Have already the third package in the use. Se good evaporator when they work for and keep. Significant variations in the quality and durability. Some are broken again after 1 day, others hold up to 2 weeks. That puts the price then clear. In each pack I have 2 duds (so I call heads only 1-2 days to keep) - ie 3 out of 5 are good. That's not really OK