3. Principles of Measurement Flashcards
Describe the components of a standard measurement system.
Measurement systems are used to detect an input, transduce the signal and display it in a form that can be used by an interpreter.
They are made up of the following:
1 > Input
– parameter chosen to be measured,
e.g. blood pressure (BP).
2
> Transducer –
device that converts
one form of energy into another,
e.g. the strain gauge inside an invasive
BP monitoring transducer.
The pressure generated by the arterial pulse
alters the shape of the diaphragm
on which strain gauges are arranged
as a Wheatstone bridge.
The deformation alters the length
of the wire in the gauges and
so their resistance is altered.
The change in electrical signal is
fed via the transmission path
into the conditioning unit.
3 > Transmission path – apparatus that carries the electrical signal to the conditioning unit.
4 > Conditioning unit – apparatus in which the electrical signal is processed, analysed and then passed to the display unit.
5
> Display unit –
monitor, gauge or dial
on which the output is displayed.
6
> Output –
final stage, and what is
viewed on the screen, dial or gauge.
The clinical context and
the limitations of the equipment used
must be taken into consideration
when interpreting the output.
Measurement systems can either be
analogue or digital
Measurement systems can either be analogue or digital.
> Analogue
– output signal is continuous,
e.g. the waveform display of an
arterial pressure trace.
> Digital – output signal is discontinuous,
e.g. the numerical display of BP
next to the arterial waveform.
Which factors affect the output of a measurement system?
1.
> Accuracy –
this determines how closely the output
reflects the true value being measured.
Machines have their accuracy quoted
as a percentage by the manufacturers,
e.g. a machine that displayed 110 units when the
true value was 100 would have an accuracy of ±10% across its working range.
2.
> Sensitivity –
this determines how small a change in input
will result in a change in output.
Less sensitive systems will be able to
operate over a wider range.
3. > Drift – as the name suggests, this is a movement of the output away from the true input value.
It is usually linear and unidirectional,
although it does not have to be.
It is usually caused by changing properties
of the components of the equipment,
e.g. ageing of thermistors.
4. > Gain – this refers to the degree of amplification of the measurement system (i.e. the output to input ratio).
What is hysteresis?
In a system with hysteresis,
the output of the system
alters depending on
whether the input is rising or falling.
In a system without hysteresis the output
can be predicted from the input alone,
but with hysteresis the operator
needs to know the input history
to estimate the output.
An example of this is
seen in the lung compliance curve,
which exhibits hysteresis because of the
elastic energy that is stored within it.
Fig. 52.1 Lung volume–pressure (compliance) curve showing hysteresis
Lung
volume (L)
Airway pressure
What is damping?
This is definitely an answer to practise aloud before you go into the exam. Although it is intuitively simple to understand, damping can be surprisingly
hard to explain.
We think it is easiest to draw the next graph (Fig. 52.2)
and use it to illustrate your answer.
Damping describes the
resistance of a system
to oscillation resulting
from a change in the input.
Damping is the result of frictional forces
working in that system.
In the perfect measurement system, any change in input would be instantly and accurately reflected in the output.
However, this is not the case in clinical systems
and it takes time for a change in input
to be reflected by a change in output.
The speed with which this happens can be measured and defined:
> The ‘response time’
is the time taken for the output
to reach 90% of its final reading.
> The ‘rise time’ is the
time taken for the output
to rise from 10 to 90% of
its final reading.
Although it is impossible to design the ‘perfect’ system described above,
we do need a system that responds as rapidly as possible to any change in input.
This in itself can create problems,
as a system that rises quickly
will have the tendency to
overshoot in its reading,
while one which rises too slowly may never reach the new elevated input value, or may simply take too long to be a useful measure of a changing input.
So, following a change in input there are several possible outcomes for the
system:
Perfect response
Under-damped
Critically damped
Over-damped
Optimally damped
Under-Damped
output changes quickly
in response to the step up in input,
but it overshoots and
then oscillates around the true value,
before coming to rest at it.
This means that it will be
some time before the true value
is displayed and the peaks and troughs
will over- and underrepresent the true value.
In a dynamic system,
e.g. intra-arterial BP, the
constantly changing input may
result in wild fluctuations,
rendering an under-damped system very inaccurate (although the MAP is still correct).
Critically damped
Critically damped –
The response and rise time of the system
are longer than an under-damped response,
but there is no significant overshoot
and
oscillations are minimal.
‘D’ is the damping factor and,
by
convention,
in a critically damped system D = 1.
Over-damped
Over-damped –
defined as damping greater than critical.
The output here could potentially
change so slowly that it
never reaches the true value.
In a dynamic system,
the response time may be too slow for the
system to be useful.
> Optimally damped –
> Optimally damped –
in reality in clinical measurement systems,
critical damping is not ideal
and
we are prepared to accept
a few oscillations
and some overshoot
to achieve a faster response time.
Hence, our systems are ‘optimally damped’
where 64% of the energy is removed
from the system and D = 0.64.
There is a 7% overshoot in this case.
What are the characteristics of the ideal invasive BP monitoring equipment?
