Pressure, level and flow measurement for automation engineers
Gauge, absolute and differential pressure; hydrostatic, radar and ultrasonic level; electromagnetic, Coriolis, vortex and differential-pressure flow — and what each means for scaling and commissioning.
Pressure
| Reference | Meaning | Example range |
|---|---|---|
| Gauge (g) | Relative to atmosphere | 0 … 10 bar(g) |
| Absolute (a) | Relative to vacuum | 0 … 2 bar(a) |
| Differential (dP) | Between two process connections | 0 … 250 mbar |
Check units and reference before scaling: 1 bar(g) ≈ 2.013 bar(a) at standard atmosphere.
Level
| Method | Principle | Watch out for |
|---|---|---|
| Hydrostatic | Pressure at the bottom = ρ·g·h | Density changes, tank pressure (use dP for closed tanks) |
| Radar (non-contact / guided) | Time of flight of microwaves | Foam, internals, dielectric constant |
| Ultrasonic | Time of flight of sound | Vapour, foam, temperature, dead zone |
| Displacer / float, capacitive, vibrating fork (point level) | Mechanical / electrical | Point level switches need proof testing |
Level in % of the tank is not volume: for horizontal cylinders and cone bottoms use a linearization table.
Flow
| Meter | Measures | Notes |
|---|---|---|
| Electromagnetic | Volume flow of conductive liquids | No pressure loss, needs full pipe and grounding |
| Coriolis | Mass flow and density | High accuracy, also gases; sensitive to installation stress |
| Vortex | Volume flow of steam, gas, liquids | Needs minimum velocity; straight runs |
| Differential pressure (orifice, venturi) | dP ∝ flow² | Square-root extraction in the transmitter or PLC — never twice |
| Ultrasonic | Velocity by transit time | Clamp-on possible |
Commissioning checks
- Range of transmitter = range in PLC scaling (same units).
- Zero check at known conditions (empty tank, pump stopped, valves closed).
- Damping in the transmitter vs filtering in the PLC — do not stack large time constants.
- Failure current behaviour (NAMUR NE 43) configured and handled — see analog signals.
Connections
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Sources
- NAMUR Recommendations and Worksheets (NE 43) — NAMUR
- SIMATIC S7-1500 Analog value processing — Function Manual 06/2014 — Siemens AG
- HART Communication Protocol — FieldComm Group
Updated 2026-10-02