Abstract: A circuit handler provides a controllable environment for monitoring of an electrical device which is being subjected to a test procedure. The controllable environment may correspond to temperature extreme on the order of -55.degree. C. and +155.degree. C. The circuit handler has a support base with a conditioning input for receiving the conditioning fluid, which may be heated air or super-cooled nitrogen or hydrogen, a control input for receiving pneumatic signals, and a pressurized air input for forming an air curtain which prevents the cold conditioning fluid from forming ice on the transport rails. The mechanical operations within the support base are performed by pneumatic solenoids to avoid the generation of stray magnetic or electrical fields which might interfere with the electrical testing process. Delivery of the conditioning fluid to the test site, and possibly a pre-soak site, is achieved by manifold depressions formed on the underside of a test site portion of the transport rail.