I bet there are interesting subtle dynamics beyond rate. Maybe interesting to record the acoustics inside the spirometer.
What's the product model? Prescription digital behavior change??
I imagine the digital version using a range finder at the top of the tube to calculate how high you raise the thing you're raising by breathing in and then calculate and report the volume of air you inhaled.
If you combine this with an app you've got a little product going. App can track progress over time. Show you some graphs. Remind you to use the spirometer.
Sell the digital spirometers to hospitals who sometimes give them to patients who have respiratory problems. Get some data to prove the digital version works better and then get insurance companies to cover it or buy it.