A critical test of the protection afford by the Parylene coating is to coat circuit board test patterns (as described in MIL-I-46058C) and subject them to insulation resistance measurements during a temperature-humidity cycle (MIL-STD-202, methods 106 and 302).
In brief, this test consists of ten cycles (one cycle per day), with each cycle consisting of seven steps. The seven steps range from low temperature, low humidity (25° C, 50% RH) to more severe conditions (65° C, 90% RH). Resistance readings are taken initially and at the 65° C, 90% RH step for each cycle of the ten day test.