Geotextile Characteristic Opening Size (COS) Tester – QualiGTX™ COS
The QualiGTX™ COS Geotextile Characteristic Opening Size Tester measures the O90 characteristic opening size of
woven, nonwoven, and composite geotextiles using a controlled wet sieving procedure. This measurement establishes
the effective pore size of a geotextile and helps engineers determine whether a filter fabric can retain the
surrounding soil while allowing water to pass through. During testing, graded granular particles are placed on a
single-layer, unstressed specimen secured within a Ø176 mm clamping ring. A continuous water spray washes particles
through the fabric as the sieve assembly vibrates. The collected material is subsequently dried and graded to
establish the characteristic opening size used in European product declarations and geotextile filter design
calculations.
Designed by Qualitest to satisfy the equipment requirements of the international wet sieving method, the QualiGTX™
COS combines controlled vibration, continuous water distribution, and corrosion-resistant construction. All
water-exposed components are manufactured from 304 stainless steel, providing reliable resistance to prolonged
contact with water while preventing corrosion from affecting the collected test material. Its 550 W motor operates
at 2800 rpm to produce a sieve deck amplitude of 1.5 mm. An integrated programmable timer supports test durations of
up to 999 seconds, allowing operators to establish consistent sieving intervals without continuous manual
supervision.
The instrument is supplied with the graded standard particle fractions specified by the test method, eliminating the
need for laboratories to source these materials separately before commissioning. It supports conformity assessment,
routine production monitoring, and filtration performance investigations for geotextile manufacturers, notified
bodies, certification laboratories, and university research facilities. With a total weight of 110 kg and dimensions
of 700 × 550 × 1100 mm, the tester can be installed in a standard laboratory workspace alongside the drying and
particle grading equipment required for the complete procedure.
Geotextile Characteristic Opening Size (COS) Tester – QualiGTX™ COS Applications
The characteristic opening size establishes how effectively a geotextile interacts with the soil it is intended to
filter. Determining this property is therefore an important step in selecting suitable filtration materials and
evaluating their performance across civil engineering and geotechnical applications.
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CE Marking and Conformity Declarations: Determines the characteristic opening size required for geotextile
manufacturers to prepare product declarations under harmonized European product standards.
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Subsurface Drainage Filter Design: Supports the selection of geotextile filters by comparing their opening
dimensions with the surrounding soil grading, helping limit soil migration and filter blockage.
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Coastal and Marine Revetment Works: Evaluates filtration fabrics installed beneath rock armor and concrete
protection units, where repeated wave action can displace fine soil particles.
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Landfill Drainage and Leachate Collection: Assesses the ability of separation geotextiles positioned above
drainage collection layers to retain fine particles from the overlying cover soil.
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Dam and Levee Filter Zones: Provides characteristic opening size measurements when geotextile fabrics are
used as alternatives to graded granular filter materials within embankment structures.
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Geotextile Manufacturing Quality Control: Monitors changes in opening size between production batches,
helping identify variations associated with fiber blends, needling density, and weaving parameters.
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Nonwoven and Woven Product Development: Examines how different manufacturing constructions influence pore
distribution and opening characteristics during the development of new geotextile products.
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Third-Party Certification Laboratories: Produces documented O90 test results for independent conformity
assessments, customer certification programs, and engineering project submissions.
Standards
The QualiGTX™ COS is designed around the equipment specifications established by the internationally recognized wet
sieving procedure for measuring the characteristic opening size of geotextiles. Its configuration supports the
prescribed specimen mounting, particle separation, and wet sieving operations required by the following standard:
- ISO 12956 – Geotextiles and Geotextile-Related Products: Determination of the Characteristic Opening Size:
Specifies the wet sieving procedure used to establish the characteristic opening size of geotextile materials.
The QualiGTX™ COS provides the apparatus configuration required to carry out this determination.
Geotextile Characteristic Opening Size (COS) Tester – QualiGTX™ COS Key Features
The QualiGTX™ COS integrates controlled mechanical agitation, continuous water spraying, and a supplied graded
particle set to support consistent characteristic opening size measurements. Its construction and operating controls
address the principal requirements of the wet sieving procedure.
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Ø176 mm Specimen Clamping Ring: Provides a defined mounting area for securing a single-layer geotextile
specimen in a flat, unstretched condition, maintaining the specimen arrangement specified by the test
method.
