Flow Calibration

Flow calibration compares a flow meter's reading against a reference with documented traceability. The method can be gravimetric (weighing the mass collected over a set time), volumetric, or comparison against a reference flow meter.

The aim is for process control and commercial transactions relying on fluid measurement to be accurate. A deviation in flow measurement directly affects both product quality and billing.

• Electromagnetic flow meters
• Ultrasonic flow meters
• Turbine and positive-displacement flow meters
• Mass (Coriolis) flow meters
• Rotameters and variable-area flow meters
• Gas meters and mass flow controllers
• Water meters

• Flow error: at selected points across the measuring range
• Repeatability
• Linearity and turndown-ratio behaviour
• Behaviour in the low-flow region
• Pressure and temperature corrections (in gas measurement)
• The effect of the flow profile

The calibration interval is not fixed by any standard and there is no general rule such as "once a year". The interval is set by weighing together the manufacturer's recommendation, how often and under what conditions the instrument is used, how critical the measurement is, the drift shown by the instrument's previous calibration results, and any legal requirement. ILAC-G24 / OIML D10 describe the methods for setting it. The sound approach is to start with a reasonable interval for the first period and then lengthen or shorten it in the light of subsequent results: an instrument that proves stable can have its interval extended, one that drifts has it shortened.

• ISO/IEC 17025
• ISO 4185 (gravimetric method — liquid)
• ISO 8316 (volumetric method)
• ISO 9300 (critical-flow nozzles — gas)
• OIML R49 (water meters)
• JCGM 100 (GUM — uncertainty)

• ISO/IEC 17025
• ISO 4185 (gravimetric method — liquid)
• ISO 8316 (volumetric method)
• ISO 9300 (critical-flow nozzles — gas)
• OIML R49 (water meters)
• JCGM 100 (GUM — uncertainty)

1. 1Receipt of the Instrument and Initial InspectionThe cleanliness and physical condition of the instrument are checked. On volumetric glassware, cracks and wear are examined; on flow meters, residues.
2. 2Conditioning of Environment and FluidThe instrument and the fluid to be used are brought to equilibrium under laboratory conditions. Temperature directly affects the density of the fluid and therefore the result.
3. 3Preparation of the Reference SystemA reference flow meter, gravimetric rig or acoustic calibrator with documented traceability is put into service.
4. 4Selection of the Measurement PointsPoints representing the instrument's operating range are selected; in flow calibration the low-flow region is addressed separately because error grows there.
5. 5Taking the MeasurementsAt each point, repeated measurements are taken once a steady flow or steady signal has been established.
6. 6Application of CorrectionsTemperature, pressure and density corrections are applied; in the gravimetric method the air buoyancy correction is taken into account as well.
7. 7Uncertainty BudgetThe reference uncertainty, repeatability, temperature stability and reading resolution are combined.
8. 8Issuing the CertificateThe results and uncertainty values are entered on the certificate; a statement of conformity with the decision rule is added if requested.

Every calibration result is given together with a measurement uncertainty; a result reported without its uncertainty is incomplete. The uncertainty is budgeted following the GUM (JCGM 100) approach: the uncertainty of the reference, repeatability, resolution, environmental effects and the stability of the instrument are combined. The certificate states the expanded uncertainty with the coverage factor (usually k = 2) and the approximate level of confidence. The smallest uncertainty a laboratory can report is limited by its calibration and measurement capability (CMC) in the accredited scope; no better uncertainty may be declared.

• The uncertainty of the reference system (the balance or the reference flow meter)
• Fluid temperature and density
• Pressure (directly affects volume in gas measurement)
• The time-measurement uncertainty (in the gravimetric method)
• Flow stability and distortion of the flow profile
• Repeatability

• Laboratory details and certificate number
• The customer and identification of the instrument calibrated: make, model, serial number
• Date of calibration and date of issue of the certificate
• The calibration method applied or reference to the procedure
• Reference standards used and traceability information
• Ambient conditions during calibration (temperature, humidity and, where relevant, pressure)
• Measurement points, reference values and instrument readings
• The deviations calculated
• Expanded measurement uncertainty and coverage factor
• The personnel who performed and who approved the calibration
• The calibrated flow points and the fluid type
• Fluid temperature, pressure and density
• Reference conditions for gas measurements (normal/standard conditions)
• The upstream and downstream straight-pipe lengths

A calibration certificate is NOT a certificate of conformity. Calibration measures and reports the deviation of the instrument; it does not correct that deviation and does not say that the instrument is "correct". To decide whether an instrument is fit for use, a tolerance (acceptance criterion) must be defined — and it is the user who sets that tolerance, not the laboratory. If a statement of conformity is requested, ISO/IEC 17025 clause 7.8.6 requires a DECISION RULE to be agreed in advance, defining how measurement uncertainty is taken into account in the decision; a conformity statement made without regard to uncertainty is misleading. The certificate also shows the condition AT THE TIME of calibration: later use, transport or shock can change the instrument's condition, and the certificate does not cover that. Where a next calibration date appears on the certificate, it is not the laboratory's decision but a reminder entered according to the interval stated by the customer.

Calibration is carried out under ISO/IEC 17025, which requires the laboratory's technical competence, impartiality and metrological traceability. Accreditation is granted per METHOD and per MEASUREMENT RANGE: a quantity or range that does not appear in the laboratory's scope annex is not accredited, and no certificate bearing the accreditation mark may be issued for that measurement. The scope also states the smallest uncertainty the laboratory can offer (CMC); no better uncertainty may be declared. EUROLAB holds internationally recognised accreditation under ISO/IEC 17025, granted by accreditation bodies that are signatories to the ILAC mutual recognition arrangement; whether your instrument can be calibrated within accredited scope is confirmed in writing at the quotation stage.

• Volume Calibration
• Acoustic Calibration
• Photometry Calibration