A simplified method for determining the frequency response of pneumotachographs used in infants

In all rapidly changing systems, an appropriate response time of all sensing devices is essential for the accurate conversion of a physiological parameters to a proportional electrical signal. The frequency response of equipment is the ability to accurately reflect both the magnitude and temporal relationship of dynamic events over a defined frequency range. The phase lag expresses the temporal delay between the physical event being measured and the output signal. The attenuation expresses diminution of the amplitude ratio of the output signal in relation to the input signal over a range of frequencies. We have developed a method that specifically addresses the measurement of frequency response and attenuation of pneumotachographs and low pressure transducers. The system consists of a 81 liter rectangular box separated in the middle by a 30.5 cm acoustic loudspeaker, the cone sealed by latex, and driven by a signal generator coupled to a low frequency amplifier. This system can produce an undistorted sinusoidal signal between 0 and 20 Hz, and the peak flow through the pneumotachograph is only minimally affected by changes in frequency. Rapid analysis is possible using an oscilloscope to produce Lissajou loops. There is no measurable attenuation between the electrical signal and the pressure generated over frequencies from 1 to 20 Hz. The system is accurate at low frequencies and can generate appropriate signals over the frequency range of interest for respiratory applications, and it can be inexpensively constructed.