ISO 3968:2017

Overview

ISO 3968:2017 - Hydraulic fluid power - Filters - Evaluation of differential pressure versus flow - defines a standardized procedure to evaluate how differential pressure (pressure drop) across hydraulic filters varies with flow rate and fluid viscosity. The standard provides a clear basis for agreement between filter manufacturers and users and covers testing of complete filter assemblies, spin-on filters and any valves in the flow stream.

Key topics and requirements

• Scope of testing: Measurement of differential pressure across relevant parts of a filter assembly (housing/head/bowl), spin-on filters and in-line valves under varying flow rates and viscosities. Typical filter types defined: Type 1 (spin-on no head), Type 2 (spin-on with head), Type 3 (replacement-element assemblies).
• Test equipment and setup: Requirements for pump, reservoir, temperature control, clean-up filter, sampling valve and mounting. Emphasis on controlling fluid temperature and viscosity during tests.
• Measurements: Procedures for pressure measurement (inlet/outlet), temperature, kinematic viscosity, flow rate and fluid cleanliness. Attention to instrument accuracy and test-condition tolerances.
• Test procedure: Steps include pipework correction, ensuring cleanliness of test circuit, test sequences for whole filters, individual elements, heads/spin-on combinations, and bypass/anti-drain back valves.
• Bypass and valve tests: Determination of full-flow characteristics, opening and closing pressures, and leakage rates for any element by-pass valves.
• Data presentation: How to record and present differential pressure vs. flow curves and identification statements. Annex A introduces the power-loss equation and calculation methods.
• Terminology and references: Uses ISO 5598 vocabulary and references to related fluid contamination and viscosity standards.

Applications and users

ISO 3968 is practical for organizations involved in hydraulic system design, maintenance and procurement:

• Filter manufacturers - to characterize product hydraulic performance and publish differential-pressure vs. flow data.
• Test laboratories and R&D teams - to validate design changes and compare media/housing impact on pressure loss.
• System engineers and specifiers - to select filters that meet pressure-drop limits at rated flow and specified fluid viscosities.
• Maintenance and reliability engineers - to assess expected impact of filters on system pressure and pump power loss.

Keywords: hydraulic filters, differential pressure, flow rate, viscosity, filter testing, ISO 3968, spin-on filters, filter assembly, contamination control.

Related standards

Normative references cited in ISO 3968 include:

• ISO 5598 (vocabulary), ISO 4406 (contamination coding), ISO 4021 (fluid sampling), ISO 3448 and ISO 2909 (viscosity), ISO 3675 (density), ISO 6415 (spin-on filter dimensions).