They work at Prestan but sometimes they don't blow themselves up for students or me. Especially after someone uses a mannequin mouth instead of "pocket mask mockup" (humidity in the hallway I guess). The oropharyngeal opening collapses, making lung filling impossible. I cut off the ribbed straws and put the pieces (2 inches) in the mannequin's mouth, which helps, but they will still break in the invisible area of the neck. I haven't found a bag that doesn't, but if someone should say prestan.
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