Abstract: Disclosed is an automatic means for entering into a cardiac output computer a computation constant for a given size catheter. The technique involves the placement of a capacitor across the output terminals of a thermodilution catheter assembly. This capacitor does not effect temperature measurements. By means of software controlled switching in the cardiac output computer circuity, the capacitance of this capacitor can be determined and can be used to indicate the catheter size. Disclosed also is the process used to set up and identify the catheter gauge.