principles of pressure transducers james peerless january 2012
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Principles of Pressure Transducers
James PeerlessJanuary 2012
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Objectives• PC_BK_56 Transducers and strain
gauges• PC_BK_64 Pressure transducers• PC_BK_65 Resonance, damping
and frequency response
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Objectives• Definitions– Pressure– Transducers
• The Wheatstone Bridge• Resonance & Damping• Invasive Blood Pressure Monitoring
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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
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Units of Pressure• SI: 1 Pa = 1 Nm-2 = 1 kg.m-1.s-2
• Other units101.3 kPa= 1 atm= 1 bar (100kPa)= 1020 cmH2O
= 750 mmHg (1 torr)
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Components of IBP setup• Arterial cannula• Tubing• 3-way tap• Pressurised bag• Strain gauge transducer• Microprocessor• Amplifier• Display Unit
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Transducer• A device which converts
one form of energy to another.
• E.g. pressure transducers convert mechanical energy to electrical energy
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Strain Gauge
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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
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• Variable resistor calibrated to zero
• Any change in unknown resistance means that current flow is detected across the galvanometer
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R1
R3
=R2
R4
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• What affects transducer signals?– Damping– Resonance and frequency
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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.
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DampingResponse 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
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Types of Damping
• Critical damping d=1• Under-damping d1• Over-damping d∞• Optimal damping: 0.64
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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
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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
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Frequency• Affecting natural frequency of a system:
• Short, wide and rigid tubing
F α d
√(l × c × ρ)
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Indications for IBP monitoring• Inaccurate NIBP– Obesity, arrhythmias
• Unstable patient• Frequent blood samples required• LiDCO
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Problems with IBP• Cannula-related– Disconnection– Haemorrhage– Infection– Thrombosis– ischaemia
• Transducer-related– Calibration– Resonance– Damping
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Summary• PC_BK_56 Transducers and strain
gauges• PC_BK_64 Pressure transducers• PC_BK_65 Resonance, damping
and frequency response
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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.