Compliance and audits
Flow meter calibration: methods, K-factor, turndown and intervals
Flow meter calibration runs fluid through the meter at several steady flow rates and compares its reading with a reference (a weighing tank, a volumetric tank, a piston or pipe prover or a master meter), then reports the error or K-factor at each flow. The result holds only for the flow range and fluid it was done in, and a normal NIST calibration uses five flows, each measured several times and repeated on a second occasion.
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What a flow meter calibration checks
A calibration compares what the meter indicates with a reference quantity of fluid collected or displaced over the same period, at steady flow, pressure and temperature. NIST reports the result as a meter factor, the ratio of the flow computed from the standard to the averaged meter indication, or its reciprocal (SP 250-73). For a meter with a pulse output, the same result is usually stated as a K-factor.
Which output you calibrate matters. A meter may give a flow rate, a totalised volume, a mass or a pulse train. The lab reads the meter's output with its own calibrated instruments, and their uncertainty goes into the budget.
Comparing the error with a limit and recording a pass or fail is a verification against a specification. Meters used for trade also have legal limits, covered below.
The standards and references
ISO 4185:1980 describes liquid flow measurement by weighing the liquid delivered into a tank in a known time, in static and dynamic forms, with the calculation of uncertainty. ISO records it as confirmed in 2019, and a second edition is registered as a work item (ISO/AWI 4185).
ISO 8316:1987 describes the same idea with a calibrated volumetric tank. ISO 9368-1 sets out how to check static weighing installations.
OIML D 36:2020 covers pipe provers, including piston provers, as standards for verifying and calibrating meters. It replaced OIML R 119 of 1996.
For meters under legal control, OIML R 117:2019 covers dynamic measuring systems for liquids other than water, OIML R 49:2024 (published jointly as ISO 4064:2024) covers water meters and OIML R 137-1&2:2012 covers gas meters. In the US, NIST Handbook 44 Section 3.37 covers mass flow meters.
National metrology institutes publish their methods. NIST describes its flow calibration standards in the SP 250 series, PTB runs a water flow test facility with gravimetric and volumetric references, and TÜV SÜD National Engineering Laboratory houses the UK national standard for flow measurement.
Four reference methods compared
In the gravimetric method, liquid is collected in a tank on a scale for a measured time. NIST's sequence sets a stable flow through the meter, zeroes the scale, switches a diverter into the tank and starts the timer, records the water temperature and the air conditions for the buoyancy correction, then reverses the diverter and takes the final mass (SP 250-73). ISO 4185 also has a dynamic form, in which the mass is taken from weighings made while liquid is still arriving.
A volumetric tank uses a calibrated volume in place of the scale (ISO 8316). Both methods stop the flow path into a receiver, so they suit a laboratory better than a working line.
A prover sweeps a known volume past detectors with a piston or sphere. OIML D 36 describes unidirectional, bidirectional, small volume and full stroke types. The prover's base volume is calibrated first, by water draw, by master meter or dimensionally, and its readings are corrected for the temperature and pressure of the liquid and the prover. D 36 recommends temperature devices with an expanded uncertainty of 0.2 °C or less and pressure devices of 0.05 MPa or less (6.3, 6.4), and enough preliminary runs to clear gas and stabilise the liquid temperature (6.6).
A master meter is a calibrated meter in series with the meter under test. OIML D 36 (5.3.2) describes the same idea for calibrating a prover: the master meter is only an intermediate link, so calibrate it against another standard and confirm that it is stable before and after the work.
| Method | Reference quantity | Described in | Usual place |
|---|---|---|---|
| Gravimetric | Mass of liquid collected in a weigh tank in a measured time | ISO 4185:1980 | Laboratory |
| Volumetric tank | Volume collected in a calibrated tank in a measured time | ISO 8316:1987 | Laboratory |
| Piston or pipe prover | Known volume swept between detectors by a piston or sphere | OIML D 36:2020 | Laboratory or in line on site |
| Master meter | A calibrated meter in series with the meter under test | OIML D 36:2020, 5.3.2 (for provers) | In line on site |
Liquid or gas: what changes
Gas is compressible, so its density depends on pressure and temperature, and a gas volume means little without them. OIML R 137 calls the pressure and temperature that a volume is converted to the base conditions (3.2.19). Check which ones the meter and the certificate use.
