National Cylinder Gas (division of Chemtron) Pulmonary Function Indicator, 1974.  Used a hot-wire transducer to measure gas flow.

From: Burton RM, Kozel WM, Penley RP, Ward GH, Chapman RS.  Application and evaluation of portable field instruments for measuring Forced Expiratory Volume of children and adults in environmental health surveys.  Environmental Health Perspectives, 1974; 8: 123-131.  Page 125.