Direct vs. Indirect
- measure desired measurand directly
if the sensor is invasive
direct contact with the measurand is possible but expensive,r,isky and least acceptable
- e.g., Blood glucose meter
- e.g., Thermometer
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
- 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
Cardiac output (volume of blood pumped per minute by the heart)
- determined from Measurements of respiration and blood gas concentration (invasive or non-invasive)
Dye dilution (invasive)
- measured from the speed of indicator substance passing from injection site to measurement site.
Morphology of internal organs
- determined from x-ray shadows (non-invasive)
• Pulmonary volumes
- determined from variations in thoracic impedance
- plethysmography (noninvasive).
Sampling and Continuous
- Measurands such as body temperature and ion concentrations that change slowly sampled infrequently
- Fastly changing measurands such as ECG, respiratory gas flow
• Measuring mode such as sampling or continuous are
determined based on
- • objective of the measurement
- • condition of the patient
- • potential liability of the physician
Generating and Modulating
measurand modulates the electrical signal which is supplied externally source
- in a way that affects the output of the sensor.
- • 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.
Generating sensor produce their signal output from energy taken directly from measurand
- Example, photovoltaic cell.
• Photovoltaic light sensor
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
Analog and Digital Modes
- Analog: continuous in time and continuous in amplitude,
- require analog-to-digital converters for digital signal processing
- Digital: discrete in time and take only a finite number of different values
require digital-to-analog converters to interface with analog display devices.
The advantages of the digital mode of operation include
- Greater accuracy
- Noise immunity
- No periodic calibration
- Readability (in display)
- 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.
•There are indirect digital sensors that use analog primary sensing elements and digital variable conversion elements
- , e.g., optical shaft encoders.
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
Real-Time and Delayed-Time
Sensors acquire signals in real time as the signals occur. The output
- of the measurement system may not display the result immediately.
• Some types of signal processing such as averaging and transformations
- need considerable input before any results can be produced.
• Example: ECG signals
- need to measured in real-time to determine an impending cardiac arrest
• Example: cell cultures
- which requires several days before any output is acquired