How to read an O-ring chemical compatibility chart

A chemical compatibility chart is the fastest way to verify whether your chosen elastomer survives the medium in your application. Rows are chemicals, columns are materials, and at the intersection is a resistance rating. To read the chart correctly you need to understand three things: what the rating tiers mean, why swell is the decisive quantity, and what the chart itself cannot tell you.

The four-tier rating scale

The standard scale (per the Parker O-Ring Handbook) has four usable tiers plus an "unknown" escape hatch. It is based mainly on volume swell and hardness change of the material in the medium.

• Excellent: volume swell up to 5 %, hardness change up to 10 %. Suitable for long-term service.
• Good: swell 5–15 %. Acceptable for normal service.
• Marginal: swell 15–30 %. Short-term or static seals only.
• Poor: swell over 30 % or rapid degradation. Avoid.
• Unknown: no published data — verify with the vendor, do not guess.

Why swell?

Swell is the headline quantity because it ties directly to seal function. A small amount of swell (up to 15 %) can even be beneficial — it tightens a static groove seal. Above 30 %, though, the ring loses elasticity, overfills the groove and, in a dynamic application, wears or extrudes quickly. Beyond swell, watch hardness change: a material whose plasticisers are leached out by the chemical hardens and cracks.

Temperature changes everything

A rating in the chart holds at a particular temperature — usually room temperature or slightly elevated. At a higher temperature the chemical reaction runs faster, so a material rated "good" at 20 °C may be only "marginal" at 60 °C. Good charts therefore quote a temperature window for each rating. Always compare that window with your actual operating temperature.

Using the chart in practice

1. Identify your medium precisely, including concentration (sulfuric acid at 10 % and at 98 % behave completely differently).
2. Find the medium’s row and read the ratings across the candidate materials.
3. Pick a material rated "excellent" or "good" at your temperature.
4. With multiple media (a mixture, or sequential contact fluids) the material must satisfy every one — the worst rating governs.
5. For an "unknown" rating, or a mixture with no data, contact the supplier rather than guessing.

Limits of the chart

The chart is an excellent first filter, but it does not replace a real-application test for critical uses. It does not account for pressure, contact time, concentrations outside the listed levels, or the synergy of several substances. Use it to narrow the choice to one or two materials, then confirm those by trial where it matters. Our online chart covers the 400 most common media across 10 materials — when a medium is missing or the rating is "unknown", get in touch.