NIST's primary gas standards are PVTt systems, which fill a tank of known volume for a measured time and work out the mass collected from pressure and temperature. The 34 L and 677 L standards have an uncertainty of 0.025% and the 26 m³ standard 0.09% (k = 2). Its Working Gas Flow Standard uses critical flow venturis and laminar flow meters calibrated on the PVTt standards, at 0.15% to 0.07%.
The gas matters as well as the flow. OIML R 137-1&2 allows verification with air or another gas only if the difference from the gas in use stays within a third of the maximum permissible error, or if a correction is applied (12.5.2.3, 13.1.3). Liquids have a similar effect through viscosity. NIST's sample turbine meter report notes that flows below about 10% of the maximum, where the bearings dominate, carry extra viscosity uncertainty. That is a rough estimate, and only a calibration in two fluids of different viscosity shows it for a given meter (SP 250-1039r1).
K-factor and meter factor
The K-factor of a pulse-output meter is its pulse count per unit of volume or mass. PTB and NIST define it as the output frequency divided by the flow rate, which is the same thing (PTB, Engel and Baade, 2006; NIST SP 250-1039). A calibration gives the K-factor at each flow, and the meter's transmitter or counter uses it to turn pulses into a quantity.
Take a turbine meter set to 100 pulses per liter. Over one run it produces 20,000 pulses while the prover delivers 198.0 L. The meter indicates 200 L, an error of +1.0%, and the K-factor from that run is 20,000 ÷ 198.0 = 101.0 pulses per liter.
The K-factor is not a constant. It varies with flow rate, and PTB's paper shows that it shifts with temperature through the thermal expansion of the meter body. NIST reports turbine meters in dimensionless form, Strouhal number against Roshko number, so the result can be used at a different liquid or temperature (SP 250-1039r1). OIML D 36 allows the error of a measuring system to be calculated from a meter factor determined under ISO 4267-2 clause 7, or directly from the proved volumes (6.9).
How many test flows and what turndown means
A normal NIST liquid calibration uses five flows, five averages of the readings at each and the whole sequence repeated on a second occasion, with the flow adjusted upward in one set and downward in the other. NIST's gas service uses five flows with three or more measurements at each, on two occasions. It rarely works with fewer than three flows and two sets of three measurements (SP 250-1039r1). Wait for stability first: NIST's piston prover procedure requires the meter factor to hold within 0.2% before data is taken.
Turndown is the ratio of the highest to the lowest flow over which a meter meets its accuracy. A meter rated at 100 L/min with a 20:1 turndown is specified down to 5 L/min. Pick calibration flows across the range the meter actually sees, including near the bottom of the turndown, and do not read the result beyond the range you calibrated. NIST states that its calibration values only apply over the calibrated range.
The legal recommendations set minimum flow ratios, shown in the table. A water meter also has zones. OIML R 49-1:2024 fixes Q2/Q1 at 1.6 and Q4/Q3 at 1.25 (4.1.5, 4.1.6), so the lower zone runs from Q1 up to Q2 and the upper zone from Q2 to Q4.
| Recommendation | Applies to | Required ratio |
|---|---|---|
| OIML R 117-1:2019 | Fuel dispensers, other than for liquefied gases | Qmax / Qmin of at least 10 |
| OIML R 117-1:2019 | Other liquid measuring systems | Qmax / Qmin of at least 5, or less with an automatic flow checking device |
| OIML R 49-1:2024 | Water meters | Q3 / Q1 from a set list that runs from 40 to 1,000 and may be extended upward (4.1.4) |
| OIML R 137-1&2:2012 | Gas meters | Qmax / Qmin of at least 50 with Qmax / Qt of at least 10, or 5 to under 50 with Qmax / Qt of at least 5 |
Error limits for meters under legal control
If a meter is used for trade or is otherwise regulated, the error limit comes from the applicable recommendation or national rule. OIML R 117-1:2019 gives one limit for the complete measuring system and a tighter one for the meter alone, for quantities of at least 2 L or 2 kg (Table 3).
