Simplified breath pressure apparatus

In Breath pressure measurements, I described my first attempt at using PVC to make apparatus for breath-pressure measurements, with a 1″ PVC tee to press against my face:

One leg of the tee holds the 2mm air leak, the stem of the tee holds the barbed fitting for connecting to the pressure sensor, and the other leg of the tee is for putting around my mouth.

I suggested

Perhaps I should try again with just a 1/2″ female threaded tee—that may be cheap enough that every student can have their own, and only the barbed fittings (which get no flow through them) would be shared.  Students wouldn’t even need to buy their own—I could have a stock of 50 of them, and wash them in a dishwasher after the lab. 

Today, when I went to the hardware store, I did not find any ½” tees with two female threads, so I simplified the design further, eliminating the screw-in plug and just drilling a 5/64″ (2mm) diameter hole into a ½” elbow (one side slip, the other female pipe thread):

The ½” elbow is small enough that I can put my lips around the opening, which would have been a bit difficult with the 1″ tee.

I was able to get similar expiratory breath pressure measurements with either apparatus, but I had trouble getting a good seal for inspiratory measurements with the ½” elbow—I kept getting leaks at the corners of my mouth. None of my measurements today (with either apparatus) got up to the pressures I observed yesterday.  I’ll practice with it a bit more—maybe adding a short length of ½” PVC that I could put a little further into my mouth.  If I can get it to work consistently, it is certainly a cheap enough solution for every student to have their own—79¢ each for the elbows at the hardware store, but only 20¢ from PVC fittings online.

I was not able to figure out where I had bought the 3/16″ barbed fittings from, but I found some for 62¢ each (in 10s) from Cole-Parmer.  At that price, I might even have the students each buy their own, with only the pressure sensors themselves being reused.