Compatibility charts are useful and widely misused. They summarise immersion tests, often at room temperature, and often for a simple pure fluid.
What a rating usually means
- A or 1 (excellent): less than 10 percent volume swell, little hardness change.
- B or 2 (good): 10 to 20 percent swell, small property change.
- C or 3 (fair): 20 to 40 percent swell or marked changes. Useful only for static, short-term use.
- D or 4 (poor): severe swell or degradation. Avoid.
Swell is only part of the picture. A compound may swell little while extractable plasticiser leaves, causing shrinkage after drying.
What charts hide
- Temperature. A chart at 23 degrees C says nothing about 150 degrees C.
- Concentration. Dilute and concentrated acids behave very differently.
- Mixtures. Real fluids contain additives, inhibitors and contaminants that charts omit.
- Duration. Many tests last 7 days. Long service may bring more change.
- Compound differences. One FKM may be rated A in a fluid where another is C.
- Dynamic effects. Swollen rubber wears faster.
- Gas permeation and decompression. Not shown in liquid immersion ratings.
Turn the chart into a test plan
- Pick candidates rated A or B at the lowest temperature.
- Soak real-size specimens in the actual fluid at maximum temperature (ASTM D471 or ISO 1817).
- Measure volume, mass and hardness before and after, and after drying.
- Check compression set in the fluid.
- Run a short functional test.
Thresholds
For static seals most users accept volume change between -3 percent and +15 percent. Dynamic seals tolerate less, around -3 to +10 percent. Shrinkage is a leak risk, since the ring pulls away from the surfaces.
Documenting
Keep the report with fluid batch, temperature, exposure time and method. Then the result can be reused for later orders.
About this article. It is original writing prepared from public sources, standards and general industry practice. It is not a copy of any book or standard and it is not engineering advice. Confirm values against the current edition of the standard and test in your own application. Only analyse material you own or are entitled to analyse.
