Test Standards DIN ISO 815-2 respectively ISO 815-2
At low temperatures rubber materials continually lose their elastic restoring force which leads to the fact that they can only bridge gap sizes to a limited degree and slow down considerably in their reset rate. For characteristic static O-ring applications it can be assumed that O-rings start leaking only above a compression set value of 90 to 100%. Therefore, it can be ensured within the specifications that a maximum compressions set value at the lower operation temperature is specified; often the value after 30 minutes relaxation is used for this purpose. However, in some standards (DIN EN 549) the functional criteria is defined a lot more exactly, namely a maximum compression set value of 50% at the lower operational temperature, testing time 72h, measurement after 30 minutes. As the recovery of the specimen takes place much slower than at high temperatures due to the low temperatures, it is possible that it is not completed after 30 minutes. Therefore, the standard DIN ISO 815-2 respectively ISO 815-2 includes measurements until 2 h after relaxation.