Principles of Pressure Transducers James Peerless January 2012

Objectives PC_BK_56Transducers and strain gauges PC_BK_64Pressure transducers PC_BK_65Resonance, damping and frequency response

Objectives Definitions – Pressure – Transducers The Wheatstone Bridge Resonance & Damping Invasive Blood Pressure Monitoring

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 = length 2 = m 2

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

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

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

Strain Gauge

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

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

R1R3=R2R4R1R3=R2R4

What affects transducer signals? – Damping – Resonance and frequency

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.

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

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

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

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

Frequency Affecting natural frequency of a system: Short, wide and rigid tubing Fα d √(l × c × ρ)

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

Problems with IBP Cannula-related – Disconnection – Haemorrhage – Infection – Thrombosis – ischaemia Transducer-related – Calibration – Resonance – Damping

Summary PC_BK_56Transducers and strain gauges PC_BK_64Pressure transducers PC_BK_65Resonance, damping and frequency response

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