There are a few issues with this machine compared with the S9:
- It is much louder. After comparing this machine with the S9, Arturo found that the S9 was almost silent in comparison. Subjectively, I rate the difference between the machines at 5dB. The HDM Z1 was so loud my wife refused to let me test it. One interesting thing is that adding the HME to the tube makes the machine quieter, which indicates that the increased noise isn't just due to the louder air pump, but also to do with the resonant frequency of the output into the hose.
- For at home use, there's no elegantly integrated humidifier like the S9. Instead, you buy a Fisher & Paykel humidifier, or you use a HME. HMEs are consumables, and each one is good for only 7 days, which at $6/pop is significant over the usual 2 year depreciation period of a CPAP machine. Since you'll almost certainly need to travel with HMEs, the HMEs add some bulk but not significant weight to the final package.
- The ResMed S9 is an auto-PAP, adjusting pressure according to how much you need to avoid apnea events. The Z1, however, is a fixed pressure CPAP, so you only get to set one setting, and live with it for the entire trip. My 95% pressure was 9, so that's what I used. I initially didn't think there was much difference for me, but at the end of the trip, I switched back to the S9 and immediately felt more refreshed after a night's sleep, indicating that the auto PAP algorithm on the ResMed is more effective and provides better sleep.
- There are reports as to the robustness of the machine, with some users reporting failure after 4 weeks of use. HDM offers a 2 year warranty, but that's of no use to you while you're traveling if your machine fails! My trip was only 3 weeks, so I decided it was worth the risk.