Volume change is either the increase (swell, as in Figure 28) or decrease (shrinkage, as in Figure 29) in the volume of a specimen which has been in contact with a fluid. This contact may range from occasional “splashing” to constant immersion. Any resulting volume change can range from minor (indicating there is a relative compatibility between the fluid and the specimen) to major (indicative of incompatibility). Volume change is typically noted as a percentage of the original volume. For example, a specimen that swells to twice its original volume is said to have undergone a 100% increase.

An elastomeric seal typically becomes softer as a result of swell, whereas shrinkage generally hardens the seal. A slightly swollen seal is, in most cases, still functional. A limited amount of swell may even compensate for other variables, such as compression set. Shrinkage, on the other hand, can exacerbate an already-existing compression set problem. With some of its soluble components (such as plasticizer) having been extracted by system fluid, an O-ring seal that has undergone shrinkage is more prone to leaks.

As described in ASTM test method D 471, volume change testing typically employs ASTM and Industry Reference Material (IRM) oils, as well as ASTM Reference Fuels, service liquids, and Type IV Reagent Water. Regardless of the liquid in use, testing involves immersing a material sample (of known properties) in the liquid for a specific period of time (e.g. 70 hours) at a specific temperature (e.g. 100° C ± 2°), both variables based on the conditions expected in service. Material deterioration (if any) is then determined based on changes in physical properties, including volume.

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