Compression set is an important parameter to evaluate the foam’s permanent deflection which determines the long term performances,. e.g. sealing, isolation.
Compression set can be measured based on the ASTM D 395 A or B, As shown in the following figure.
In practice, method B is more frequently used to measure compression set. It is worthy to note that Cb is calculated by (to-tn), NOT t0. This mistake can be seen in Internet. The samples are placed in the testing apparatus shown the following to ensure tn. And the detailed tests can be obtained from ASTM website.
A good video might be helpful to guide correct measurements.