ISO 8426-2:2025

Hydraulic fluid power - Determination of derived displacement of positive displacement pumps and motors - Part 2: Zero-pressure intercept method

This document specifies the zero-pressure intercept method for the determination of the derived displacement of hydraulic fluid power positive displacement pumps and motors under steady state conditions. A single value for the derived displacement is determined from measurements at a single shaft speed.

Transmissions hydrauliques — Détermination de la cylindrée calculée des pompes et moteurs volumétriques — Partie 2: Méthode d'interception à pression nulle

General Information

• Status: Published
• Publication Date: 11-Aug-2025
• ICS: 23.100.10 - Pumps and motors
• Technical Committee: ISO/TC 131/SC 8 - Product testing
• Drafting Committee: ISO/TC 131/SC 8 - Product testing
• Current Stage: 6060 - International Standard published
• Start Date: 12-Aug-2025
• Due Date: 21-Sep-2025
• Completion Date: 12-Aug-2025

Relations

• Revises: ISO 8426:2008 - Hydraulic fluid power - Positive displacement pumps and motors - Determination of derived capacity
• Effective Date: 02-Jul-2022

Overview

ISO 8426-2:2025 - Hydraulic fluid power - Determination of derived displacement of positive displacement pumps and motors - Part 2: Zero-pressure intercept method - defines a standardized test and calculation method to determine the derived displacement (also called derived capacity) of positive displacement pumps and motors under steady‑state conditions. The standard specifies obtaining a single derived displacement value from measurements at a single shaft speed by extrapolating high‑pressure flow to zero differential pressure using the zero‑pressure intercept method.

Key topics and technical requirements

• Scope: Applies to positive displacement pumps and motors tested under steady‑state conditions; produces one derived displacement value per set of test conditions (shaft speed, fluid temperature, displacement setting).
• Measurements: Record high‑pressure flow rates (qve,i), inlet/outlet/case pressures, differential pressures (Δp), shaft speed (n) and inlet temperature (θ1) in accordance with ISO 4409:2019 test practices.
• Data points: A minimum of five steady‑state differential pressures is recommended, increased in equal increments from minimum to maximum differential pressure.
• Special cases:
  • Variable displacement units: follow ISO 4409:2019 subsections for variable units.
  • Reverse flow: if displacement depends on flow direction, determine derived displacement for each direction.
  • Pump vs motor mode: test in the intended mode; values can differ between modes.
• Calculation:
  • Use linear regression (ordinary least squares) on qve versus Δp to determine the zero‑pressure intercept qve,0.
  • Convert intercept to derived displacement: V = qve,0 / n.
• Reporting: Test report must include test ID, unit description, shaft speed and pressure ranges, test circuit diagram, fluid properties, measurement accuracy, measured and derived values (including correlation coefficients) and reference to ISO 8426-2:2025.

Applications and who uses it

• Hydraulic component manufacturers use ISO 8426-2:2025 for product characterization, performance data sheets and quality control.
• Test laboratories and certification bodies apply the method to produce repeatable, comparable displacement measurements.
• System designers and engineers use derived displacement figures for sizing, simulation and troubleshooting of hydraulic circuits.
• R&D and validation teams rely on the zero‑pressure intercept method for performance benchmarking and correlation with volumetric efficiency.

Related standards

• ISO 4409:2019 - Methods of testing pumps, motors and transmissions (referenced for measurement procedures)
• ISO 5598 - Fluid power vocabulary (terms and definitions)
• ISO 1219‑1, ISO 4391 - Graphical symbols and parameter symbols (referenced for symbols/units)

Keywords: ISO 8426-2:2025, hydraulic fluid power, derived displacement, zero‑pressure intercept method, positive displacement pump testing, hydraulic testing standards.

Standards Content (Sample)

International
Standard
ISO 8426-2
First edition
Hydraulic fluid power —
2025-08
Determination of derived
displacement of positive
displacement pumps and motors —
Part 2:
Zero-pressure intercept method
Transmissions hydrauliques — Détermination de la cylindrée
calculée des pompes et moteurs volumétriques —
Partie 2: Méthode d'interception à pression nulle
Reference number
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
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Phone: [phone]
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Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and units. 1
5 Test procedure . 2
5.1 General requirements .2
5.2 Number of steady-state differential pressures .3
5.3 Variable displacement unit .3
5.4 Reverse flow unit .3
5.5 Unit used as pump or motor .3
6 Calculation of derived displacement . 3
7 Test report . 4
Bibliography . 6

iii
Foreword
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This document was prepared by Technical Committee ISO/TC 131, Fluid power systems, Subcommittee SC 8,
Product testing.
This first edition of ISO 8426-2 cancels and replaces the second edition (ISO 8426:2008), which has been
technically revised.
A list of all parts in the ISO 8426 series can be found on the ISO website.
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iv
Introduction
In hydraulic fluid power systems, power is transmitted and controlled through a liquid under pressure
within an enclosed circuit. Two types of components of such systems are the positive displacement pumps
and motors. One of the technical parameters of these components is the derived displacement, also known as
derived capacity. This document is intended to describe the zero-pressure intercept method for determining
the derived displacement of hydraulic fluid power positive displacement pumps and motors. The term
derived displacement is preferred over derived capacity.
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