Direct vs Indirect Methods in Measurement Systems

Direct vs. Indirect Methods

• Direct methods measure a quantity directly using a standard. Examples include using a ruler for length and a thermometer for temperature.
• Indirect methods involve measuring related quantities and calculating the value of the desired quantity. Examples include using trigonometry for height and using Ohm's law for resistance.

Direct Methods - Summary

• Simple, single-purpose instruments are used.
• Usually more accurate than indirect methods.
• Minimal mathematical involvement.

Indirect Methods - Summary

• Multiple instruments or calculations are needed.
• Accuracy depends on the precision of all measurements.
• Significant mathematical involvement using formulas, equations, and relationships.

Measurement Systems

• Measurement systems are used in engineering applications for determining quantities or variables.
• These systems are modular and consist of several components.

Components of a Measurement System

• Measured Variable (Measurand): The physical quantity being measured (e.g., temperature, pressure, force).
• Sensor: Detects or senses the measurand and converts it into an electrical signal.
• Variable Conversion Element: Converts the signal generated by the sensor into a different form (e.g., mechanical to electrical).
• Signal Processing: Improves accuracy and makes the signal easier to interpret by amplifying, filtering, or digitizing it.
• Output Measurement: The processed signal is the measurable output that can be used.
• Signal Transmission: Transmits the processed signal to a remote location using wired or wireless means.
• Use of Measurement at Remote Point: The transmitted signal is used for controlling systems, logging data, or further analysis.
• Signal Presentation or Recording: Displays or records the final processed signal for users.
• Output Display/Recording: The final output allows users to read, observe, or store the data.

Measured Variable (Measurand)

• Refers to the physical quantity being measured (e.g., temperature, pressure, force).

Sensor

• Detects or senses the measurand.
• Converts the measurand into a corresponding signal, typically electrical.

Variable Conversion Element

• Modifies or converts the signal generated by the sensor.
• Handles changes from mechanical signals to electrical ones.

Signal Processing

• Improves accuracy or simplifies interpretation of the signal after conversion.
• Involves signal amplification, filtering out noise, or digitizing an analog signal.

Output Measurement

• The processed signal becomes the output measurement.
• This measurement can be transmitted to a remote location.

Signal Transmission

• Transmits the processed signal to a remote location for further use or display.
• Uses wired or wireless methods.

Use of Measurement at Remote Point

• Where the transmitted measurement is utilized.
• Could be for system control, data logging, or further analysis.

Signal Presentation or Recording

• The final processed signal is displayed or recorded for users.
• Shown on a screen, chart, or other output devices

Output Display/Recording

• The final stage where the measured value is displayed or recorded.
• Allows users to read, observe, or store the data.

Evolution of Instruments

• Mechanical instruments are reliable for static and stable conditions but lack rapid responses to dynamic or transient conditions.
• Electrical instruments are faster than mechanical ones.

Types of Measuring Instruments

• Deflection Type
  • Simple to operate and read.
  • Examples: Analog Ammeter, Pressure Gauge, Spring Balance.
• Null Type
  • More complex and may require skilled operation.
  • Used for precise and sensitive measurements.
  • Examples: Wheatstone Bridge, Potentiometer, Balance Scale.

Control a Process/Operation

• Measurement of a variable and its control are interconnected.
• Accurate measurement is crucial for controlling process variables like temperature, pressure, or humidity.

Experimental Engineering Analysis

• Used to find solutions to engineering problems.
• These problems can be theoretical designs or practical analysis.

Types of Instrumentation Systems

• Intelligent Instrumentation: Data is refined for presentation purposes.
• Dumb Instrumentation: Data requires processing by the observer.

Static Characteristics of Instruments and Measurement Systems

• Static characteristics apply to measurements of quantities that are constant or vary slowly over time.

Accuracy

• Closeness of a measured value to the true or accepted value.
• Indicates how correctly an instrument measures the actual quantity.

Active Transducers

• Need an external source of power for operation.
• Not self-generating.

Passive Transducers

• Produce an output signal in response to the measurand.
• Variations in resistance, capacitance, inductance, or other electrical parameters.

Capacitive Transducer

• Converts the measurand to a change in capacitance.
• Consists of two parallel conducting plates separated by a dielectric material.

Piezoelectric Transducers

• Convert mechanical stress or pressure into an electrical signal.
• Piezoelectric materials generate an electrical charge when subjected to mechanical stress.
• Suitable for high-frequency applications like ultrasound.
• Can be brittle and break under excessive mechanical stress.
• Compact and lightweight.

Strain Gauge

• Measures the amount of strain (deformation) in a material.
• Consists of a thin, conductive wire or foil that changes resistance when stretched or compressed.
• Acts as a secondary transducer by converting displacement into changes in resistance.