An Electrically Calibrated Plethysmograph for Direct Measurement of Limb Blood Flow

This paper describes an electrically calibrated plethysmograph which may be used with all lengths of mercury-in-rubber strain gauges. Due to the very low resistance of these strain gauges, electrical calibration of previously available plethysmographs has suffered from errors caused by lead-wire resistance. The present instrument eliminates lead-wire errors by a design which effectively places the strain gauge at the corners of the measurement bridge. Linearity of the output for large changes in gauge resistance has been insured by the incorporation of a constant-current bridge supply.