hood the tube 89 is tested in the following manner the dewar flask lever valve 85 is opened the pressure regulator 82 is adjusted until liquid nitrogen exits the orifice 94 at the end of the test sample tube the discharge of liquid nitrogen is readily confirmed by placing an expanded ptfe membrane in the path of the exiting nitrogen and noting wetting of the membrane all fittings and connection are examined to ensure that no leaks are present the tube 89 is then examined for gaseous permeation of nitrogen through its wall along the length of the tube as evidenced by a plume of condensed water vapour in the vicinity of the tube the applied pressure is adjusted until such a steady plume is observed a steady plume indicates both gas permeation and that the air is still in the test environment the plume as described demonstrates that gaseous nitrogen is exiting along the length of the tube 89 which is indicative of distributed evaporative cooling note that the pressure increase in the dewar flask 80 resulting from the evaporation of the nitrogen alone may be sufficient to pressurise the tube 89 the tube under test is allowed to chill for a.