The Electrical Capacitance Diaphragm Manometer

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Abstract
A new type of diaphragm manometer for direct pressure measurements in the arterial blood stream and in other fluid systems is described using a radiofrequency, crystal‐controlled oscillator and a pressure‐sensitive condenser in the pick‐up unit. The potential drop due to the plate current of the oscillator tube flowing through a resistor is amplified, and is observed and photographed by means of a cathode‐ray tube. Sensitivity control is obtained by the use of an inverse feed‐back network in the amplifier. The range of sensitivity is from a maximum of one mm Hg per inch of cathode‐ray tube spot deflection to any desired minimum. The approximate cut‐off frequency of the manometer is 580 c.p.s. when used with a number 20 needle, 475 c.p.s. with a number 22 needle and 205 c.p.s. with a number 26 needle, each one inch long. These values are higher than those of previous diaphragm manometers with the same size liquid system and the same size needle. Sample records of the arterial pressure are presented.

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