Most likely an abbreviated (jargon) term for testing using alternating humidity cycles with condensation (in an engineering context, 'wissel' normally means 'alternatinig'). A possible English 'equivalent' would be 'cyclic humidity testing with condensation' or possibly 'cyclic condensation (testing)'

Sample refs (Google hits):

Testing Equipment
... QCT condensation tester. ... The tester expose the materials and specimens to alternating cycles of ... and concentration or air at elevated humidity and temperature. ...
www.itlinc.com/sEnvironmentalEquip.html

Matco Associates, Inc. - Humidity Chamber
... Humidity chamber testing is widely used, and it emphasizes the sensitivity of ... so that the specimen is exposed to alternating humid air and condensation. ...
www.matcoinc.com/humidity_chamber.php

Sharetree Systems Humidity Testing
... Cyclic chambers can be programmed to vary temperature and humidity separately or
together and even ensure condensation on the product. ...
www.sharetree.co.uk/product_env_humid.html