Principles of measurement

Question 1

Describe the basic components of a standard measurement system?

Answer 1

measurement systems are designed to measure a variable, process it and display it for interpretation

consist of
* input - e.g. pressure
* transducer - converts one form of energy to another e.g. current
* transmission path - cables to transmit current
* processor - amplification, filters, AD conversion
* display unit

Question 2

what is the difference between analogue and digital ?

Answer 2

2 types of data represenation / signals

analogue = continous waveform e.g. BP waveform
digital = single numerical value e..g MAP

Question 3

what are the requirements for an ideal clinical measuring system?

Answer 3

economical
- cheap, easy to use, does not require frequent calibration

accuracy
- highly accurate
- reliable
- high signal to noise

outputs are of convienent measurement e.g. not many decimal points.

Question 4

define accuracy

Answer 4

the degree of how close the measured value is to the actual value.

calibration is required for accuracy

Question 5

define sensitivity

Answer 5

the measurement system picks up small changes. This may or may not be beneficial. e.g. if recording large range of values, a small change may be a hinderance.

Question 6

define linearity ..

Answer 6

the input is proportional to the output

Question 7

define non-linearity..

Answer 7

the output value is not proportional to the input. it changes disproportionately e.g. exponential relationship

Question 8

define drift

Answer 8

Drift refers to a deviation from the true value over time i.e. the accuracy is lost overtime this may result in

it may be …
* linear/offset error - offset always by the same amount. requires 1 point calibration
* gain/gradient error - increase / decrease in drift as input values increase. requires 2 point calibration

caused by changes in the measurement equiptment e.g. aging.

Question 9

define hysteresis

Answer 9

this is the phenomena where a state of a system or its output depends not only on the input value but also the history of the system e.g. if it is increasing or decreasing.

Question 10

define precise

Answer 10

Precision relates to the reproducibiltiy of repeated measurements. i.e. less random error

Question 11

define bias..

Answer 11

depends on context

in data /measurement
Bias is the difference between the expected (average) result of a measurement or estimation and the true value. i.e. if thermometer is off by 2 degrees this is its bias.

in statistics it is the tendancy to favour /disfavour an outcome based on unfair / uncontrolled variables.

Question 12

draw graphs for:
* accurate and precise data
* accurate and imprescise

Answer 12

Question 13

draw graphs for:
* inaccurate and precise data
* inaccurate and imprescise

Answer 13

Question 14

describe accuracy in terms of bias and precision …

Answer 14

an output needs to be both precise and non-bias (i.e. not drifted) to be accurate

Question 15

what types of measurement errors are there and calibration methods for these..

Answer 15

• linear/offset error - offset always by the same amount. requires 1 point calibration
• gain/gradient error - increase / decrease in drift as input values increase. requires 2 point calibration

Question 16

what is calibration?

Answer 16

the process of comparing the output of a measuring device against a known standard to ensure accuracy of this device.

Different devices vary on levels of calibration (one point, 2 point, multi point) and how often it is required (depends on drift)

Question 17

when should calibration be performed?

Answer 17

when manufacturing

regular periods depending on how prone to drift

machine is giving unexpected values

Question 18

can you give examples of when one point and 2 point calibration is used

Answer 18

1 point calibration - for when the error is always by a set amount. e.g. when the arterial transducer is zerod by defining atmospheric pressure as 0mmHg.

2 point calibration - for when there is a gain error. e.g. pH electrode is calibrated using 2 known pH samples. and Fuel cell for 2 known O2 samples (21% and 100%)

multipoint - non linear relationship between true values and output. e.g. thermistors

Question 19

what is meant by intrapolation and extrapolation?

Answer 19

intrapolation = measured values lie within calibrated points

extrapolation = measured values lie outside the range of calibrated points = less likely to be accurate e.g. sats probe below 70%

Question 20

what is an amplifier?

Answer 20

A device that increases the amplitude of the signal output for easier interpretation and use.

Question 21

what is gain?

Answer 21

the degree of amplification
e.g. if doubled, gain is 2

Question 22

what is noise?

Answer 22

unwanted signal that can disrupt wanted output signal and complicate interpretation.

Question 23

what is meant by signal to noise ratio?

Answer 23

during the amplification process both the signal and noise can be amplified.
The SNR is the degree of wanted signal to unwanted noise.
this can be used as a measure of performance of a system.
e.g. 1:1 is poor
3:1 = 3 decibels is a higher SNR and therefore better.

