Principle of action of solid state strain gauge transducer

Outline of Wheatstone bridge configuration with excitation by regulated supply followed by stable, fixed gain amplification.

Description of errors, overdamping, overshoot, causes & remedial actions

 

1.                  Resistance

a)                  Ohms law:

b)                 Increased resistance

i)                    heating wire resistor

ii)                  cooling semiconductor

iii)                 stretching (making longer & thinner) wire resistor

a)                  ‘Strain gauge’: used in pressure transducer

c)                  To monitor & measure the change in resistance — use of ‘Wheatstone Bridge’ circuit

1.                  Wheatstone Bridge consists of:

a)                  Source of electrical potential (battery)

b)                 Galvanometer

c)                  resistors

i)                    R1: set resistor

ii)                  R2: set resistor

iii)                 R3: variable resistor

iv)                R4: measured resistor

 

2.                  Theory of Wheatstone Bridge

a)                  The object is to measure R4

b)                 Circuit consists of 2 resistive limbs:

i)                    R1-R2

ii)                  R3-R4

c)                  Current flows via both resistive limbs

d)                 Some current will flow via galvanometer (X®Y or X¬Y) unless points X & Y are exactly equipotential

e)                  A state of balance (indicated by a ‘null’ indication on the galvanometer) will exist when the ratio of the resistors R1:R2 = R3:R4 since the supply voltage will in each case be divided by the same fraction

f)                   Thus, if R1 = R2, adjust known resistances of R3 until a balance occurs will indicate that R4 = R3

 

3.                  Fixed gain amplification

a)                  The current through the galvanometer is not linearly related to R4, so that it appears that a bridge is only accurate at balance

i)                    The output from a bridge will be linear if points X & Y are led to a differential amplifier whose input resistance is very high compared with the resistances in the bridge. Under these conditions, an altered value for R4 does not cause a current flow from X to Y, so that the output is directly proportional to R4

b)                 Pressure transducer with diaphragm

i)                    The force per unit area is sensed by the movement of the diaphragm

ii)                  Relationship between applied pressure and the movement of the diaphragm is governed by the stiffness of the diaphragm

iii)                 A relatively stiff diaphragm is needed because undue distortion of the diaphragm causes the response to become nonlinear and because the frequency response of the transducer is intimately related to the stiffness of the diaphragm

 

4.                  Advantage of Wheatstone Bridge

a)                  High sensitivity

i)                    Small changes in resistance at R4 will result in large deflections of the galvanometer

b)                 The potential at the galvanometer is then amplified before it is displayed or recorded

 

5.                  Strain Gauge Pressure transducer using Wheatstone Bridge

a)                  As pressure is applied to the strain gauge, the wires increase in length and decrease in diameter, increasing the resistance to flow of current through the wires of the Wheatstone Bridge

b)                 This change in the wires electrical resistance causes a voltage change that can be quantified to reflect the amount of pressure that changed the wires length & diameter.

c)                  The electrical signal can be amplified & measured; when calibrated, it is proportional to the pressure change

d)                 By combining strain gauge elements of which some stretch while others simultaneously compress, exaggeration of the signal within the Wheatstone bridge can be achieved

 

6.                  ERRORS

a)                  Haemodynamic pressures consist of three components:; dynamic pressure; hydrostatic pressure

i)                    Residual/static pressure

a)                  pressure we which to measure

ii)                  Dynamic pressure

a)                  pressure due to imparted kinetic energy from blood directly flowing into catheter tip

b)                 Minimised by pointing catheter tip away from blood flow direction

iii)                 Hydrostatic pressure

a)                  pressure due to differences in height between transducer and catheter tip

b)                 Eliminated by positioning transducer at level of catheter tip: levelling

b)                 Zeroing

i)                    i.e. zeroing to atmospheric pressure, therefor all pressures are made relative to atmospheric pressure

ii)                  Done by opening transducer to air and adjusting calibration on monitor to display zero

c)                  Calibration

i)                    Monitor

a)                  Adjust control knob on display to read appropriate readout value (eg 200 mmHg)

ii)                  Transducer

a)                  Use of a Hg sphygmomanometer attached to pressure transducer; correlated between sphygmomanometer readings & monitor

b)                 If discrepancies occur, transducer is out of calibration

d)                 Resonant frequency

i)                    Frequency at which oscillations have their maximum amplitude

ii)                  Most catheter—plumbing systems have a low natural/resonant frequency

iii)                 Best catheter-tubing systems have a high natural/resonant frequency so that important input frequencies are well below the natural harmonic range

iv)                Determined by size, shape and material of plumbing system

v)                  If pressure waveform contains being transmitted contains a frequency response that is near the system’s resonant frequency, the system will tend to resonate resulting in overshoot  of systolic pressures, lowered diastolic pressures and the appearance of numerous small oscillations in the waveform

vi)                Note that intraarterial pressure monitoring, especially at high heart rates has more higher frequency components than eg CVP monitoring

vii)               Avoided by using a plumbing system whose natural (resonant) frequency is as far away from the frequency of the signal as possible (as high as possible)

a)                  Minimise catheter tubing length

(1)               Increased length lose energy from pressure wave due to increased frictional resistance resulting in a reduced amplitude (poiseuille’s law)

(2)               Increased length reduces the resonant frequency of the system to a similar region as that of the pressure signal

b)                 Use stiff, noncompliant tubing

(1)               Compliant tubing reduces amplitude of pressure waveform

c)                  Use large diameter catheters

(1)               Narrow bore catheters lose energy from pressure wave due to increased frictional resistance resulting in a reduced amplitude  (poiseuille)

d)                 Eliminate bubbles & clots

(1)               Bubbles & clots are compressible thereby causing loss of pressure wave’s energy thereby lowering the natural frequency resulting in overestimation  actual pressure

(2)               Use a continuous heparin flush

e)                  Damping coefficient

i)                   

ii)                  Describes how quickly an oscillating system comes to rest

iii)                 Most catheter—plumbing systems are underdamped ; have a high damping coefficient

iv)                Because optimal damping is difficult, every effort should be made to increase the natural frequency of the system

v)                  Underdamping:

a)                  Pressures with a frequency close to the resonant frequency will be exaggerated

vi)                Overdamping

a)                  High frequency oscillations will be damped out so that true pressure changes will be underestimated

vii)               Optimising damping

a)                  Minimise bubbles & clots

b)                 Use stiff walled catheters

f)                   Assessing of systems natural frequency & damping coefficient

i)                    Fast flush—square wave 47 Daily; 173 Sykes