Abstract
Attempts to use photoconductive cells in a direct writing earpiece and cuvette densitometer were made. The major advantages of this instrument are simple electric circuit, high sensitivity and stability, and immediate linear response to changes in dye concentration. The design of the circuit, by obtaining the difference of each output voltage of the two independent red and infrared channels, made it possible to minimize the drift and pulsatile fluctuations of the baseline caused by factors other than dye concentration. Direct writing necessitated only a low gain amplifier, the baseline drift of which during five minutes was within two per cent of the total deflection. Response to a rapid change in light intensity was within 0.24 second for 90 per cent of the maximum deflection, which was proved to bear a linear relationship to changes in dye concentration over a range of practical use. Dye dilution curves from the earpiece in patients with intracardiac shunts presented accurate information in detecting their abnormalities.
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