Anaesthesia & intensive care medicine
Volume 10, Issue 2 , Pages 102-105, February 2009

Principles of pressure transducers, resonance, damping and frequency response

M B Wilkinson, MBBS, FRCA, is a Consultant Anaesthetist in Northampton, UK, with an interest in vascular anaesthesia, medical education and simulation. He has a certificate in Education in association with the University of Dundee, UK, and is a Primary FRCA examiner. Conflicts of interest: none declared

M D Outram, MA, MBBS, FRCA, is a Senior Registrar in Anaesthetics and Intensive Care, currently working at the John Radcliffe Hospital, Oxford, UK. He has a special interest in intensive care medicine and is in his advanced year of dual accreditation training. Conflicts of interest: none declared

Abstract 

The most common invasive pressure transducer we use is a strain gauge. This consists of short pieces of wire attached to a diaphragm which is distorted during transmission of the pressure impulse. Consequent changes in the length of the wire alter their resistance, which, when connected to a Wheatstone bridge circuit, allow us to produce an electrical signal for display. Resonance may occur when the frequency of the pressure waves in the incoming impulse matches the natural frequency of the transducer thereby causing superimposition of pressure waves. The natural frequency of the system may be increased by using a stiff diaphragm and short wide-bore tubing. Fourier analysis allows us to breakdown a complex waveform into its component harmonics. Faithful reproduction of the waveform is possible using the first ten harmonics. Therefore a transducer system with a natural frequency response of 30 Hz would allow us to reproduce pressure waves of 180 beats/min. Damping reduces the high-frequency noise to allow a more accurate reproduction of the wave form. Too little damping allows oscillations which distort the results while too much damping delays the signal.

Keywords: damping, frequency, invasive, resonance, transducer, Wheatstone bridge

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PII: S1472-0299(08)00255-5

doi:10.1016/j.mpaic.2008.10.007

Anaesthesia & intensive care medicine
Volume 10, Issue 2 , Pages 102-105, February 2009

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