Flow meter

01

Pick the measuring principle from the fluid

The US Bureau of Reclamation explains that a magnetic flowmeter senses the voltage induced in the flowing liquid, so the liquid needs sufficient conductivity; temperature, viscosity, density and solids do not change its calibration, but dissolved chemicals can deposit on the electrodes. It measures in both directions with negligible head loss, and the electrodes should be horizontal so air does not break the circuit. [2]

The US Department of Energy's metering guide describes two ultrasonic types: transit-time meters compare upstream and downstream signal times and are designed for clean fluids, while Doppler meters need bubbles or solids in the liquid to reflect the signal. Ultrasonic meters can be clamped outside the pipe, for temporary or permanent metering. [1]

Vortex meters sense the frequency of vortices shed by a bluff body in the stream, which is proportional to velocity; turbine meters count the rotation of an impeller. Positive-displacement meters need no straight run but have a higher pressure drop at peak flow. For gas, DOE notes that volume must be corrected to standard temperature and pressure, so meters may include temperature and pressure compensation. [1]

Variable-area meters (rotameters) read the height of a float in a tapered tube; Reclamation notes they have few moving parts but can have large head loss and must be installed vertically. [4]

Give the supplier the fluid, its conductivity (for magnetic meters), whether it carries bubbles or solids, viscosity, and for gas or steam the pressure and temperature at the meter.

02

Check how accuracy is stated and over what turndown

DOE notes that accuracy may be stated as a percentage of reading or a percentage of scale, and that turndown is the range of flows over which a meter keeps its stated accuracy and repeatability: a steam meter accurate from 1,000 to 25,000 lb/h has a 25:1 turndown. [1]

DOE's water metering table gives turndown figures of 10:1 for positive displacement and turbine meters, below 5:1 for orifice and venturi meters, 20:1 for vortex meters and 10:1 or 20:1 for Doppler or transit-time ultrasonic meters. [1]

As an editorial calculation example only: a meter stated at ±1% of full scale on a 100 m³/h range can be off by 1 m³/h at any flow, which is ±5% of reading at 20 m³/h. A meter stated at ±1% of reading stays at ±0.2 m³/h at that flow. Ask which basis the data sheet uses.

Reclamation reports that later magnetic meters can hold about ±1.0% from minimum to maximum discharge, and that propeller meters should be chosen to run near the middle of their range because bearing wear raises errors at low velocity. [2] [3]

03

Plan the straight pipe the meter needs

DOE explains that elbows, tees, filters, valves and fittings distort the flow profile, so velocity and differential-pressure meters need unobstructed straight pipe upstream and downstream, expressed in pipe diameters and in addition to the meter's own length. Flow conditioners can reduce it, and isolation valves make later removal easier. [1]

One manufacturer's engineering table gives conservative minimums: a vortex meter 25 diameters upstream and 5 downstream after a fully open gate valve, rising to 30 upstream after a partially open valve or regulator; a thermal meter 15 upstream, rising to 40; a turbine meter 10 upstream and 5 downstream. It states that positive-displacement and Coriolis meters are not influenced by upstream and downstream fittings. [6]

Reclamation adds that a fully developed velocity profile needs about 20 to 30 diameters, that spiral flow from bends and valves can cause large errors in either direction, and that the straight approach and straightening vanes used in calibration must be reproduced in the field. [3]

Send a sketch of the pipe with fittings and distances. If the run is short, ask the supplier which principle or flow conditioner suits it before choosing the size.

04

Plan inspection and recalibration

DOE recommends monthly checks for leaks, unusual noise, blocked strainers and cleanliness, and calibration as the manufacturer recommends or when trended data suggest drift. For ultrasonic meters it adds checking transducer position, coupling and pipe vibration. [1]

Reclamation notes that water measured with mechanical meters must be free of foreign matter and that meters should be inspected for wear and corrosion; some magnetic meters have wipers or electrode cleaners against deposits. [5] [2]

05

Sanitary magmeters for dairy lines, Coriolis and thermal meters for fuel and gas

BCST and Aister each cover magnetic, vortex, turbine, ultrasonic, Coriolis and thermal mass meters in one catalogue; Wepower's range runs from magnetic and turbine meters to oval-gear units. [7] [8] [9]

For food and beverage lines, Artang lists a 3-A sanitary electromagnetic flowmeter for milk and beverages, Sentec a sanitary food and beverage electromagnetic flowmeter, and Wepower hygienic tri-clamp magnetic meters. [10] [11] [9]

For fuels and gases, Bluesky lists dispenser flow meters and Coriolis mass meters, and Chang AI lists thermal mass flowmeters, which it states cover 0.1 to 250 Nm/s with a 1:2500 range ratio — a supplier claim to verify on the data sheet. [12] [13]

06

Sensor, tube, transmitter and the parts that change with principle

The sensor row is where the principles described above part ways; gas and steam meters add the temperature and pressure compensation DOE describes. [1]

07

Line conditions to send: size, flow range, accuracy basis, pressure, outputs

Size the meter to the flow rather than the pipe, and say whether accuracy is meant as percentage of reading or of scale; the earlier calculation shows how far the two drift apart at low flow. [3] [1]

08

Send the fluid, range and pipe data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Fluid: [liquid, gas or steam; conductivity; viscosity; bubbles or solids; corrosive or hygienic]
Flow: [minimum; normal; maximum; units; volume or mass]
Conditions: [pressure; temperature; standard reference conditions for gas]
Accuracy: [required accuracy; % of reading or % of scale; calibration certificate]
Pipe: [diameter; material; sketch with fittings and straight runs upstream and downstream; orientation]
Connection: [flange standard; tri-clamp; clamp-on; insertion]
Electrical: [power supply; outputs; protocol; compact or remote transmitter; IP rating; hazardous area]
Documents: [data sheet; calibration certificate; material and hygiene certificates]

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