There are seven principles in common industrial use for online density and concentration. None is universal. This page is the map; the six-step selection guide is the route.
The comparison
| Principle | Physically measures | Wins when | Ruled out by |
|---|---|---|---|
| Ultrasonic — acoustic impedance | Reflection at a boundary, plus sound speed | Abrasive slurry, large bore, gas present, no source wanted | Media where solids and liquid have almost no impedance contrast |
| Ultrasonic — attenuation | Energy lost across a transmission path | Cannot tap the pipe; retrofit and trial installs | Entrained gas; bore above DN500 |
| Ultrasonic — time of flight | Speed of sound | Clean binary solutions with a monotonic curve | Slurry; any medium where the curve turns over |
| Tuning fork | Resonant frequency of an immersed fork | Tanks and lines, wide chemical range, good value | Heavy abrasion; media that build up on the fork |
| Coriolis | Tube resonant frequency, plus phase shift for mass flow | Clean liquids needing density and mass flow together | Two-phase flow; abrasive slurry; large bore economics |
| Optical | Refractive index, or absorbance spectrum | Clean liquids; chemical specificity; lab traceability | Opaque abrasive slurry |
| Differential pressure | Hydrostatic head over a known height | Simple, inexpensive, well-understood | Variable velocity, moving installations, gas in the leg |
| Microwave | Phase shift through the medium | High solids where water content dominates | Media whose conductivity swamps the permittivity signal |
| Gamma / radiometric | Attenuation of gamma photons | Nothing may touch the process; extreme duty | Licensing, transport and disposal obligations |
Three questions that eliminate most of the table
- Is there abrasive suspended solid? If yes, Coriolis and optical are out, and tuning fork needs a wear-resistant tip.
- Is gas present, or does the process create it? If yes, transmission ultrasonic and Coriolis suffer most, and you should ask specifically how the candidate instrument behaves — and whether it tells you.
- Can the pipe be tapped? If no, the feasible set narrows to clamp-on ultrasonic, externally mounted gamma, or a bypass loop.
Working through those three before comparing accuracy figures saves most of the effort, because accuracy is rarely the binding constraint — survival in the duty and behaviour under upset usually are.
Accuracy is not comparable across principles
A specification of ±0.001 g/cm³ on a clean-liquid instrument and ±1 % FS on a slurry instrument are not on the same scale, and neither number tells you what the instrument will do on your process. Ask instead: what was the reference method, what medium was it calibrated on, and what happens when the process moves away from those conditions. A calibration certificate on the actual process liquid is worth more than a smaller number on a datasheet.