Understanding interferences

A detector placed at 90° detects the scattering caused by the particles. The greater the scatter, the higher the turbidity value. Depending on the application or industry in which the device is used, additional scattered light detectors can be used at other angles.

However, there can also be disturbances in the measurement.

Below is a list of the most common interferences to consider when measuring turbidity.

Our devices are designed to mitigate these interferences as much as possible.

Disturbances

What is it / Why is it happening

Effects on Measurements

Bubbles

Bubbles are generally caused by sample outgassing or by adding air to a sample (e.g. by shaking a sample vial).

Bubbles reflect light and can be a significant error, leading to higher readings if not corrected.

Colour

Color is mostly an “aesthetic parameter” and is often caused by decaying organic material (like plants). Most frequently
in surface water, but can also occur in groundwater.

The color of the sample absorbs the incident light, causing false low readings. Please note that devices with an IR light source are not affected by this interference.

Settling of particles

Some particles are too large and/or too heavy to remain in suspension and sink to the ground. Most common in samples with a turbidity value greater than 10 NTU.

May cause stratification of a sample and may result in false low or high turbidity readings.

Scale & Putrefaction

Limescale deposits are usually calcareous and form a mineral deposit on a surface. Rot is often biological and is the "slime" that forms on surfaces.

These are generally more for process monitoring. The problem is that these materials detach from the surfaces and cause spikes in readings that are not representative of the sample.

Condensation

Water that collects on the outside of the sample cup. Forms when moisture in the air comes into contact with a surface that is colder than the air.

Condensation can reflect light and can be a significant error, resulting in higher readings.

Scattered light

Any light that does not come from the sample and reaches the detector is called stray light. Examples are internal reflections or reflections off glass and the penetration of light into the sample chamber. This leads to a positive error in the turbidity measurement.

A measuring device's ability to control the stray light in the measuring chamber is the most important factor for accurate results.

 

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