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304 Stainless Steel Wetted Components: Protects water-contact surfaces against corrosion during repeated wet
sieving operations while minimizing the risk of contamination in the recovered particle fraction.
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Graded Standard Particle Set Included: Supplies the standard particle fractions required for testing,
allowing laboratories to prepare for operation without arranging separate material purchases.
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1.5 mm Vibration Amplitude: Maintains the specified sieve deck movement, equivalent to 3 mm total travel, to
deliver consistent mechanical action to the granular material throughout testing.
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50 Hz Vibration Frequency: Maintains continuous particle movement across the specimen, increasing the
opportunity for individual grains to encounter different fabric openings during the test.
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550 W Motor at 2800 rpm: Provides the drive power needed to maintain steady sieve agitation throughout the
selected testing period.
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Programmable Timer to 999 Seconds: Allows operators to define and repeat test durations accurately, reducing
variations associated with manual timing.
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Continuous Water Spray System: Directs water through the particle bed to transport fine material across the
geotextile and help release particles that become lodged within its structure.
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Compact 110 kg Frame: Combines a 700 × 550 × 1100 mm overall footprint with a laboratory-oriented design,
allowing space for the subsequent drying and grading operations.
Theory and Method
The filtration function of a geotextile depends on its ability to transmit water without allowing unacceptable
quantities of surrounding soil to pass through. Its internal opening dimensions therefore play a central role in
determining filtration behavior. The wet sieving procedure evaluates these openings by using the geotextile specimen
itself as the separation medium.
For the test, a single layer of geotextile is secured across the clamping ring without applying tension. A measured
quantity of graded granular material is distributed over the specimen before the sieve assembly begins vibrating
under a continuous water spray. As the particles move across the fabric, grains small enough to pass through its
interconnected openings are carried into a collection pan. The recovered material is then dried and separated by
grading to establish its particle size distribution. The diameter corresponding to the point at which 90% of the
passing material is finer is reported as the characteristic opening size, designated O90.
Water is essential to the separation process because fine particles can adhere to geotextile fibers and to
neighboring grains through surface-charge and electrostatic effects. Under dry sieving conditions, these
interactions can prevent particles from passing through the fabric even when their dimensions are small enough to
negotiate its openings. The continuous spray reduces these adhesion effects, transports fine particles toward the
available openings, and helps release grains that have become partially lodged within the fabric structure. This is
particularly important when testing needle-punched nonwoven geotextiles, where the thick and complex fiber network
can retain particles under dry conditions and produce less representative results.
Maintaining the specimen without tension is another important aspect of the procedure. Applying a stretching force
can separate the fibers within a nonwoven structure and enlarge its openings, resulting in measurements that
overstate the opening size the fabric would exhibit under soil loading. The QualiGTX™ COS avoids this effect by
holding the specimen flat across the Ø176 mm ring without introducing strain.
Throughout the test, the sieve deck operates at a 1.5 mm amplitude, corresponding to 3 mm of total travel, with a
vibration frequency of 50 Hz. This controlled movement keeps the granular material circulating over the geotextile
rather than allowing it to remain concentrated in one area. As particles repeatedly move across the specimen, they
encounter different openings, supporting a representative assessment of the fabric's characteristic opening size.
Geotextile Characteristic Opening Size (COS) Tester - QualiGTX™ COS Technical Specifications
| Parameter |
Specification |
| Test Principle |
Wet sieving, geotextile used as the sieve medium under water spray |
| Measured Property |
Characteristic opening size (O90) |
| Applicable Materials |
Woven, nonwoven, and composite geotextiles and related products |
| Specimen Clamp Inner Diameter |
Ø176 mm |
| Specimen Mounting |
Single layer, unstressed |
| Vibration Frequency |
50 Hz |
| Vibration Amplitude |
1.5 mm (3 mm vibration height) |
| Timer Range |
0 to 999 s |
| Motor Speed |
2800 rpm |
| Motor Power |
550 W |
| Wetted Component Material |
304 stainless steel |
| Included Accessories |
Graded standard particle set |
| Voltage |
AC 380 V, 50 Hz |
| Dimensions (W × D × H) |
700 × 550 × 1100 mm |
| Weight |
110 kg |
