Compression Set
Also known as: permanent set · C-set · CS%
Compression set (CS%) measures how much of an O-ring's original thickness is permanently lost after sustained compression — low compression set (≤25%) indicates the compound will maintain sealing force over time.
Technical Detail
Compression set is measured per ASTM D395 Method B (constant deflection) or ISO 815. A sample is compressed to 25% of its original thickness, held at elevated temperature (typically 70–200°C depending on compound) for 70 hours, then released and measured after 30 minutes recovery. CS% = (t0 – tr) / (t0 – t1) × 100, where t0 = original thickness, t1 = compressed thickness, tr = recovered thickness. Ideal value = 0% (full recovery); practical acceptance limits: ≤20% for dynamic seals (reciprocating pistons, rotating shafts); ≤25% for static O-ring seals; ≤35% for gaskets in low-vibration static flanges. High compression set causes leakage as sealing force drops below minimum. Compression set is significantly affected by cure system: peroxide-cured FKM and EPDM show CS 10–20% at 200°C; bisphenol-cured FKM shows 25–40% at same conditions. Always request CS test data at service temperature, not just ambient.
Reference Standards
- ASTM D395 Method B
- ISO 815-1:2019
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