How can we attach a Spirometer to OpenBCI?

In the www.openbci.com forum it says «The OpenBCI boards are primarily for measuring biosignals using a differential amplifier (plus and minus poles): such signals as EEG, ECG, EMG, EOG, etc. These signals are in the range of say tens (or more) of millivolts (EMG and ECG) down to 1 microvolt (lowest EEG). One might expect …