ASTM D6855 Determination of Turbidity Below 5 NTU in Static Mode

Get Certified ASTM D6855 Testing for Better Water Quality Control 

Turbidity testing shows how clear a water sample is. As a result, treatment plants and manufacturers can tell when the water needs more treatment. Moreover, low-level turbidity data support process control and water-quality checks. Experienced analysts at Matestlab follow standard methods and report clear results across water testing services. Therefore, you can make confident decisions about water treatment. Reliable data also supports consistent product quality.

The primary purpose of ASTM D6855 is to give a consistent way to measure low-level turbidity. Treatment plants and manufacturers use turbidity data to decide when, how, and to what extent they must treat water. However, users must confirm that the method works for water types that ASTM did not test.

The scope of ASTM D6855 includes:

• Material type: Water, including municipal drinking water, ultra-pure water, and low turbidity samples. ASTM states that it tested the method on these water types. Users must confirm that the method is valid for waters of untested matrices.
• Purpose: Determining the turbidity of clean water in static mode. The method gives a turbidity value for low-level measurements under 5.0 NTU. Laboratories and manufacturers use that value to judge water clarity.
• Property measured: Turbidity, expressed in NTU. The standard defines its calibration standards in NTU values. Other assigned turbidity units are assumed to be equivalent, so a 1.0 NTU formazin standard equals a 1.0 FNU standard.
• Method: Static determination of turbidity with a turbidimeter. The analyst calibrates the instrument with turbidity standards and then measures the water sample. ASTM lists the turbidimeter technologies that suit this measurement range.
• Result: A turbidity value below 5.0 NTU for the water sample. The analyst reports the value in NTU. Other assigned turbidity units are treated as equivalent to NTU.
• Test limitations: The method applies only to turbidities under 5.0 NTU. ASTM tested it on specific water types, so users must check validity for other waters. For turbidity above 1 NTU, ASTM D7315 covers static measurement.
• Methodology: Static determination of water turbidity against NTU-defined calibration standards. The analyst compares the sample reading with the calibrated instrument response. The full standard gives the instrument and calibration requirements.
• Applications: Monitoring clean water where clarity matters. Typical examples include drinking water, process water, and high purity industrial water. Manufacturers and laboratories also use it for water-dependent processes.

What are the Uses of ASTM D6855 Testing?

ASTM D6855 gives a standard way to measure the turbidity of clean water. Turbidity is undesirable in drinking water, plant effluent waters, and water for food and beverage processing. Therefore, the method supports several practical uses. The main uses include the following:

• Measuring low-level turbidity: Determines turbidity in water below 5.0 NTU
• Drinking water: Checks the clarity of municipal drinking water
• Process water: Checks water used in manufacturing processes
• High purity industrial water: Measures turbidity in ultra-pure and high purity water
• Quality control: Supports consistent monitoring of water clarity
• Method selection: Suits clean water, while ASTM D7315 suits turbidity above 1 turbidity unit

Why is ASTM D6855 Important?

Turbidity makes water cloudy, which is undesirable in drinking water and in water used for food and beverage processing. Low-level measurements need a consistent method because small differences matter in clean water. ASTM D6855 provides that method for turbidity below 5.0 NTU. Therefore, laboratories and manufacturers can compare results on the same basis. In addition, calibration standards defined in NTU values keep readings consistent between instruments. 

Testing Technique, Process, and Data Collection Guide

To begin, the analyst selects the sample and prepares the turbidimeter. Next, the analyst calibrates the instrument with standards defined in NTU values. After that, the analyst measures the sample in static mode and records the reading. Finally, the laboratory reports the turbidity in NTU. Therefore, data collection centers on one turbidity value for each sample. 

Common Challenges and Troubleshooting in ASTM D6855

The main challenge is the limited range of tested water types. ASTM tested the method on municipal drinking water, ultrapure water, and low-turbidity samples. As a result, users must check that it works for other water types. Sample color is another challenge. Color can interfere with white light instruments. Therefore, analysts choose a near-IR design for colored samples. To troubleshoot, laboratories choose a technology from Table and record it with every result. In addition, they report the matching units, so results compare fairly with historical data. Moreover, samples above 5.0 NTU fall outside the scope, so laboratories use another method, such as ASTM D7315. Users also set their own safety practices, since the standard does not cover every safety concern.

• Laboratory reports turbidity in nephelometric turbidity units (NTU), with lower values generally indicating clearer water and higher values indicating greater cloudiness.
• Analysts confirm that measured turbidity falls within the method’s applicable range, reported here as below 5.0 NTU, and selects an appropriate method for samples outside that range.
• Technician records turbidity readings, instrument identification, calibration standards, and relevant sample details to support reliable interpretation of measurements.
• Analysts verify that the method is suitable for the tested water matrix because sample characteristics can affect turbidity measurements and their interpretation.
• Analysts compare results only when samples are measured using equivalent methods and comparable calibration conditions, documenting any differences that may affect the comparison.
• Engineers interpret turbidity values against applicable water-quality criteria or project requirements rather than treating a single turbidity threshold as a universal acceptance limit.