Start with the flow profile
Ask for minimum sustained flow, normal flow, maximum continuous flow and short-duration peak flow. Include peak duration, daily duty, seasonal variation and any reverse flow.
A daily total alone is insufficient. A site using 24 m³/day could draw 1 m³/h continuously or 12 m³/h for two hours. Those cases have the same daily consumption but different capacity requirements.
Oversizing can leave useful low flows below the meter's specified range. Undersizing can exceed its continuous capacity or consume too much pressure. Select from the manufacturer's flow and pressure-loss data, then reconcile the selection with pipework and available connections.
Understand Q1, Q2, Q3, Q4 and R
Under OIML R49, Q1 is minimum flow, Q2 is transitional flow, Q3 is permanent flow and Q4 is overload flow. R = Q3/Q1. Q2/Q1 is 1.6 and Q4/Q3 is 1.25. For accuracy class 2, maximum permissible error is ±5% from Q1 up to Q2; from Q2 through Q4 it is ±2% at 0.1–30°C and ±3% above 30°C, within rated conditions. Class 1 limits differ. Orientation-specific ratings must be checked. OIML R49-1:2024, sections 3–4 and marking requirements
Worked example: a hypothetical Q3 = 4 m³/h, R160 meter has Q1 = 4/160 = 0.025 m³/h, or 25 L/h. Using the ratios above, Q2 = 40 L/h and Q4 = 5 m³/h. These are calculated examples, not a recommendation to operate continuously at Q4.
At the same Q3, R400 gives Q1 = 10 L/h. Check the remaining application requirements too.
Starting flow is not guaranteed accurate flow
A meter may begin registering below its specified minimum measuring flow. Detecting movement is different from measuring within the stated error band. For small-leak monitoring or low-consumption billing, ask for the actual low-flow performance and alarm logic.
Legacy Class A/B/C markings and older Qmin/Qt/Qn/Qmax tables must be interpreted using their stated standard and edition. Do not convert “Class B” directly into “R160,” or assume that identically named columns in unrelated catalogues define identical capacities.
Compare accuracy on the same basis
“±0.5%” is incomplete without the basis and operating conditions. Percentage of reading, percentage of full scale, repeatability and total installed uncertainty answer different questions.
Suppose two hypothetical instruments have a 100 m³/h span. At an actual 10 m³/h:
| Stated basis | Calculated error magnitude | Relative to actual flow |
|---|---|---|
| ±0.5% of reading | ±0.05 m³/h | ±0.5% |
| ±0.5% of 100 m³/h full scale | ±0.5 m³/h | ±5% |
Real specifications can also include a fixed velocity or zero term. Read the complete expression. A repeatable meter can consistently repeat a biased value; repeatability is not proof of accuracy.
Check velocity and head loss
For a full circular bore, Q = v × A. With actual internal diameter d in metres, Q in m³/h = v × πd²/4 × 3,600.
For an actual 0.100 m bore at 1 m/s, calculated flow is approximately 28.27 m³/h. Nominal DN100 is a connection designation; it is not always the exact internal diameter.
This calculation checks plausibility. It does not replace the model's approved flow limits or establish a universal “best velocity.” Include the pressure losses of reducers, strainers, valves and fittings, especially in low-pressure or gravity-fed systems.