Acoustic impedance (symbol Z) is the property that lets an ultrasonic density meter read slurry density without a radioactive source. This explainer covers the units, the formula, how the reading is produced, and — the part most articles skip — where the method runs out of road.
Definition and formula
Specific (characteristic) acoustic impedance is the product of a medium's density and its speed of sound:
Z = ρ × c
where ρ is density (kg/m³) and c is the speed of sound in the medium (m/s). It is a bulk material property, not a property of the instrument, which is why an impedance reading transfers between installations in a way that a raw sensor voltage does not.
Units
- SI: pascal-second per metre, Pa·s/m, equivalently kg/(m²·s).
- The rayl: 1 rayl = 1 Pa·s/m. Practical values are large, so the megarayl (MRayl) is normal usage.
| Medium | Approx. Z (MRayl) |
|---|---|
| Air | 0.0004 |
| Water (20 °C) | 1.48 |
| Typical mineral slurry | 1.6 – 2.6 |
| Quartz | 15 |
| Stainless steel | 45 |
Two things follow from that table. Water and slurry sit close together, so the instrument has to resolve a small difference well — that is the engineering problem. Air is four orders of magnitude away from water, which is why entrained gas matters so much; a small void fraction moves Z more than a large change in solids does.
How the reading is produced
At the boundary between a known reference and the slurry, the fraction of an ultrasonic pulse that reflects depends on the impedance mismatch across that boundary:
R = (Z₂ − Z₁) / (Z₂ + Z₁)
Measuring that reflection gives the slurry's Z. Combined with the measured sound speed, the meter solves for ρ. Because it reads a material property rather than a radioactive count rate, an acoustic-impedance density meter such as the PS7000 needs no source, no licence, and no disposal route at end of life.
The PS7000 uses a single sensor that both transmits and receives, with a linear frequency-modulated (chirp) excitation. One practical consequence: the measurement happens at one face rather than across the pipe, so pipe diameter does not limit the method the way a through-transmission arrangement does — DN50 to DN1000 is the standard range.
Why it suits slurry
Impedance contrast between the carrier liquid and the suspended solids is large, so the bulk Z tracks solids concentration closely. That makes the method well matched to mining slurry, FGD limestone and gypsum slurry, coal-washing heavy media, and tailings — the cases where a radiometric gauge has historically been the default.
What degrades the reading
Being explicit about this is more useful than a specification table:
- Entrained gas. The dominant error source, for the reason in the table above. Bubbles raise the apparent solids reading. Instruments that report a gas index alongside the density value let you see this rather than guess at it.
- Coating on the sensor face. A scale or clay layer sits between the reference and the slurry and biases the boundary reflection. This is an installation-position problem more than an instrument problem — it is why flow past the sensor face matters.
- Temperature. Sound speed in water changes roughly 3 m/s per °C near ambient, so temperature has to be measured and compensated, not assumed.
- Settled or stratified flow. If solids have dropped out of suspension at the measuring point, the meter reads what is in front of it, and that is not the line average. Horizontal runs at low velocity are the classic trap; a vertical upflow section is the usual fix.
Common confusions
Impedance is not attenuation. Acoustic impedance is a reflection-based property; attenuation is how much energy the medium absorbs and scatters along a transmission path. They are different measurements with different failure modes — see the attenuation coefficient explainer and the PS7000 vs PS7010 comparison.
Impedance is not sound speed alone. Time-of-flight instruments measure c and infer concentration from a calibrated c–concentration curve. That works well on clean binary solutions and poorly on slurry, where the curve is not single-valued.
"Non-intrusive" is not "non-contact". The PS7000 sensor face is wetted and mounted flush with the bore. Nothing protrudes into the flow, so there is no obstruction and no wear surface in the stream — but the sensor does touch the medium.
Where to go next
- How to choose an industrial online density meter — the six-step selection method.
- Mining slurry density measurement overview.
- PS7000 ultrasonic slurry density meter — full specification.