When to use
When preparing the instrument list for a water, effluent, slurry or chemical line and you need to know whether the meter can sit at line size; when the process gives a minimum and maximum flow and you need to check both ends; and when reviewing a vendor proposal that reduces the meter bore relative to the pipe.
Velocity, not flow, is what sizes the meter
A magnetic flow meter measures the voltage induced in a conductive liquid crossing a magnetic field. That voltage is proportional to the mean velocity, not to the volumetric flow. The same 60 m³/h produces 2.1 m/s in a DN100 sensor and 0.53 m/s in a DN200 — and the larger sensor, carrying the same flow, works with a signal four times weaker.
That is why the tool’s first step is the equation v = (Q/3600)/(π·(D/1000)²/4), evaluated at both ends of the operating range. The line velocity uses the real internal diameter from the pipe catalogue; the meter velocity uses the nominal DN, which is the bore that actually sets the sensor size. Each value is then classified against two bands: the recommended band of the fluid class and the measurable envelope of 0.3–10 m/s. Inside the recommended band the result is “OK”; between the band and the envelope it is “Caution”; outside the envelope it is “Not OK”.
Three bands, and the turndown each one can hold
The tool works with three fluid classes taken from manufacturer practice. Clean liquid uses 1.5–3.0 m/s. Abrasive service keeps the same floor but caps the ceiling at 2.0 m/s to spare the liner. Fouling service raises the floor to 2.0 m/s so the flow scours the electrodes.
The consequence is easy to miss. At a fixed bore, velocity is proportional to flow, so the velocity ratio between Qmax and Qmin is exactly the turndown. The clean band holds at most 2:1, the fouling band 1.5:1, the abrasive band 1.33:1 — and only if a bore of exactly the right size exists. A 4-inch line at 30–60 m³/h shows it: DN100 gives 1.06–2.12 m/s, DN90 gives 1.31–2.62 m/s, DN80 gives 1.66–3.32 m/s. Turndown is 2.0, the band ratio is 2.0, and still no size in the series closes both ends. The tool proposes DN80 as the closest, labels it a partial improvement, and says that the limit is rangeability, not size.
The line commands; the sweep proposes a remedy
By default the meter is installed at line size. The tool then sweeps the whole DN series, from DN15 to DN2000, and scores each bore with a penalty in which falling below the floor weighs three times more than exceeding the ceiling. Low velocity is what kills a measurement; high velocity is wear that can be monitored.
A smaller bore is proposed when it genuinely lowers that penalty. A larger bore is proposed only with a material excess: the maximum velocity at line size must exceed the ceiling by more than 30 % (3.9 m/s in clean service) or leave the 10 m/s envelope. A 4-inch line taking 266 m³/h with no minimum flow declared runs at 9.4 m/s in DN100, and the tool proposes DN200 at 2.35 m/s. Between the ceiling and that threshold the answer is a wear warning, not an expander.
When the sweep finds nothing better, the tool says so. Silence would read as “all is well”; an explicit “no bore in the series brings the velocity into the band” tells the engineer the problem is the operating range.
What a bore change costs
Installing the meter in a bore different from the line adds two concentric reducers, the straight run the manufacturer requires around the sensor (here ≥ 5×DN upstream and ≥ 2×DN downstream, with the pipe always running full) and a permanent head loss across the reducers, which belongs in the pump sizing of the line and not only on the instrument datasheet. That is why a reduction never produces an “OK” verdict: even when it brings both ends into the band, it is a decision with a price, and the verdict stays at “Caution” until someone accepts it.
What this tool does not cover
The sizing is purely by velocity and bore for a magnetic meter. It does not check fluid conductivity, choose liner or electrode material, compute the head loss of the reducers, estimate measurement accuracy at a given flow, or select between measuring principles (Coriolis, vortex, ultrasonic, differential pressure). It does not handle gas or partially filled pipes. Those checks remain with the process data sheet and the manufacturer’s selection tool.
If the choice falls on a differential-pressure meter instead, the primary element is sized with the ISO 5167 orifice plate calculator — the how to size an orifice plate walkthrough shows the steps, and the orifice plate lesson explains the physics — while the transmitter side is checked with the differential pressure transmitter accuracy tool, which also shows why a dP loop loses accuracy so quickly at low flow.
Formulas and fundamentals
v = (Q/3600) / (π·(D/1000)²/4) Q in m³/h, D in mm, v in m/s. For the LINE the tool uses the real internal diameter resolved from the pipe catalogue (material + DN + schedule/SDR); for the METER and for every candidate bore it uses the nominal DN in mm.
