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Direct vs. Indirect
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Direct
- measure desired measurand directly
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if the sensor is invasive
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direct contact with the measurand is possible but expensive,r,isky and least acceptable
- e.g., Blood glucose meter
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Noninvasive
- e.g., Thermometer
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When the desired measurand is not accessible, we can use
- either another measurand that bears a known relation to the desired one
- some form of energy or material that interacts with the desired measurand to generate a new measurand that is accessible
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Indirect
- measure a quantity that is accessible and related to the desired measurand
- assumption: the relationship between the measurands is already known
- often chosen when the measurand requires invasive procedures to measure directly
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Cardiac output (volume of blood pumped per minute by the heart)
- determined from Measurements of respiration and blood gas concentration (invasive or non-invasive)
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Dye dilution (invasive)
- measured from the speed of indicator substance passing from injection site to measurement site.
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Morphology of internal organs
- determined from x-ray shadows (non-invasive)
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• Pulmonary volumes
- determined from variations in thoracic impedance
- plethysmography (noninvasive).
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Sampling and Continuous
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Sampling
- Measurands such as body temperature and ion concentrations that change slowly sampled infrequently
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Continuous
- Fastly changing measurands such as ECG, respiratory gas flow
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• Measuring mode such as sampling or continuous are
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determined based on
- • objective of the measurement
- • condition of the patient
- • potential liability of the physician
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Generating and Modulating
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Generating
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• Modulating
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measurand modulates the electrical signal which is supplied externally source
- in a way that affects the output of the sensor.
- Example
- • Photoconductive Cell
- is called the Light Dependent Resistor or LDR.
- • A Photoconductive light sensor does not produce electricity
- but simply changes its physical properties when subjected to light energy.
- The most common type of photoconductive device
- is the Photoresistor
- which changes its electrical resistance
- use the measurand to alter the flow of energy from an
- external response to changes in the light intensity.
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Generating sensor produce their signal output from energy taken directly from measurand
- Example, photovoltaic cell.
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• Photovoltaic light sensor
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is the Solar Cell.
- convert light energy directly into DC electrical energy in the form of
- a voltage or current to a power a resistive load
- such as a light, battery or motor.
- also known as self-powered mode
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Analog and Digital Modes
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Analog sensors
- Analog: continuous in time and continuous in amplitude,
- require analog-to-digital converters for digital signal processing
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Digital sensors
- Digital: discrete in time and take only a finite number of different values
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require digital-to-analog converters to interface with analog display devices.
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The advantages of the digital mode of operation include
- Greater accuracy
- Repeatability
- Reliability
- Noise immunity
- No periodic calibration
- Readability (in display)
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Quasi-digital sensors
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Quasi digital
- variable frequency, pulse rate, or pulse duration that are easily converted to digital signals
- combine a simplicity and universality that is inherent to analog devices and accuracy and noise immunity, proper to sensors with digital output.
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•There are indirect digital sensors that use analog primary sensing elements and digital variable conversion elements
- , e.g., optical shaft encoders.
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Many clinicians prefer analog display
- to check whether a physiological variable is within certain limits or to observe fastly changing variable
- such as heart rate since it is annoying to observe it on digital display
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Real-Time and Delayed-Time
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Sensors acquire signals in real time as the signals occur. The output
- of the measurement system may not display the result immediately.
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• Some types of signal processing such as averaging and transformations
- need considerable input before any results can be produced.
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• Real-time
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• Example: ECG signals
- need to measured in real-time to determine an impending cardiac arrest
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• Delayed-time
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• Example: cell cultures
- which requires several days before any output is acquired