Flow calibration
Flow meters are checked against a reference that measures the same fluid: a weighed tank, a master meter or a prover. Fluid properties and the pipe around the meter matter as much as the meter.
Instruments and typical accuracy
| Instrument | Typical range | Typical accuracy | Where it is used |
|---|---|---|---|
| Coriolis mass flow meter | Wide, mass flow | ±0.05 to ±0.2 % of reading | Custody transfer, batching, chemicals |
| Electromagnetic meter | Conductive liquids | ±0.2 to ±0.5 % of reading | Water, wastewater, slurries |
| Ultrasonic, in-line | Liquids and gases | ±0.5 to ±1 % of reading | Water, oil and gas pipelines |
| Ultrasonic, clamp-on | Liquids in pipes | ±1 to ±3 % of reading | Checks without cutting the pipe |
| Turbine meter | Clean low-viscosity fluids | ±0.25 to ±1 % of reading | Fuel, water, gas |
| Vortex meter | Steam, gas and liquid | ±1 to ±1.5 % of reading | Process plants, steam |
| Differential pressure (orifice) | Many sizes | ±1 to ±2 % of reading | Gas and steam, with ISO 5167 |
| Positive displacement meter | High viscosity liquids | ±0.1 to ±0.5 % of reading | Oil, fuel, dispensing |
| Thermal mass or rotameter | Gas, low flow | ±1 to ±5 % (rotameters of full scale) | Gas dosing and indication |
Accuracy usually degrades at low flow: check the turndown ratio in the specification. The accuracy figures are typical ranges to help you compare instruments. Always use the specification of the instrument in front of you.
Reference standards and traceability
- Gravimetric method. Collect the liquid in a weighed tank for a measured time. Traceable to mass and time, with density from temperature. ISO 4185 describes it for liquids.
- Master meters and provers. A calibrated reference meter or a piston or small-volume prover in series with the meter under test.
- Meter standards. ISO 5167 for differential pressure devices, OIML R 117 for liquid measuring systems and ISO 17089-1 for ultrasonic meters.
- Quality system. ISO/IEC 17025, scope listing fluid, flow range and uncertainty.
How it is calibrated
Typical procedure
- Stabilise liquid temperature and flow, and purge air from the line and the meter.
- Check the pipe: straight lengths, alignment of gaskets, full pipe and the right direction.
- Run at several flows, commonly 10, 25, 50, 75 and 100 % of the range, with at least three runs each.
- Collect for a long enough time so the start and stop timing is negligible, typically 30 to 60 s or more.
- Compare the meter volume or totaliser with the reference, and compute the error at each flow.
- Report error against flow, with the fluid, temperature and uncertainty, and adjust the K-factor or calibration factor if required.
Where it goes wrong
- Air in the line. Bubbles read as flow in many meters and ruin the reference.
- Poor flow profile. Elbows and valves just upstream distort the flow.
- Wrong fluid. Viscosity and density change meter response, especially turbines.
- Timing and diverter. A diverter that opens late adds volume to the collection.
- Pulsation and cavitation. Pumps create pulsations that bias many meters.
What goes in the uncertainty budget
- Reference uncertainty: weighing, density, timing or master meter.
- Temperature and pressure of the liquid.
- Repeatability of the meter and of the runs.
- Installation effects and flow profile.
- Diverter and start and stop timing.
Combine the contributions as in the uncertainty budget calculator, then decide pass or fail with a guard band as explained on the calculators page.
Calculators
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