This paper is concerned with the development of an improved form of cuvette densitometer used in the measurement of cardiac output by the dye‐dilution method.

In this method the output is calculated from the relationship expressing rate of dilution of a dye introduced in the heart as a function of flow rate. The role of the densitometer is that of recording the time‐concentration function of the dye in blood. This recording is made as whole blood is led in a continuous flow from a peripheral artery of the subject through the transparent cuvette of the densitometer.

The densitometer to be described is an improvement over previous units in its stability, sensitivity, size, and convenience of application. A complete description of the principles of operation, circuit details, mechanical arrangement, and performance characteristics is included along with recommendations for future improvements.

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