered in industrial practice.
- Non-conductive fluid (oil, pure water) – Electromagnetic only fails. No conductive path for electrode sensors.
- Multi-phase flow (gas-liquid) – All except Coriolis fail. Volumetric technologies measure voids as flow.
- Very low flow (10% of range) – Vortex and DP fail. Physics breaks down at low Reynolds number.
- High viscosity (>500 cP) – Turbine, vortex, electromagnetic fail. Flow dynamics change.
- Slurries (abrasive particles) – Turbine, PD, and orifice fail. Erosion of moving or sharp surfaces.
Next Steps: Which Metre Is Right for You?
This chart gives you the landscape. To narrow down to a specific metre:
- Eliminate by fluid type – Non-conductive? Coriolis or vortex only.
- Eliminate by accuracy – Need ±0.2%? Coriolis, turbine, or PD only.
- Eliminate by cost – Budget £2k? Electromagnetic or vortex most likely.
- Evaluate remaining candidates – Get datasheets, check installation constraints.
- Verify with manufacturer – Confirm your specific fluid/flow combination works.