We use the lung volume bags, and a peak flow meter. I bought a carton of disposable mouthpieces from a local chemist, and the kids get one, label it and keep it to use on both pieces of equipment. It is useful to compare lung functionality (peak flow) with lung capacity. One of the very fit PE teachers here was disappointed to discover that the late 40's not real fit lab tech could blow wa-a-a-ay more than her on a peak flow meter.

Ah, but I have had asthma for years, and have spent time doing breathing exercises.
Once the kids have used both pieces of equipment, the mouthpieces can be chucked out, since they're only cardboard anyway.
We also have used a datalogger with CO2 gas and O2 gas probes to measure how much oxygen was in a breath. It's good for first aid etc.