In the connection and encapsulation of substrates such as cables by covering members such as heat-recoverable sleeves, typically provided with adhesive or mastic linings, the formation upon heating of a desired hermetic seal between the substrate and the covering member is checked by using a device which can detect the excess air pressure which, once the hermetic seal has been made, builds up within a space between the covering member and the substrate. Preferably this is done by means of a probe which passes between the covering member and the substrate into said space and which is connected to a pressure sensitive device such as a balloon or a manometer. After use, the probe, which is preferably made from a plastics material such as nylon, can be fused or deformed to ensure that the hermetic seal remains intact.