Selecting a Flow Meter

Quick pick (TL;DR)

Conductive liquid (water, slurry) → electromagnetic (no pressure loss, handles solids).
Need mass / custody / hard fluids → Coriolis (best accuracy, costly).
Steam & clean gas/liquid → vortex.
Large pipes / no intrusion → ultrasonic.
Cheap & universal where DP is acceptable → orifice (DP).
Always check the meter spans the min–max flow (turndown) and the required straight run.

Step 1 — Select by fluid

Fluid / need	Best meter	Why
Conductive liquid	Electromagnetic	No ΔP, solids OK
Mass / custody	Coriolis	True mass, best accuracy
Steam, clean	Vortex	Wide range
Large / non-intrusive	Ultrasonic	Clamp-on, no ΔP
Budget	Orifice (DP)	Cheap, has ΔP loss

Step 2 — Worked selection

Cooling water, 4in

Process data fluid = cooling water (conductive) flow = 30 / 100 / 130 m³/h (min/norm/max) Step 1 — Eliminate conductive water → magnetic ideal no ΔP budget loss wanted → not orifice Step 2 — Turndown turndown = max / min = 130 / 30 = 4.3:1 magnetic handles ~20:1 → OK Step 3 — Velocity v = Q / A , A(4in) ≈ 0.0081 m² v = (100/3600) / 0.0081 = 3.4 m/s 1–5 m/s ideal for magnetic → OK Answer → electromagnetic, ±0.5%, 4-20mA+HART

Fluid points to magnetic; turndown & velocity checks confirm it.

Step 3 — Don't forget

Straight run (5D/3D); Coriolis needs ~none.
Liner/body suit the fluid.
Output, Ex rating, connection.

Velocity & turndown calculator

Pipe velocity & flow turndown

v = Q/A (1–5 m/s for a magmeter); turndown = max/min.

Normal (m³/h)

Min (m³/h)

Max (m³/h)

Line ID (mm)

1–5 m/s suits a magmeter (rangeability ~20:1).

Common mistakes

Magmeter on non-conductive fluid.
Insufficient straight run.
Oversized meter → low-flow inaccuracy.

Requisition & datasheet

Fluid, size, flow range, P, T.
Type, accuracy, output, materials, Ex.

Frequently asked questions

What should I know about Selecting a Flow Meter?

Conductive liquid (water, slurry) → electromagnetic (no pressure loss, handles solids). Need mass / custody / hard fluids → Coriolis (best accuracy, costly). Steam & clean gas/liquid → vortex. Large pipes / no intrusion → ultrasonic. Cheap & universal where DP is acceptable → orifice (DP). Always check the meter spans the min–max flow (turndown) and the required straight run.

Which standards or codes apply?

ISO 5167 (orifice/DP), ISO 4064 (water meters), AGA 3/9 (gas), API MPMS (custody), IEC 60534 references for control loops.

Standards — ISO 5167; ISO 4064; AGA 3/9; API MPMS.