1
> A short,
stiff,
wide cannula.
This helps to keep its natural frequency high
(see explanation of resonance and natural frequency below).
2
> No connections in the system,
again to keep the natural frequency high.
3 > No air bubbles in the system as these are compressible and therefore decrease energy transmission up the column of fluid resulting in a damped trace.
What is calibration?
Calibration is a process in which the output of a measuring device is compared to a standard of known units of measure to determine the accuracy of the measuring device.
For example, a blood gas machine may be primed with a solution of known pH and its output is compared with the known value of the solution.
If the measuring device is
proved to be inaccurate
it can be reset accordingly.
Calibration should not be done against
just one standard;
at least two must be used.
The more standards that are used
for comparison, the more accurate
will be the resultant measuring device.
Calibration should be performed when:
> A predetermined period of time has elapsed.
> The machine has been used a
predetermined number of times.
> The machine gives unexpected results.
> The machine is moved,
undergoes vibration or damage.
What is ‘signal noise’?
Signal noise describes unwanted
external information that is fed
unintentionally into a transducer,
resulting in the output being altered.
We see this on the ECG display when
diathermy is being used.
Often here, the noise is so ‘loud’
that it actually obscures the ECG display.
The magnitude of noise is described
by comparing the two amplitudes
to give the ‘Signal:Noise (S/N) ratio’.
How can we overcome noise?
> We can add ‘filters’
into measurement systems,
so that signals above or below
given frequencies are ‘ignored’
and not processed to produce an output.
• High pass filters ignore signals
below a given frequency.
• Low pass filters ignore signals
above a given frequency.
• Notch filters ignore signals at a given frequency,
.g. 50 Hz – mains frequency.
> We can average out the signal
in cases where the signal is repetitive
(as in most biological systems)
and the noise is intermittent.
This is useful
when the noise is very loud
compared to the desired signal.
Resonance and natural frequency
Resonance is the
tendency of a system to oscillate
at maximum amplitude at certain frequencies.
This resonant frequency is determined
by the mass and stiffness of the system.
The greater the mass,
the slower the oscillations,
and the stiffer the system,
the faster the oscillations.
The components of a measurement system
will oscillate at their own
natural frequency.
Imagine that we are able to give energy
to the system at the perfect moment
to allow us to increase
the amplitude of these oscillations.
As the oscillations got bigger,
it would be impossible to stop
them being transmitted to the output reading.
In this way, the method of measuring
the true value would interfere
with the output value displayed.
For the ‘physics challenged’ among us,
an easier way to visualise this is
to imagine yourself on a swing.
If you push off, and swing your legs at a steady rate, your swinging, or oscillation, will remain fairly constant.
If, however, a friend comes
and pushes you
when you are at your highest point,
they give energy to your system
and you will swing higher and higher –
your oscillations will increase in amplitude.
If you were attached
to an output monitor,
you would see the units displayed increase.
The key to this concept is that your friend has to push you at just the right point in your swing; too soon or too late, and they will not help increase your amplitude, and may even decrease it.
For our clinical example, think of the invasive BP transducer. If its natural frequency was near that of the input being measured ie swings up and down of the arterial pulse)
it is easy to see how its own
natural oscillations could be augmented
by those of the pulse, which could
act like the friend pushing the swing.
The natural frequency of the arterial pulse is around 20 Hz (because it is comprised of lots of sine waves ‘stacked’ on top of each other, each with a frequency of approximately 2 Hz).
For this reason, the manufacturers try to make the natural frequency of the transducer kit out of this range, and in fact most kits have a natural frequency of around 45 000 Hz.
This would work well,
were it not for the way we then
choose to use the carefully designed apparatus.
We do not have the transducer in close proximity to the artery; instead we attach a long column of saline to the end of the transducer and attach this to the arterial cannula.
This is for convenience,
but it dramatically reduces the natural frequency
of the measuring system.
The long,
heavy saline column reduces the
natural frequency from
45 000 Hz to around 15 Hz
and this brings the frequency
of the measuring system
very close to
that of the input.
Hence, it is now easy to see how the oscillations in BP can augment the oscillations of the measuring system and so give us falsely elevated and depressed peaks and troughs in our output display.
This is the practical
example of under-damping,
when the arterial pressure is inappropriately
over-represented.
At the beginning of the section on damping,
we said that it was
caused by friction in
the measurement system.
Using the example of
being on the swing again,
if you become frightened that your friend is
pushing you too hard,
and you are swinging too high, what do you do?
You scrape your feet along the ground,
increasing friction, dissipating
energy and therefore reducing
the amplitude of your oscillations.
The things that increase damping
in our invasive BP monitoring system
have been listed above.
As users of the system, we cannot
actually affect the natural frequency of the system.
The only thing we can do theoretically is alter the length of the saline column, but even this is predetermined by the length of the tubing in the ‘arterial line’ packs.
Increasing the length of the saline column
will add to damping which
you would think would be helpful
given that we are worried about
augmenting the system’s natural oscillations.
However, this will decrease the system’s natural frequency to a more significant degree and so we tend to move towards an under-damped system, the longer the saline column.
