Haze Meter

Method for Measuring Turbidity at Low Solid Concentrations

In the clean water and pharmaceutical industries, controlling the liquid’s turbidity is an extremely critical factor. When the required turbidity is below 1 unit, we are dealing with suspended solids that are nearly invisible. At this level of purity, even a very minor error in the measurement procedure can lead to an incorrect assessment of product quality. The following article shares practical experience to achieve the best measurement results for ultra-pure water.

Why Measuring Turbidity Below 1 Unit is a Technical Challenge

When turbidity is already very low, the suspended particles in the water are small and sparsely distributed. The scattered light collected by the meter is very weak, and the biggest problem is that the actual signal is easily contaminated by noise.

Every measuring device generates a certain amount of inherent stray light emitted by the internal components. When measuring highly turbid water above 50 units, the true signal is strong, so the noise is negligible.

How to Deal with Fingerprints and Air Bubbles on Sample Cells

Practical experience shows that over 80% of measurement errors in the low range do not originate from the device but from the sample preparation stage.

A single fingerprint or a small water streak on the body of the sample cell can cause scattered light to deviate and result in an error. Always wipe the cell with a lint-free tissue in one direction, and only hold it by the cap or the neck.

Tiny air bubbles are also a common cause of error because they scatter light very strongly, causing the machine to mistake them for solids. The simple solution is to gently tap the side of the cell or let the sample rest for a few minutes to allow all the bubbles to escape. Some specialized meters have a specially designed measuring chamber or a continuous measurement and automatic averaging mode to minimize the impact of residual air bubbles.

Infrared Lamp Technology Helps Eliminate the Influence of Water Color

In natural water or treated wastewater, a faint yellow color sometimes exists due to dissolved organic compounds. This color can absorb a portion of the light used for turbidity measurement, thereby skewing the results.

Low-range turbidimeters often use an infrared (IR) light source because infrared light is less absorbed by organic colors than white light. Consequently, the measurement result accurately reflects the turbidity caused by suspended particles instead of being affected by the water's background color. This is an important technical criterion when selecting equipment for mildly colored water samples.

For labs that need to monitor low-level turbidity, a small NTU range, and require stable repeatability, the CHN SPEC DH-10 model is a suitable choice. The device uses an IR source according to the ISO standard, provides stable measurement for clean water and drinking water, features a simple interface, and is quick to operate, making it suitable for repeated daily measurements.

See more products at: CHN SPEC Turbidity Meter

The Benefit of Measuring Light at Multiple Angles with Ratio Technology

Standard meters only measure scattered light at a 90-degree angle, and this is prone to interference.

More sophisticated meters use Ratio Technology, which means they collect light at multiple angles: 90-degree scattered angle, 0-degree transmitted light, and sometimes even 180 degrees. From this data, the meter calculates the ratio between the signals to eliminate stray light and reduce the influence of sample color.

Ratio Technology is especially necessary when working with water turbidity below 0.1 unit, as it helps the results become much more stable than the traditional single-angle measurement method.

When to Switch to an Automatic Online Turbidity Monitoring System

Measuring samples manually with a handheld meter only provides a single point-in-time snapshot of water quality.

In large water treatment plants, pharmaceutical, or beverage facilities, continuous monitoring is mandatory to safeguard the entire process. Online turbidimeters are installed directly on the pipeline, capable of measuring every minute or even every second.

This system helps immediately detect issues such as filter membrane tears and generates timely alerts. Through automatic monitoring, you can avoid the risk of ruining an entire product batch due to a short-term incident and maintain more stable output water quality.