TD = Q_max / Q_min Velocity is proportional to flow at a fixed bore, so the velocity ratio v(Qmax)/v(Qmin) equals TD whatever the size. If TD exceeds the ratio of the band itself, no bore in the series can hold both ends inside it.
clean 3.0/1.5 = 2.0 | abrasive 2.0/1.5 = 1.33 | fouling 3.0/2.0 = 1.5 The largest turndown that fits inside the recommended band, and only if a bore of exactly the right size exists. With a discrete DN series the usable turndown is usually smaller.
pen = 3·max(0, v_rec,min − v_min) + 1·max(0, v_max − v_rec,max) + 1000·(excursion outside 0.3–10 m/s) Each DN of the series is scored on this penalty; ties are broken by how close the geometric mean √(v_min·v_max) is to the band centre √(v_rec,min·v_rec,max). Falling below the floor weighs three times more than exceeding the ceiling, because low velocity loses accuracy while high velocity only costs wear.
v_max(DN line) > 1.30 · v_rec,max or v_max > 10 m/s A smaller bore is proposed whenever it lowers the penalty. A bore LARGER than the line is only proposed with a material excess over the ceiling — 3.9 m/s for clean service, 2.6 m/s for abrasive — because enlarging costs two reducers, straight run and head loss.
Q = v · π·(DN/1000)²/4 · 3600 The inverse of the first equation, used for the tool's flow-range table per DN at 0.3 m/s (minimum full scale), ~2.5 m/s (nominal) and 10 m/s (maximum).
Standards & methods
- ISO 6817 — measurement of conductive liquid flow in closed conduits, electromagnetic method (method and performance; does not prescribe a sizing velocity)
- ISO 9104 — methods of evaluating the performance of electromagnetic flowmeters for liquids
- ASME B36.10M / ISO 6708 — DN ↔ NPS correspondence used for the bore series
- Manufacturer practice (Endress+Hauser Promag, Siemens SITRANS F M, Emerson Rosemount 8700) — origin of the velocity bands and the 0.3–10 m/s envelope
Typical reference values
| Quantity | Typical range | Note |
|---|---|---|
| Recommended velocity, clean liquid | 1.5 – 3.0 m/s | manufacturer optimum is 2–3 m/s; Siemens states 1–3 m/s nominal |
| Recommended velocity, abrasive (slurry, sand) | 1.5 – 2.0 m/s | the ceiling drops to limit liner and electrode wear |
| Recommended velocity, fouling / coating | 2.0 – 3.0 m/s | the floor rises so the flow keeps deposits off the electrodes |
| Measurable envelope | 0.3 – 10 m/s | below ~0.3 m/s accuracy degrades; some sensors are rated to ~12 m/s |
| Largest turndown inside one band | 2.0 (clean) · 1.5 (fouling) · 1.33 (abrasive) | theoretical ceiling; the discrete DN series usually gives less |
| Flow in DN100 at 0.3 / 2.5 / 10 m/s | 8.5 / 70.7 / 283 m³/h | Q = v·A·3600 with D = 100 mm |
| Straight run around the meter | ≥ 5×DN upstream · ≥ 2×DN downstream | manufacturer guidance; the pipe must run full |
Worked example
Cooling water in a 6-inch line, 45 to 80 m³/h (FT-01)
Inputs
- Line pipe
- 6" carbon steel STD/40 —
- Internal diameter (from catalogue)
- 154.08 mm
- Minimum / maximum flow
- 45 / 80 m³/h
- Fluid class
- Clean band 1.5–3.0 m/s
Results
- Line velocity at Qmin / Qmax
- 0.67 / 1.19 m/s
- Velocity in a DN150 meter
- 0.71 / 1.26 m/s
- Turndown Qmax/Qmin
- 1.78 —
- Proposed bore
- 6" → DN100 (4") reduction
- Velocity in DN100
- 1.59 / 2.83 m/s
- Verdict
- Caution reduction required
At line size the meter would run below the 1.5 m/s floor across the whole range, so the tool proposes reducing to DN100, where both ends fall inside the band (1.59 and 2.83 m/s). The verdict stays at "Caution" on purpose: the reduction resolves the velocity but has a cost — two reducers, the straight run around them and their head loss — that someone has to accept. Note that DN125 would not do: 1.02–1.81 m/s still misses the floor at Qmin. With a turndown of 1.78, below the 2.0 ceiling of the clean band, a bore that fits exists; with 30–60 m³/h (TD = 2.0) in a 4-inch line it would not.
