Ultrasonic Density Meter

One beam, three measurements

Fire an ultrasonic pulse into a liquid and three quantities track density: the fraction reflected at the interface (acoustic impedance), the fraction that survives the crossing (attenuation), and the time it takes to cover a fixed path (sound velocity). All three correlate with density, but they fail in completely different ways — and treating them as one product category is where most selection mistakes start.

Acoustic impedance — built for bubbly, abrasive slurry

Acoustic impedance is Z = ρ·c, density times sound velocity. The sensor emits a pulse and measures its reflection coefficient at the boundary between the sensor face and the medium. The important consequence: the measurement happens within a few millimetres of the sensor face. The beam never has to cross the pipe. Entrained air, coarse particles and scale destroy the signal of any through-transmission method, but they do not stop a reflection at the interface. That is why impedance instruments keep working in coal-washing slurry, FGD gypsum, and cyclone overflow — and why they are the usual replacement for a Cs-137 gauge. PS7000 uses this method.

The trade-off is temperature sensitivity: sound velocity itself moves with temperature, so compensation and the calibration model — not the transducer — are what separate a good instrument from a bad one.

Attenuation — when the pipe cannot be touched

Sound loses energy exponentially as it crosses a medium, and the attenuation coefficient tracks solids content. Because the beam has to cross, this method can be built clamp-on: sensors strap to the outside of the pipe, no tapping, no shutdown, no wetted parts. PS7010 is that instrument. The trade-off is equally clear: bubbles scatter, and scattered energy reads as higher concentration; pipe material, wall thickness and couplant condition all sit in the signal path. Clamp-on is for sites where "we cannot cut the line" outranks accuracy, with low and stable entrained gas.

Sound velocity — the precision route for clean liquids

In a single-phase liquid, sound velocity against concentration is a clean single-valued curve, and a time-of-flight measurement resolves it finely. PS7020 takes this route, for acid and caustic concentration, solvents, and sugar solutions — media with no solid phase. Introduce particles or bubbles and the premise is gone. That is not degraded accuracy; it is the wrong instrument.

Against the other principles

The ultrasonic route's structural advantage is that it is non-nuclear. No source means no NRC general licence and no state agreement-state registration, no leak testing schedule, no EU BSS notification, and no disposal liability at end of life — the cost that surprises buyers is rarely the gauge, it is the twenty years of custody that follow it.

Against a tuning fork: forks are simpler and very reliable in clean liquid, but the tines abrade in slurry, catch fibre, and grow crystal — service life in mineral duty is limited. Against Coriolis: better accuracy and mass flow in the same instrument, but price and pressure drop climb steeply with line size, which rules it out on large slurry mains. Against differential pressure: cheap and transparent, but it depends on a stable level and a stable line, and impulse lines plug in slurry.

Three things that decide the outcome on site

• Where the sensor goes matters more than which model it is. Pump discharge riser, full pipe, no gas pocket, enough straight run from the last elbow. Miss those and no principle will read correctly. We have diagnosed a site whose reading dropped to zero every ten minutes; the cause was a sensor pointed at a spot that periodically collected gas.
• Sampling and calibration. The meter measures a physical quantity; the customer wants concentration, and a material-specific curve sits between them. Eight to twelve grab samples across the real operating range at commissioning are worth more than any factory calibration.
• Temperature. Sound velocity has a substantial temperature coefficient. How well compensation is done is what decides whether the summer and winter readings agree.

Buying from China

Instruments ship with CE marking; ATEX / IECEx and UL / CSA status varies by model and should be confirmed for your specific configuration before you order. Quotations are issued in USD or EUR on EXW, FOB Shanghai or CIF terms, and we can supply the HS code and a declaration of conformity with the quotation. Ask for the field-installation drawing at the same time — the mounting decision above is easier to get right before the spool is fabricated.