Principles of Pressure Transducers

James PeerlessJanuary 2012

Objectives• PC_BK_56 Transducers and strain

gauges• PC_BK_64 Pressure transducers• PC_BK_65 Resonance, 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 = length2 = m2

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)

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

R1

R3

=R2

R4

• What affects transducer signals?– Damping– Resonance and frequency

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.

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

Types of Damping

• Critical damping d=1• Under-damping d1• Over-damping d∞• Optimal damping: 0.64

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

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

F α 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_56 Transducers and strain

gauges• PC_BK_64 Pressure transducers• PC_BK_65 Resonance, damping

and frequency response

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.