groove however single spring strips have been found to be preferred because of their simpler structure for a good understanding of fig3 and the structure shown in fig1 and 2 the method of construction is considered in particular plug tube 10 is first provided with a set of spirally disposed grooves 15 extending approximately half way through the tube material such grooves do not significantly weaken the strength of plug tube 10 however a circumferential groove in plug tube 10 of the desired depth would extend most if not all of the way through the vent tube itself accordingly plug tube 10 is provided with a coaxially disposed joining ring 33 which is affixed to the inside of plug tube 10 at the desired location for the circumferential groove or grooves joining ring 33 is then affixed to plug tube 10 by any convenient means such as by brazing welding or the use of adhesive materials with joining ring 33 in place a circumferential groove of appropriate depth is machined into plug tube 10 and also preferably at least partially into joining ring 33 spring strips 20 and 30 are then disposed in their respective circumferential grooves and their ends attached so as to hold the strips in place this joining is typically accomplished by soldering the common portion of the.