Turbidity/Solids Concentration Measurement Solutions

When simple 90° scatter technique turbidimeters are used in applications with fluids that less than almost clear (i.e. drinking water), coating/fouling quickly forms on the optical window surfaces.  Fouling cuts down the amount of excitation light entering the fluid and reduces the amount of scattered light being received by the instruments’ photodetector.  This causes both drift and a reduction in sensitivity.  Using a ratio based 90° scatter instrument provides compensation for window fouling and keeps the measurement stable and online.

Making turbidity measurements at high pressure can be challenging.  Optical windows in any turbidity measurement circuit must be rated to the design pressure of the plant. This is a problem for instruments that utilize windows with a large surface area.  Kemtrak inline turbidity measurement cells, because they are fiber optic based, need only small windows into the process line and are therefore able to meet extremely high pressures.

Turbidity probes, particularly small ones, use a backscatter or transmission technique to measure turbidity.  Backscatter measurement techniques do not work well at low turbidity levels.  Transmission measurement can work well at low turbidity levels, but requires a long optical path length to do so – something not generally found on optical probes. If you need to use a probe rather than a flow through cell, Exner EXspect sensors are available with a 20mm pathlength and perform well at low turbidity levels.

Many turbidity sensors have “high temperature” options, but these are really just standard sensor parts mounted with some sort of plastic insulator.  It’s not an effective way to measure turbidity at high temperature, and sensor failures are common.  The only way to resolve this problem is to use a fiber optic-based system that has no electrical or electronic parts in the sensor housings.  Kemtrak systems use industrial fiber optics to connect the measurement cell to sensitive optical components in the electronic unit.

Calibrating turbidity systems is not a simple task.  The main problem is that calibration solutions constantly settle, so readings are always on the move.  Agitating to re-suspend particulate and pouring into and out of measurement cells can introduce air bubbles that create errors.  Kemtrak has a simple calibration system using sealed formazin (or polymer bead) standards that are inserted into window optical paths using a unique positioning fixture, ensuring repeatable and consistent results.

The chemical stability of formazin suspensions is dependent on storage conditions. Exposure to heat or direct sunlight will denature the polymer structure, while prolonged contact with ambient air will allow the fluid supernatant to evaporate while air-borne contaminants enter. Formazin standards should be stored in a cool, dark place and refrigeration will provide extended life. When using your standards, do not leave containers open longer than necessary and always seal them tightly for storage. Never pour standards back into their storage containers as this will introduce contamination and could lead to erroneous future calibrations.  As a rule, a 4000 NTU formazin suspension, when stored correctly, will be stable for about one year.