"Optical concentration meter" covers two quite different instruments. Both look at light through a sapphire window; what they do with it, and therefore what they can measure, has almost nothing in common.
Route 1 — refractive index (critical angle)
Light is directed at the interface between a sapphire prism and the process liquid. Beyond a certain incidence angle — the critical angle — light is totally internally reflected. That angle depends on the refractive index of the liquid, which in turn depends on dissolved solids concentration. Imaging where the shadow line falls gives refractive index nD, which is converted to concentration or degrees Brix through a calibration for that medium.
Key characteristics:
- Reads only the thin layer at the prism face, so turbidity, colour, bubbles and suspended solids do not blind it. This is the property that sells it.
- Direct traceability to a laboratory refractometer — the same physical quantity, so plant and lab agree.
- Excellent resolution on dissolved-solids systems: the PS7110 resolves 0.00002 RI / 0.02 Brix and is specified at ±0.00017 RI / ±0.15 Brix.
- Blind to anything that does not change refractive index, and unable to distinguish two components that move nD the same way.
- Prism fouling shows up as a slow drift, so a self-cleaning position or a wash provision matters.
Route 2 — spectral absorption
Light across a wide band is passed through the medium and the absorbance spectrum is recorded. Different species absorb at different wavelengths, so the spectrum carries chemical identity, not just a single scalar. A calibration model maps spectral features to concentration — the PS7100 covers 200–1700 nm at 33 nm resolution and reports concentration, density and full spectrum together.
Key characteristics:
- Can separate components that a single-scalar method cannot. If two species must be told apart, this is usually the only optical route that works.
- Can flag a wrong medium, not merely a wrong concentration — the spectrum either matches the expected substance or it does not. On a shared line running several products, that is a safety function as much as a measurement.
- Needs a calibration model built on real samples, and the model is only as good as the sample set it was built from.
- Strong colour or high turbidity attenuates the path and constrains where the model is valid.
Choosing between them
| Requirement | Refractive (PS7110) | Spectral (PS7100) |
|---|---|---|
| Single dissolved species, clean binary | Preferred | Works, more than needed |
| Two or more species to separate | Not possible | Preferred |
| Must confirm which medium is in the line | Not possible | Preferred |
| Highly coloured or turbid | Preferred | Check feasibility first |
| Lab cross-check by refractometer | Direct | Indirect |
| Brix / sugar / syrup | Preferred | Works |
What optical methods cannot do
Neither route measures suspended solids concentration in an abrasive slurry. Light does not get far in mineral slurry, and the prism face would not survive the duty. For those services the choice is between ultrasonic, differential pressure, microwave and gamma. Optical instruments belong on clean liquids, and the honest framing is that they trade universality for resolution and chemical specificity on the media they do suit.