Optical Concentration Meter

Two optical routes that have almost nothing in common

Refractive index — how much is dissolved

Light crosses a sapphire prism towards the liquid and undergoes total internal reflection beyond the critical angle. The position of the resulting shadow line is set by the refractive index of the liquid, and a CCD reads that position. Refractive index against total dissolved concentration is a stable single-valued curve, so this instrument answers "how much is dissolved" — Brix, brine strength, acid concentration, glycol content, coolant ratio. PS7110 takes this route.

Its strength is often misunderstood: the measurement happens within a few micrometres of the prism face, so it is indifferent to suspended solids, bubbles and colour. Dark, opaque liquid measures perfectly well, because the light was never going to cross it. Its weakness is equally sharp: the prism has to stay clean. Once a film, an oil layer or crystal forms, the critical angle is set by that layer and not by the process.

Spectroscopy — which component

Different molecules absorb characteristically across the UV, visible and near-infrared. Measuring absorbance at several wavelengths and fitting a calibration model separates components — free chlorine and free alkali reported at the same time, or two acids distinguished in a mixture. PS7100 takes this route.

What a spectroscopic analyser can do is set by its calibration: it is accurate over the conditions the model has seen and extrapolates badly beyond them. Commissioning takes longer than for any other principle here, and the payoff is a multi-component answer nothing else on this page can give.

Against the other principles

A refractometer and a fork or Coriolis are not measuring the same thing: those measure density, this measures refractive index. In a binary solution the two are interchangeable; add a third variable — temperature, suspended solids, a second solute — and they diverge, and which one is "right" depends on what you are controlling.

Against ultrasonic sound velocity: both target clean liquid. Sound velocity does not care about a dirty window but does care about bubbles; refractive index does not care about bubbles or particles but does care about films. Dirty and gas-free points to optical; gassy and clean points to ultrasound.

The window is the whole story

Every optical concentration meter is accurate on the bench. On site, the difference between them is almost entirely whether the window stays clean. Three rules that hold up:

• Mount where the liquid moves and cleans the face. A probe in a dead zone fouls within a week.
• For film-forming media, fit steam or liquid purge and put the purge interval into the control logic rather than into someone's memory.
• Treat zero drift as a fouling alarm, not an instrument fault. A periodic water zero check is the single most effective maintenance routine for this class of meter.

Buying from China

CE marked; ATEX / IECEx varies by configuration and hygienic (3-A / EHEDG) options exist for food and beverage duty — confirm both against your specification before ordering. Quotations in USD or EUR on EXW, FOB Shanghai or CIF, with HS code and declaration of conformity. For a spectroscopic application, send the medium composition and its expected range with the enquiry: the calibration plan, not the hardware, decides whether the project succeeds.