- OIML R 49-1:2024 water meters: class 1 allows ±1% in the upper flow zone and ±3% in the lower zone, class 2 allows ±2% and ±5%. Above 30 °C the upper zone limits become ±2% and ±3% (4.2.2, 4.2.3).
- OIML R 137-1&2:2012 gas meters at type evaluation and initial verification: from the transitional flow up to Qmax, class 0.5 allows ±0.5%, class 1 ±1% and class 1.5 ±1.5%. From Qmin up to the transitional flow the limits are ±1%, ±2% and ±3% (Table 2).
- NIST Handbook 44 Section 3.37, Table T.2, for mass flow meters: accuracy class 0.3 has an acceptance tolerance of 0.2% and a maintenance tolerance of 0.3%, and class 0.5 has 0.3% and 0.5%.
- Repeatability: OIML R 117-1:2019 limits a meter's repeatability error to two-fifths of the line A value for quantities of at least 5 times the minimum measured quantity, and Handbook 44 T.3 limits the range of repeated tests to 40% of the maintenance tolerance.
| Accuracy class | Measuring system (line A) | Meter alone (line B) |
|---|---|---|
| 0.3 | ±0.3% | ±0.2% |
| 0.5 | ±0.5% | ±0.3% |
| 1.0 | ±1.0% | ±0.6% |
| 1.5 | ±1.5% | ±1.0% |
In situ or in the laboratory
A laboratory gives controlled flow, temperature and fluid, but it does not reproduce your pipework. NIST lists installation effects, long-term changes and temperature effects on the meter among the uncertainty components the user has to add in service (SP 250-1039r1). OIML R 49-1:2024 classifies water meters by their sensitivity to irregular velocity profiles upstream and downstream.
VSL, the Dutch national metrology institute, notes that calibrating on site needs special process connections for the mobile equipment, and that where a meter cannot be removed during operation the work may have to wait for a shutdown. Otherwise the meter goes to a calibration lab or back to the manufacturer. In practice an in-situ calibration usually means a prover or a master meter in series with the installed meter.
Regulators accept in-situ checks too. In US wastewater permits, EPA's NPDES Compliance Inspection Manual has the facility's flow measurement systems calibrated by a qualified source at least once a year, and an inspector may compare a meter with a portable strap-on meter and expect the two to agree within ±10 percent (Chapter 6, EPA 305-K-17-001).
As-found readings, adjustment and a meter out of tolerance
Calibrate before you adjust. The as-found K-factor or error at each flow shows how the meter measured while it was in use. Changing a K-factor in a transmitter is an adjustment, so record the old value and the new one, then repeat the flows as left. OIML R 117-1:2019 says an adjustment device may only be used to bring errors as close to zero as possible, and forbids adjusting with a bypass of the meter.
If an as-found error is outside its limit, the meter was out of tolerance for an unknown part of the time since its last good calibration. ISO 9001 clause 7.1.5.2 expects you to look back at what it measured and deal with anything that may be wrong. For a flow meter, look at what its readings controlled, such as batch quantities, dosing, custody transfer volumes or figures on a permit report.
How often to calibrate a flow meter
No standard sets one interval for all flow meters. VSL says the interval is set so that a meter stays within the accuracy and tolerance limits in the operating procedures of the company using it. ILAC-G24:2022 / OIML D 10:2022 lists the factors to weigh, among them the tendency to wear and drift, the extent and severity of use and the environment (5.1).
Regulators do set periods for some meters. Germany's Mess- und Eichverordnung (MessEV, Anlage 7) gives water meters a verification period of 6 years and meters for liquefied gases 1 year. Gas meters get 5 years unless another row applies, with diaphragm meters at 8 to 16 years depending on size. The EPA expects at least once a year for wastewater flow measurement systems.
How a meter drifts depends on its design. OIML R 117-1:2019 requires an endurance test only for meters with moving parts in the liquid, and says Coriolis, ultrasonic and electromagnetic meters do not need one. Use each meter's own as-found history, and recalibrate after a repair, a change of fluid or a change of installation.
What the certificate and record should show
Whether the meter goes to a lab or stays in-house, keep a record that lets someone else repeat the calibration and reach the same decision. ISO/IEC 17025:2017 clause 7.8 sets the general content of a calibration certificate. NIST's reports add the uncertainty of the calibration factor and the conditions of the test.
