Choosing flow meters for water, oils, chemicals, gases and steam

01 / Start with the fluid

01 / Start with the fluid
Fluid / service	Starting point	Benefit and boundary
Conductive water / cooling water / chemicals	630 electromagnetic	Continuous measurement without a rotating impeller; requires a full pipe, adequate conductivity and compatible liner/electrodes.
Clean low-viscosity liquids / gases; local reading	616 metal-tube variable-area	Direct local scale; density and gas pressure/temperature must match calibration conditions.
Clean liquid with visible flow behavior	643 glass-tube variable-area	Visible float and flow; check tube and wetted-material compatibility and pressure/temperature limits.
Clean oils / more viscous liquids; totals	624A oval gear / 624B Roots	Positive-displacement options for oil metering; check particles, wear, pressure loss and viscosity range, with suitable filtration.
Clean low-viscosity liquids	624C turbine	Evaluate instantaneous and total flow; bearings, cleanliness, viscosity and flow profile matter. The high-viscosity uses of 624A/B do not apply automatically.
Machine cooling-water monitoring	600 paddlewheel sensor	Pair with a compatible display/transmitter; check diameter, velocity, installation and impeller wear.
Gas / steam / differential-pressure installations	680 / 681 / 682 orifice elements	Needs calculation, differential-pressure measurement, pressure taps and any required compensation; includes permanent pressure loss.
Cooling-flow presence only	649 paddle flow switch	Switching supervision, not a precise flow value or a complete safety interlock.

02 / Compare technologies beyond the accuracy headline

02 / Compare technologies beyond the accuracy headline
Principle	Reason to consider it	Conditions needing another approach
Electromagnetic	Continuous conductive-liquid measurement without moving measuring parts.	Not for ordinary oils, gas or steam; deal with bubbles, partially filled pipes, grounding and electrode coating.
Variable-area	Simple local reading with configurations for clean liquids or gases.	Scales are fluid/condition-specific; check orientation, sticking and pulsation.
Positive displacement	Counts repeated known volumes; useful to evaluate for viscous oils.	Particles, two-phase flow, excessive viscosity or unacceptable pressure loss may rule it out.
Turbine / paddlewheel	Monitoring clean low-viscosity flow with a suitable display or control output.	Moving parts are affected by abrasive particles, fibres, deposits, low velocity and disturbed profiles.
Orifice / differential pressure	Can be engineered for liquid, gas or steam and integrated with differential-pressure systems.	Requires calculation and supporting instruments; assess low-flow sensitivity, straight runs and permanent loss.
Coriolis mass flow (general comparison)	Direct mass flow, with density measurement, for batching and mass-based metering.	Assess cost, pressure loss, pipe stress and entrained gas. Talk to a specialist
Clamp-on ultrasonic (general comparison)	Evaluate flow without opening the pipe, for checks or retrofits on existing water lines.	Pipe material, wall thickness, lining, full-pipe conditions and acoustic coupling matter. Talk to a specialist
Vortex (general comparison)	An option for steam, gas or liquid flow; steam duties often also need pressure/temperature information.	Check low velocity, vibration, wet steam and straight runs. Talk to a specialist

03 / Four common selection traps

Using electromagnetic measurement for oil: ordinary oils lack the required conductivity. Assess 624A/B positive displacement, 624C turbine or an appropriate variable-area configuration by viscosity and cleanliness.

Specifying gas only in L/min: state whether it is actual volume at operating conditions or volume referenced to specified standard conditions. Gas volume changes with pressure and temperature; variable-area calibration must match the intended conditions.

Selecting wastewater by conductivity alone: also assess solids, fibres, bubbles, liner/electrode fouling and a consistently full pipe. Paddlewheel and float instruments need additional attention to entanglement and sticking.

Providing only average flow at a metering-pump discharge: pulsation can produce float oscillation, unstable readings or contact chatter. Specify the pump, damping and pulsation conditions so response and mounting can be assessed.

04 / Use the flow range to assess velocity and size

Minimum measurable flow can matter more than maximum capacity. List startup, normal, low-load and maximum duties, and distinguish a low-flow reading from a total or a no-flow alarm.

The tool relates actual internal diameter, full-pipe mean velocity and volume flow. It is not a model-selection program. Verify allowable velocity, range, pressure loss and accuracy for the actual instrument and service.

DOERS ENGINEERING / FLOW

Explore diameter, velocity and flow

D Actual internal diameter, not nominal DN · v Cross-sectional mean velocity

Assumes full-pipe, circular, single-phase flow. Q(m³/h) = π/4 × [D(mm)/1000]² × v(m/s) × 3600. No gas reference conversion, pressure-loss or instrument-error calculation; this value alone cannot select a meter.

05 / Installation conditions and inquiry data

Provide pipe orientation, full-pipe status, upstream pumps/bends/control valves, available straight runs and acceptable pressure loss. Variable-area mounting requirements differ by model. Check electromagnetic grounding/lining and differential-pressure impulse-line/transmitter arrangements.

For visual confirmation only, see the 644 sight-flow indicator. For a contact, see 645 / 649 switches. For values or totals, specify a meter and its display/output. These are different tasks.

Include fluid composition, liquid density/viscosity or gas reference conditions, solids/bubbles, minimum/normal/maximum flow, operating/design pressure and temperature, pipe size/material, layout, supply/output and measurement purpose.

DOCUMENTS & NEXT STEPS

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