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Questions and Answers
What is the primary function of medical instrumentation?
Which of the following is an example of a physiological variable?
What does a transducer convert?
Which principle is NOT associated with active transducers?
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How does the magnetic induction principle operate in active transducers?
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What type of transducer is a pulse transducer classified as?
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Which of the following statements about transducers is correct?
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What is an example of a biomedical application using the magnetic induction principle?
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What is the purpose of a strain gauge?
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What is the Gauge factor (G) for metals approximately?
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What does the capacitive transducer use to convert mechanical movement into an electrical signal?
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Which factors cause changes in capacitance in a capacitive transducer?
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What is the value of absolute permittivity (ε0) in F/m?
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How does a strain gauge measure changes caused by physical deformation?
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In a capacitive transducer, which dielectric constant represents plastics?
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Which statement about strain gauges is correct?
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What happens to certain non-conductive materials when pressure is applied?
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What is the primary function of a thermocouple?
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What generates the thermoelectric electromotive force in a thermocouple?
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What is the Seebeck effect in relation to thermoelectric devices?
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What occurs during the Peltier effect?
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Which phenomenon relates to the emission of electrons when light hits a material?
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What is required to maintain a reference temperature in thermocouples?
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What do passive transducers primarily utilize for their operation?
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What is defined as the minimum change or smallest increment in the measured value that can be detected with certainty by the instrument?
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Which term refers to the maximum value of a quantity that an instrument is designed to measure?
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What is the formula for calculating span?
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Which of the following best explains hysteresis in instruments?
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What does repeatability in measurement signify?
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How is reproducibility different from repeatability?
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What aspect does an instrument's sensitivity measure?
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What does the term 'backlash' refer to in mechanical systems?
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What does drift in an instrument indicate?
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Which of the following factors does NOT contribute to instrument drift?
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What is zero drift in an instrument?
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Calibration is primarily aimed at achieving which two objectives?
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Span drift occurs when the calibration changes in what manner?
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When should a measuring device be calibrated?
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Zonal drift occurs when drift happens over which part of the instrument's calibration?
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What is the primary reason why calibration is critical for measuring devices?
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Study Notes
Physiological Variables
- Physiological variables include body temperature, heart electrical activity, arterial blood pressure, and respiratory airflow.
- Medical instrumentation's major function is to measure these physiological variables.
- Various forms of physiological variables consist of ionic potential, mechanical movements, hydraulic pressures, temperature variations, and chemical reactions.
Transducers
- Transducers convert one form of variable into another, typically into electrical signals for further amplification or processing.
- Two principles of transduction:
- Passive Transducers: Control a DC voltage or AC carrier signal.
- Active Transducers: Utilize energy conversion or carrier signal modulation.
Active Transducers: Magnetic Induction
- Voltage is induced in a conductor moving through a magnetic field, proportional to the change in magnetic flux.
- Used for measuring linear or rotary motion; output voltage corresponds to linear or angular velocity.
- Biomedical applications include heart sound microphones and electromagnetic blood flow meters.
Active Transducers: Piezoelectric Effect
- Electrical voltage is generated when pressure deforms certain non-conductive materials, causing charge separation.
- This reversible effect allows materials to function as both microphones and speakers.
Active Transducers: Thermoelectric Effect
- A thermoelectric e.m.f. occurs when two dissimilar metals joined in a circuit have different temperatures, creating a current.
- A thermocouple measures temperature differences, often using a reference point like the freezing point (0°C or 32°F).
- The Seebeck effect converts thermal energy to electrical energy, while the Peltier effect involves heating and cooling junctions.
Active Transducers: Photoelectric Effect
- The photoelectric effect occurs when light hits a material, causing electron emission (photoelectrons).
- Selenium cells are used to measure light intensity in photographic meters; the output current correlates with light intensity.
Passive Transducers
- Use a passive circuit element whose value changes based on the physical variable to be measured.
Passive Transducer Using Resistive Element: Strain Gauge
- A strain gauge comprises a zigzag metallic strip to measure small resistance changes due to physical stress.
- The Gauge factor (G) is the ratio of resistance change (ΔR/R) to length change (ΔL/L), with values around 2 for metals and 120 for silicon.
Passive Transducer Using Capacitive Element
- Contains parallel metal plates separated by a dielectric, measuring mechanical movement as a change in capacitance.
- Factors affecting capacitance include the area of overlap, distance between plates, and dielectric constant.
Accuracy vs. Precision
- Resolution: The smallest detectable measurement change by the instrument.
- Threshold: The range of input values where there is no change in output.
Range and Span
- Range: Maximum quantity an instrument can measure.
- Span: Difference between minimum and maximum measurable values (Max - Min).
Sensitivity
- Defined as the output change divided by input change, indicating the instrument's responsiveness within a linear range.
Hysteresis
- Refers to different instrument responses for increasing and decreasing input, leading to potential measurement errors.
Backlash
- Maximum distance traveled by a mechanical part without causing movement in the next part, minimized by close tolerances.
Repeatability and Reproducibility
- Repeatability: Consistency in measurements under same conditions and by same observer.
- Reproducibility: Agreement between measurements by various individuals, locations, and instruments.
Linearity
- Ability of an instrument to produce outputs linearly proportional to inputs, analyzed through calibration point deviation.
Drift
- Variation in instrument output over time, unrelated to operational conditions; causes include wear, temperature, vibrations, and fields.
Types of Drift
- Zero Drift: Overall instrument calibration shifts uniformly.
- Span Drift: Calibration changes proportionately from zero.
- Zonal Drift: Drift occurs only within part of the instrument span.
Calibration
- Involves comparing instrument measurements against known standards to ensure accuracy and traceability.
- Calibration is essential for maintaining measurement accuracy over time, improving product quality, and should follow manufacturer recommendations.
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Description
Explore the principles of active transducers, focusing on the piezoelectric and thermoelectric effects. Understand how pressure and temperature changes can generate electrical voltage in different materials. This quiz will help you grasp the reversible nature of these effects and their applications.