Great to see this development but surprised more mHealth developers aren’t trying to commercialise efforts in this area as it’s going to be key to treatment of many common conditions (eg Asthma, COPD, CF, etc) in the very near future.
From some research work I’ve been involved in I think it’s likely that more reliable data is going to be found through the analysis of sound produced by a calibrated 50c whistle and it could be even less intrusive for patients to use (pitch of whistle can be >20kHZ and so out of range of humans), possibly more hygenic (the airways can provide harbour to virulent bacteria) and could open the opportunity to quantify some more complex expiratory maneuvers but I’d really like to see an EveryHeartBeat style project to solve this challenge.
HatTip: Eric Topol
mHealth Insight 
As a life long suffer of asthma and a physiology professor, I think this is great! You can measure lung volume and be proactive about treatment.