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Principles of Pressure Transducers

Principles of Pressure Transducers. James Peerless January 2012. Objectives. PC_BK_56 Transducers and strain gauges PC_BK_64 Pressure transducers PC_BK_65 Resonance, damping and frequency response. Objectives. D efinitions Pressure Transducers The Wheatstone Bridge Resonance & Damping

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Principles of Pressure Transducers

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  1. Principles of Pressure Transducers James Peerless January 2012

  2. Objectives • PC_BK_56 Transducers and strain gauges • PC_BK_64 Pressure transducers • PC_BK_65 Resonance, damping and frequency response

  3. Objectives • Definitions • Pressure • Transducers • The Wheatstone Bridge • Resonance & Damping • Invasive Blood Pressure Monitoring

  4. Pressure • Force per unit area • Force: that which changes a body’s state of rest or motion (SI: N = kg.m.s-2) • 1 Newton = the force required to accelerate a mass of 1 kg by 1 metre per second per second • Area = length2 = m2

  5. Units of Pressure • SI: 1 Pa = 1 Nm-2 = 1 kg.m-1.s-2 • Other units 101.3 kPa = 1 atm = 1 bar (100kPa) = 1020 cmH2O = 750 mmHg (1 torr)

  6. Components of IBP setup • Arterial cannula • Tubing • 3-way tap • Pressurised bag • Strain gauge transducer • Microprocessor • Amplifier • Display Unit

  7. Transducer • A device which converts one form of energy to another. • E.g. pressure transducers convert mechanical energy to electrical energy

  8. Strain Gauge

  9. Wheatstone Bridge • An electrical circuit for precise comparison of resistors. • Used to measure an unknown resistance • Null deflection technique • Two known resistors • One variable resistor • One unknown resistor • Sensitive to small changes

  10. Variable resistor calibrated to zero • Any change in unknown resistance means that current flow is detected across the galvanometer

  11. R1 R3 = R2 R4

  12. What affects transducer signals? • Damping • Resonance and frequency

  13. Damping • Damping • The tendency to resist oscillation through dissipation of stored energy • Caused by • Air bubbles • Blood • Soft diaphragm • Soft tubing • Damping describes how a system responds to the input.

  14. Damping Response time: time taken to reach 90% of final reading • Ideal: monitor system would reflect the input instantaneously. • Under-damped: the response time is fast but there is too much overshoot and oscillation around the value • Over-damped: there is little/no overshoot, but the response time is too long

  15. Types of Damping • Critical damping d=1 • Under-damping d1 • Over-damping d∞ • Optimal damping: 0.64

  16. Resonance • Resonance • The tendency of an object to oscillate • Natural Frequency • The frequency at which a body will resonate at maximum amplitude • Resonance occurs when input frequency is similar to natural frequency of the monitoring system

  17. Resonant Frequency of a System • Should be at least 10 times the fundamental frequency • The fundamental frequency of this system is the heart rate (first harmonic: 1-2 Hz) • The first 10 harmonics contribute to the waveform • If the natural frequency is less than 40 Hz, it falls within the range of the blood pressure

  18. Frequency • Affecting natural frequency of a system: • Short, wide and rigid tubing F αd √(l × c × ρ)

  19. Indications for IBP monitoring • Inaccurate NIBP • Obesity, arrhythmias • Unstable patient • Frequent blood samples required • LiDCO

  20. Problems with IBP • Cannula-related • Disconnection • Haemorrhage • Infection • Thrombosis • ischaemia • Transducer-related • Calibration • Resonance • Damping

  21. Summary • PC_BK_56 Transducers and strain gauges • PC_BK_64 Pressure transducers • PC_BK_65 Resonance, damping and frequency response

  22. References • Al Shaikh B, Stacey S (2007). Essentials of Anaesthetic Equipment; 3rd Edition. Elselvier, Edinburgh. • Davis P, Kenny G (2003). Basic Physics and Measurement in Anaesthesia; 5th Edition. Butterworth Heinemann, London. • Wijayasiri L, McCombe K, Patel A (2010). The Primary FRCA Structured Oral Examination Study Guide 1. Radcliffe, Oxford.

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