Question 24

how can noise of a system be reduced?

Answer 24

either by prevention or removal

prevention:
* shielding from electomagnetic interference
* isolating component to prevent inductance/ capacitance coupling
* high impedence system to prevent drawing in of stray current

removal
* filter - high/ low pass, notch, bandwidth
* common mode rejection / differential amplifiers.

Question 25

Describe the process of common mode rejection / differential amplifiers...

Answer 25

2 or more inputs into amplifier. the amplifier amplifies differences and not signals that are common to both e.g. 50Hz from mains. the higher the CMR, the better at removing noise

Question 26

what are the different methods for filtering noise..

Answer 26

low pass = allows frequencies below a certain wavelength high pass = above certain wavelength bandwidth = removes signals outside of a specified bandwidth notch filter = removes a given frequency

Question 27

what is the mechanism behind how filters work?

Answer 27

use principles of capacitance and inductance to favour high / low frequency currents e.g. capacitors favour high frequency AC e.g. inductors favour low frequency or DC also put into circuit with resistors e.g. RC low pass filter - the capacitor is put on the limb towards earth such that low frequencies will favour route of resistor and be measured, high frequencies will be filtered out by capacitor e.g. RC high pass - put capacitor on route of signal output such that high frequencies are not filtered out.

Question 28

are amplifiers always electronic?

Answer 28

no can be mechanical with levers e.g. bourdon gauge.

Question 29

define natural frequency...

Answer 29

The frequency at which a system or object oscilates freely when no external force is applied. this depends on the systems own properites e.g. mass/ stiffness of the system all measurement components will have a natural frequency.

Question 30

define resonance...

Answer 30

resonance is a phenomena whereby a force is applied to a system and has frequency the same as the natural frequency and this causes it to oscilate at maximal amplitude. as input frequency increases, the output amplitude increases up to a maximum = resonant frequency

Question 31

define resonant frequency...

Answer 31

resonant frequency is the frequency at which resonance occurs. i.e. occurs at natural frequency of that system.

Question 32

what is meant by damping?

Answer 32

damping is the reduction in amplitude of oscillations of a system through resistive/frictional forces. hence energy is lost from the system. without damping a system would oscillate indefinitely - theoretical, not realistic

Question 33

describe the key features of a dynamic measurement system...

Answer 33

a dynamic measuring system has input values constantly changing with time. these input values will result in output measurement. however this process is not instantaneous and the output osccilates around the true output value until it settles. damping will reduce these oscillations and can help reach the output value more accurately and quicker. however too much damping will cause an increase in rise time and again can result in accuracies/ under reading. optimal damping is required

Question 34

what are the different levels of damping in a dynamic measuring system?

Answer 34

damping of a system can reduce osccilations and help it to reach its final value sooner. under damped = a lot of oscilations around end point, takes a while to settle. the output will be record as either an over estimate or an under estimate. over damped = too slow rise, no oscilations and takes too long to ever reach end point. under estimation of output critical damping = level of damping which achieves the output without any oscilations. good that there are no oscilations but too slow for dynamic changing system. optimal damping = rapid response time, 2 oscilations around true value and then settles. good balance between speed and accuracy. this is the aim.

Question 35

what is the response time and rise time of a dynamic measurement system.

Answer 35

response time = time taken for output value to 90% of its final reading rise time = time taken for output value to rise from 10 to 90% of final reading

Question 36

what is the damping coefficient?

Answer 36

a value that quantifies the level of damping D= 1 = critical damping D=0 = no dampin g over damped >1 under damped 0 -0.3 optimal damping = 0.64

Question 37

draw a graph for optimal damping...

Answer 37

1 or 2 oscilations

Question 38

how is an analogue signal converted to a digital signal?

Answer 38

Fouriers analysis analysis of complex sine waves using trigonometric functions.

Question 39

what is the difference between dynamic and static response to measurement...

Answer 39

static measurement system - is measuring a static input e.g. temp of a room that isnt changing (or changing very slowly) dynamic measurement system - measuring input values that are changing with time.

Question 40

what is meant by 0 order, 1st order and 2nd order responses

Answer 40

A 0th order system is one that responds instantly to any input change, meaning it has no time delay or dynamic response. 1st order = takes time to reach the output value = exponential growth curve 2nd order = oscilates around the output value