My main problem has been that it is too much work to exhale using any kind of portable machine. I heard that some machines adjust the air pressure as required, but none was of enough help. Trying the HDM Z1 auto travel, it seems to be fine in all respects. I traveled to New York and found it was of great help. However, one thing seems odd. So long as you breathe rhythmically in and out, it works perfectly. Probably it adjusts itself to your breathing pattern and settles into a pressure level. But I am human. I don't breathe exactly the same all the time. Once I take a bit deeper breath and exhale, it changes the pressure *and stays* at that level, making it heavier to exhale after that. What am I missing here? I know it has got algorithms, but don't even get me started on all that rocket science. If there is an issue with this machine, I am sure many would have noticed it (faced it). If not a simple solution, at least I want to know what really is the problem first. Thank you very much for any helpful ideas.