ISO 25188:2026

• Abstract:

  This document specifies two methods for the determination of the density of glass fibre: Method A: Gas pycnometer method; Method B: Liquid pycnometer method.

• Status: Published
• Publication Date: 12-Nov-2026
• ICS: 59.100.10 - Textile glass materials
• Technical Committee: ISO/TC 61/SC 13 - Composites and reinforcement fibres
• Drafting Committee: ISO/TC 61/SC 13/WG 1 - Reinforcements and reinforcement products
• Current Stage: 6060 - International Standard published
• Start Date: 13-Nov-2026
• Completion Date: 13-Nov-2026

ISO 25188 is an ISO standard for measuring the density of glass fibre. It specifies two pycnometer-based methods for glass fibre tested as a yarn: a gas pycnometer method and a liquid pycnometer method. The document is written for laboratories, manufacturers, and users who need a practical way to determine density for material characterization and composite work.

What does ISO 25188 specify?

ISO 25188 specifies how to determine the density of glass fibre using Method A, the gas pycnometer method, or Method B, the liquid pycnometer method. The scope applies to glass fibre tested as a yarn, and the methods are also described for powder, chopped fibre, yarn, and roving.

The document is organized into six clauses: Scope, Normative references, Terms and definitions, Conditioning and test conditions, Test methods, and Test report. Clause 5 contains the two measurement routes, while Clause 6 states what the report has to record so results can be traced and compared.

What are the key requirements of ISO 25188?

ISO 25188 focuses on consistent specimen preparation, controlled temperature and humidity, and a defined calculation route for each method. In practice, that means the laboratory must control conditioning, choose the right sample preparation for the fibre form, and record the method details needed to make the result usable.

Conditioning and test atmosphere

Clause 4 requires test specimens to be conditioned in the standard test atmosphere from ISO 291, and the apparatus and specimens are then kept at the same conditions during testing. The preferred conditions are 23 C and 50 % relative humidity, which matters because density measurements are sensitive to temperature and moisture.

Method A - gas pycnometer method

Clause 5.1 bases Method A on Boyle's law and uses a gas pycnometer to determine specimen volume from pressure changes. Helium is preferred because it penetrates small pores more effectively, but nitrogen is also allowed if it is non-corrosive and non-adsorbing.

ISO 25188 requires the gas pycnometer to comply with ISO 12154:2014 and to be calibrated as specified there. The sample chamber is weighed before and after filling, the chamber is checked for leaks, the air is replaced by measurement gas or removed with a vacuum pump, and pressure readings are taken for the calculation. The result is the mean of five density values once their standard deviation is below 0.000 5 g/cm3, so the operator has a repeatability check built into the method.

Method B - liquid pycnometer method

Clause 5.2 uses Archimedes' law and a liquid pycnometer filled with distilled water. This method is built around the displaced-water volume of the fibre, so the preparation steps are important for removing size and air.

The document calls for size removal, grinding the specimen to a short fibre length, washing with water and ethanol, drying, and cooling in a desiccator before weighing. The pycnometer is then filled with debubbled distilled water and held at 23 C for at least 1 hour before the final weighing steps. The calculation uses the density of distilled water at the measured temperature, so temperature control is part of the measurement, not just a background condition.

Test report

Clause 6 requires the report to identify the fibre sample, state whether Method A or B was used, note whether the fibre was size-free and, if it was desized, give the calcination temperature. It also records the gas used or the density of distilled water, the number of specimens, the mean density, any extra operations or incidents, and the test date.

What terms does ISO 25188 define?

ISO 25188 defines density as mass per unit volume at a specified temperature. That definition matters because the result is only meaningful when the temperature is stated.

The document also uses these working terms:

• Desized - size has been removed from the fibre, typically by calcination as referenced in ISO 1887.
• Size-free - fibre that has had its size removed.
• Gas pycnometer - an instrument that determines specimen volume from the pressure change of a confined gas.
• Liquid pycnometer - a pycnometer used with distilled water to determine volume by liquid displacement.
• Standard test atmosphere - the conditioning atmosphere taken from ISO 291.
• Reference chamber - the known calibrated volume used with the sample chamber in Method A.
• Sample chamber - the chamber that holds the fibre specimen during the gas pycnometer measurement.

Who uses ISO 25188?

ISO 25188 is used by glass fibre manufacturers, composite material laboratories, and quality control teams that need density data for glass fibre products. It also supports users who calculate mechanical properties, fibre volume content, and composite structural design parameters. Buyers and suppliers can use it to agree on a common test method when density data forms part of material acceptance or comparison.

Which standards are used with ISO 25188?

ISO 25188 cites several companion standards that support the test methods:

• ISO 291 - provides the standard atmosphere for conditioning and testing.
• ISO 1887 - provides the calcination method used to remove size.
• ISO 3696 - specifies the distilled water used in Method B.
• ISO 12154:2014 - gives the gas pycnometer basis and calibration method used in Method A.

What does the ISO 25188 document contain?

ISO 25188 contains two complete measurement procedures, each with its own principle, apparatus list, specimen preparation, calculation route, and reporting items. It includes a gas-pycnometer formula based on chamber masses and pressures, and a liquid-pycnometer formula based on pycnometer masses and the density of water.

The document also gives practical operating details such as leak checking, conditioning, gas purity, temperature control, removal of air bubbles, and the repeatability rule for Method A. For Method B, it provides the sequence for cleaning, drying, filling, equilibrating, and weighing the pycnometer.

International
Standard
ISO 25188
First edition
Glass fibre — Determination of
2026-11
density
Fibre de verre — Détermination de la masse volumique
Reference number
© ISO 2026
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Conditioning and test conditions . 1
5 Test methods . 2
5.1 Method A: Gas pycnometer method .2
5.1.1 Principle .2
5.1.2 Apparatus and materials .2
5.1.3 Test specimens .2
5.1.4 Procedure .2
5.2 Method B: Liquid pycnometer method .3
5.2.1 Principle .3
5.2.2 Apparatus and materials .3
5.2.3 Test specimens .4
5.2.4 Procedure .4
6 Test report . 4

iii
Foreword
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iv
Introduction
The density of glass fibres is an important parameter for determining the physical properties of glass fibres
and for calculating the mechanical properties of glass fibre composites. Determination of the density of glass
fibres affects the characterization of the strength and modulus of glass fibres and composites, the testing
and calculation of the volume content of fibres in composites, and the structural design of composites.
Glass fibre manufacturers can measure the density of glass before it is made into fibre. In addition, glass
fibre users can measure the density of glass fibre according to ISO 12154. However, since ISO 12154 gives a
general method for measuring the skeleton density of a solid material samples and is not optimized for glass
fibre including pre-treatment, a density measurement standard for glass fibre should be established.
The two test methods given in this document are applicable to the glass fibre powder, chopped fibre, yarn
and roving.
v
International Standard ISO 25188:2026(en)
Glass fibre — Determination of density
1 Scope
This document specifies two methods for the determination of the density of glass fibre:
— Method A: Gas pycnometer method;
— Method B: Liquid pycnometer method.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 291, Plastics — Standard atmospheres for conditioning and testing
ISO 1887, Textile glass — Determination of combustible-matter content
ISO 3696, Water f
...