All solids concentration meters use indirect methods (such as optical, ultrasonic and nuclear). Indirect methods correlate the solids concentration with a measurable factor.
The limitation of not relating perfectly to the quantity of suspended matter does not seriously affect the analysers ability to produce a repeatable signal of great value in process control.
When a light beam is directed on to liquid containing suspended particulates, the suspended particulates scatter some of the light.
The nephelometer used to observe and measure the amount of light that the particulate matter scatters.
Nephelometer
Typical image as shown in Figure.
Source: WEF,2008
The amount of scattered light relates approximately to the amount of particulate matter, particle size and surface optical properties.
This is a photoelectric device that uses an incandescent light source (lamp), which produces light in wavelengths from blue to red.
The light is directed to a liquid and if the liquid contains particles, some of the light strikes the particles and scatters.
By placing a photocell or light detector at an angle to the light beam rather than directly in front of it, the detector receives only light scattered by the suspended particulate matter.
Most nephelometers have the photo detector placed at a 90-degree angle to the incandescent light source.