Several respiratory sensors can be used like belts (measuring the extension of the thoracic/abdominal circumference, see also 145 ), differential pressure (measuring the air flow at the nostril level) or temperature sensors (measuring temperature changes at the nostril level).
How to choose?
Johnson, B. N., Russell, C., Khan, R. M., & Sobel, N. (2006). A comparison of methods for sniff measurement concurrent with olfactory tasks in humans. Chemical senses, 31(9), 795-806.
BreathMetrics is a Matlab toolbox for algorithmic extraction of the full set of features in human respiratory flow recordings.
Reference paper: Noto,T., Zhou, G., Schuele, S., Templer, J., & Zelano, C. (In press). Automated analysis of breathing waveforms using BreathMetrics: a respiratory signal processing toolbox, Chemical Senses, , bjy045, https://doi.org/10.1093/chemse/bjy045