Electronic Instrumentation - General Configuration of Instruments

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Introduction to Instrumentation

• Definition: Instrumentation is a technology of measurement serving science, all branches of engineering, medicine, and almost every human endeavor.

• Primary Uses: Measurement is basically used to monitor a process or operation, or to control a process.

• Central Challenge: The major problem encountered with any measuring instrument is the error.

• Fundamental Principle: The basic concern of any measurement is that the measuring instrument should not affect the quantity being measured. However, in practice, an instrument always extracts some energy from the measured medium, meaning the measured quantity is always disturbed. It is impossible to make a perfect measurement.

• Selection Criteria: Knowledge of the performance characteristics of an instrument is essential for selecting the most suitable device for specific measuring jobs.

Functional Elements of an Instrument

A generalized measurement system consists of several functional elements that process information from the measured medium to the observer:

• Primary Sensing Element: It receives energy from the measured medium and produces an output depending on the value of the measured quantity.

• Variable Conversion Element: A device used for determining the value or magnitude of a quantity or variable.

• Variable Manipulation Element: Also known as the signal conditioning element. it converts the output of the primary element into a signal suitable for transmission.

• Data Transmission Element: Required when the elements of an instrument are physically separated; it performs the function of transmitting data from one element to another.

• Data Presentation Element: Conveys information about the measured quantity to personnel for monitoring, control, or analysis. This includes visual display devices.

Null and Deflection Methods

Electrical measurement systems generally employ one of two principles for instrumentation:

• Deflection Type System:

  • Principle: The quantity to be measured produces an effect (voltage or current) that creates a torque resulting in mechanical deflection.

  • Example: The PMMC (Permanent Magnet Moving Coil) instrument.

  • Mechanism: A spring system counters the torque with a restoring torque that increases with deflection. Equilibrium is reached when the pointer stops. Calibration is obtained by equating these torques mathematically.

  • Advantages: Convenient for use and readily used for calibration.

  • Disadvantages: Generally less accurate than null-type methods.

• Null Type Instrument:

  • Principle: The quantity to be measured is compared against an already calibrated effect of another system to achieve a "null" condition.

  • Example: The Wheatstone bridge used for measuring electrical resistance.

  • Mechanism: A sensitive galvanometer shows deflection for any difference between the measured and calibrated effects. The calibrated effect is varied (manually or automatically) until the galvanometer shows no deflection (zero/null).

  • Advantages: Extremely high accuracy.

  • Disadvantages: Can be difficult to use due to the addition of different weights/components involved in the balancing process.

Sources and Types of Measurement Errors

• General Sources of Error:

  1. Insufficient knowledge of process parameters and design conditions.

  2. Poor design.

  3. Change in process parameters, irregularities, and upsets.

  4. Poor maintenance.

  5. Human error by the operator.

  6. Certain design limitations.

• Categorization of Errors:

  • Gross Errors: Caused by human mistakes and improper use of instruments (e.g., misreading a scale or parallax error). They are avoided through great care in recording data and taking multiple observations.

  • Systematic Errors: Occur due to shortcomings of the instrument (defective/aging parts) or environmental effects. These are subdivided into:

    • Instrumental Errors: Inherent in the mechanical structure (e.g., friction in bearings, irregular spring tension, or stretching). To avoid these: zero the instrument, select suitable equipment, apply correction factors, or calibrate against a standard.

    • Environmental Errors: Due to external conditions such as temperature, humidity, pressure, or magnetic/electrostatic fields. To avoid these: use air-conditioning, hermetically seal components, or use magnetic shields.

    • Observational Errors: Introduced by the observer, most commonly parallax error. Avoided by reading the scale directly perpendicular to its face.

  • Random Errors: Also known as unsystematic errors or noise. These remain after gross and systematic errors are accounted for. They result from a large number of small effects and statistical fluctuations. Treated mathematically by averaging a large number of data points.

Error and Accuracy Calculations

• Absolute Error ($e$):   $e = A_n - A_t$   Where $A_n$ is the measured value and $A_t$ is the true value.

• Percentage Error ($\% \text{ Error}$):   $\% \text{ Error} = \frac{A_n - A_t}{A_t} \times 100$

• Relative Accuracy ($A$):   $A = 1 - \left| \frac{A_n - A_t}{A_t} \right|$

• Accuracy Percentage ($a$):   $a = A \times 100$

• Example calculation:

  • Expected value ($A_t$) = $80\,V$. Measured value ($A_n$) = $79\,V$.

  • Absolute error = $1\,V$.

  • Percentage error = $1.25\, \%$.

  • Relative accuracy = $0.9875$.

  • Percentage of accuracy = $98.75\, \%$.

Loading Effects and Limiting Errors

• Loading Errors: To minimize the loading effect:

  • Voltmeter: Internal resistance must be very large (Mega Ohms) because it is connected in parallel.

  • Ammeter: Internal resistance must be very small (approximately zero) because it is connected in series.

• Limiting Errors: The manufacturer guarantees accuracy within a certain percentage of the full-scale reading.

  • Example: A $600\,V$ voltmeter accurate within $\pm 2\, \%$ at full scale has a limiting error of $\pm 12\,V$ ($0.02 \times 600$). If used to measure $250\,V$, the percentage limiting error increases: $\frac{12}{250} \times 100 = 4.8\, \%$.

• Tolerance: A resistor of 600\, ̩ with an absolute error of \pm 60\, ̩ has a relative error of $\pm 10\, \%$. This is also referred to as a tolerance of $\pm 10\, \%$.

Analog vs. Digital Instrumentation Systems

• Analog System Components:

  • Primary Element (Transducer): Converts a physical quantity into a proportional electrical signal (voltage, current, resistance change).

  • Secondary Element (Signal Processing): Amplifies, filters, or modifies the weak transducer output (e.g., amplifiers, A/D converters).

  • Final Element (Output Unit): Indicates value via a CRO, digital computer, or pointer.

• Digital System Components:

  • Transducer: Converts parameters (temp, pressure) to electrical signals.

  • Signal Conditioning: Balancing circuits and calibrating elements.

  • Scanner/Multiplexer: Sequentially provides multiple analog signals to the measuring instrument.

  • ADC: Converts analog signals to digital for the recorder.

  • Digital Recorder: Stores data for analysis.

  • Auxiliary Systems: Computers and software for management and optimization.

• Comparison Table:

  • Analog: Continuous output, less accuracy, high power consumption, higher sensitivity, cheaper, highly portable, lower resolution.

  • Digital: Discrete output (finite values), high accuracy, lower power consumption, lower sensitivity, expensive, less portable, higher resolution.