- The meter's ID, size, type and the output calibrated, with the K-factor or span set in it as found.
- The method, such as gravimetric or master meter, and the reference used with its ID and calibration due date.
- The fluid, with its temperature, pressure, density and viscosity where they affect the result.
- For gas, the base conditions and the gas used.
- Each flow with the reference quantity, the meter reading, the error or K-factor and the repeatability.
- The installation, if the work was done in situ, including any upstream straight run or flow conditioner.
- The as-found and as-left results and the K-factor before and after any adjustment.
- The uncertainty, the date, who did the work and the next due date.
How to calibrate a flow meter, step by step
- Identify the meter, its output, its range and the limit that applies from the maker's specification, the contract or the regulation.
- Choose the reference: gravimetric, volumetric, prover or master meter, and confirm it has a current calibration and a suitable uncertainty.
- Decide on the fluid and on in-situ or laboratory work, and note the fluid properties that matter, such as temperature, pressure, density and viscosity.
- Fill the line, clear any gas and run until the temperature and flow are stable.
- Set the first flow and wait until the meter reading is stable.
- Measure at least five flows across the range the meter sees, with several runs at each, reading the meter and the reference over the same period.
- Work out the error or K-factor at each flow and the repeatability. Compare them with the limit and record these as-found results before any adjustment.
- If you change a K-factor or adjust the meter, record the old and new values and repeat the flows as left.
- If the meter was out of tolerance, assess what it measured since its last good calibration. Then set the next due date.
Where Axiospec fits
Axiospec is calibration management software. It keeps each flow meter with its calibration history, certificates and next due date, and each calibration names the reference standard used. One calibration record holds up to 100 test points. Each point has its nominal value, its tolerance as an absolute band, a percentage of nominal or a percentage of span, and the as-found and as-left readings, so a five-flow calibration fits in one record. The certificate Axiospec produces lists those points.
On an in-house calibration, a final reading outside its tolerance sets the result to Fail, and recording a pass anyway takes a written reason. A Fail puts the meter in quarantine. A Fail, or a Pass with an as-found reading out of tolerance, asks you to write down what the meter measured since its last good calibration, and any not yet written show in Needs attention. Axiospec does not run the test, calculate the K-factor or choose the method or the interval. Those stay with you or your lab.
The free plan covers up to 50 active instruments. A new workspace starts with a 45-day trial of Professional, with room for 1,500 active instruments and no credit card. We load your existing records for you at no charge on every plan.
Common questions
- How do you calibrate a flow meter?
- Run the fluid through the meter at several steady flows, typically five across its range, and compare the meter reading with a reference over the same period. The reference can be a weighing tank, a volumetric tank, a piston or pipe prover or a master meter. Record the error or K-factor at each flow before any adjustment.
- How often should flow meters be calibrated?
- No standard sets one interval for all flow meters. Set it from the meter's as-found history, its service and the accuracy you need, and follow any contract or regulation. The EPA expects wastewater flow measurement systems to be calibrated at least once a year.
- What is a flow meter K-factor?
- It is the number of pulses a meter produces per unit of volume or mass. A calibration finds the K-factor by dividing the pulses counted during a run by the reference quantity delivered. The K-factor changes with flow rate and temperature, so it is reported at each calibration flow.
- What does turndown mean on a flow meter?
- Turndown is the ratio of the highest flow to the lowest flow at which the meter meets its accuracy. A meter with 20:1 turndown and a maximum of 100 L/min is specified down to 5 L/min. OIML R 117 requires a ratio of at least 5 for most liquid measuring systems.
- What is the difference between gravimetric and volumetric flow calibration?
- A gravimetric calibration collects the liquid on a scale and works from its mass, while a volumetric calibration collects it in a tank of known volume. ISO 4185 covers the weighing method and ISO 8316 the volumetric tank method. Both are normally laboratory methods because they collect the flow in a receiver.
- Can a flow meter be calibrated in place?
- Yes, with a prover or a master meter in series with the installed meter, or with a portable comparison meter for a check. The installation then counts as part of the result, but the method needs suitable connections and often a shutdown. National institutes and accredited labs generally calibrate at controlled conditions instead.
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