Common mistakes
- Picking the meter by the flow in the catalogue table without checking the minimum flow. A meter that reads 2.8 m/s at Qmax can sit at 0.7 m/s at Qmin, and the accuracy stated on the datasheet does not hold down there.
- Assuming the meter must always match the line size. A 6-inch line carrying 45–80 m³/h runs at 0.7–1.3 m/s — below the floor along the whole range. The remedy is a smaller sensor, not a smaller pipe.
- Inferring the nominal size from the internal diameter. A 4-inch HDPE SDR 11 pipe has 90 mm inside, which a "nearest DN" rule reads as DN80. The nominal DN comes from the declared size, not from the wall thickness.
- Expecting a bore change to fix a wide turndown. If Qmax/Qmin is larger than the band ratio (2.0 for clean liquid), every bore misses one end; changing the size only buys reducers. Split the range across two meters or accept the excursion in writing.
- Forgetting what the reduction costs. Two concentric reducers, the straight run required around them and their permanent head loss have to enter the piping layout and the pump sizing.
- Specifying a magnetic meter for a liquid without confirming its conductivity. The sizing by velocity says nothing about whether the meter can measure the fluid at all — that is a separate check.
Frequently asked questions
What is the minimum velocity for a magnetic flow meter?
Two numbers matter. The measurable floor is about 0.3 m/s: below that the induced voltage is so small that accuracy degrades. The recommended floor, where the datasheet accuracy is comfortable, is 1.5 m/s for clean and abrasive service and 2.0 m/s for fouling fluids. The tool flags a velocity below the recommended floor as "Caution" and below 0.3 m/s as "Not OK".
Can the flow meter be smaller than the pipe?
Yes, and it is the standard way to raise velocity when the line is oversized for the flow. The tool keeps the meter at line size by default and proposes a smaller bore only when it actually reduces the deviation from the band. The reduction adds two concentric reducers, the straight run the manufacturer requires and a permanent head loss.
When does the tool propose a meter larger than the line?
Only when the maximum velocity at line size exceeds the band ceiling by more than 30 % — 3.9 m/s for clean liquid — or the 10 m/s envelope. Between the ceiling and that threshold the meter still measures; the tool issues a wear warning instead of buying an expander.
Why can't any bore cover my whole flow range?
Because at a fixed bore the velocity ratio equals the flow ratio. A clean band of 1.5–3.0 m/s holds at most a 2:1 turndown; an abrasive band of 1.5–2.0 m/s, only 1.33:1. If Qmax/Qmin is larger, one end always falls outside, whatever the size. The tool says so explicitly instead of proposing a bore change that does not solve it.
Why are the velocity bands different for abrasive and fouling fluids?
For abrasive service the ceiling drops to 2.0 m/s because wear of the liner and electrodes grows quickly with velocity. For fouling or coating fluids the floor rises to 2.0 m/s because a faster flow keeps deposits from building up on the electrodes. Both bands come from manufacturer practice — there is no single prescriptive standard.
Does this replace the manufacturer's sizing tool?
No. It checks velocity, turndown and bore at the project stage, before a vendor is chosen. Accuracy at a given flow, liner and electrode material, conductivity limits and the pressure rating of the sensor still come from the manufacturer's datasheet.
Glossary
- Recommended velocity band
- Range of mean velocity in which the sensor works with its stated accuracy and acceptable wear. It depends on the fluid class.
- Measurable envelope
- Outer limits (0.3–10 m/s) beyond which the meter no longer measures reliably. Leaving it gives a "Not OK" verdict.
- Turndown (rangeability)
- Ratio between maximum and minimum operating flow, Qmax/Qmin.
- Line bore
- Nominal size of the pipe where the meter is installed. In this tool it commands: the meter sits at line size unless a change is justified.
- Bore reduction
- Installing a meter smaller than the pipe, between two concentric reducers, to raise the velocity inside the sensor.
- Liner
- Non-conductive lining of the measuring tube of a magnetic meter. Its wear rate is what limits velocity in abrasive service.
- Straight run
- Length of straight pipe required upstream and downstream of the meter so the velocity profile is developed — here ≥ 5×DN and ≥ 2×DN.
- DN / NPS
- Nominal diameter in millimetres and nominal pipe size in inches; DN100 corresponds to NPS 4 regardless of wall thickness.