Instrumentation Basics - Part 2: Types of Instruments

 

In Part 1, we covered the fundamental concepts of instrumentation. Now, we'll delve into the various types of instruments used to measure common process variables.

1. Temperature Measurement

• Temperature is one of the most frequently measured variables in industry.
• Thermocouples:
  • Measure temperature by generating a voltage proportional to the temperature difference between two dissimilar metals (Seebeck effect).
  • Types: J, K, T, E, R, S, B (each with different temperature ranges and characteristics).
  • Advantages: Wide temperature range, rugged.
  • Disadvantages: Non-linear output, require cold junction compensation.
• Resistance Temperature Detectors (RTDs):
  • Measure temperature by the change in electrical resistance of a metal (usually platinum) with temperature.
  • Advantages: Accurate, linear output.
  • Disadvantages: Narrower temperature range than thermocouples, more fragile.
• Thermistors:
  • Semiconductor devices whose resistance changes significantly with temperature.
  • Advantages: High sensitivity.
  • Disadvantages: Non-linear, limited temperature range.
• Infrared Pyrometers:
  • Measure temperature by detecting the infrared radiation emitted by an object.
  • Advantages: Non-contact measurement.
  • Disadvantages: Affected by emissivity of the object.

2. Pressure Measurement

• Pressure is defined as force per unit area.
• Bourdon Tube:
  • Measures pressure by the deflection of a curved tube that straightens out when pressure is applied.
  • Types: C-type, spiral, helical.
  • Advantages: Simple, relatively inexpensive.
  • Disadvantages: Limited accuracy.
• Diaphragm Gauge:
  • Measures pressure by the deflection of a flexible diaphragm.
  • Types: Sealed, open.
  • Advantages: Can measure low pressures.
  • Disadvantages: Susceptible to damage from overpressure.
• Bellows Gauge:
  • Measures pressure by the expansion or contraction of a bellows.
  • Advantages: High sensitivity.
  • Disadvantages: More complex and expensive than Bourdon tubes.
• Strain Gauge:
  • Measures pressure by the change in electrical resistance of a wire or semiconductor when it is strained.
  • Advantages: Accurate, can measure high pressures.
  • Disadvantages: Requires signal conditioning.
• Piezoelectric Transducer:
  • Generates a voltage when pressure is applied to a piezoelectric crystal.
  • Advantages: Fast response.
  • Disadvantages: Sensitive to temperature changes.

3. Flow Measurement

• Flow rate measures the quantity of fluid moving past a point per unit time.
• Differential Pressure Flowmeters:
  • Measure flow rate by measuring the pressure drop across a restriction in the flow path.
  • Types: Orifice plate, venturi tube, pitot tube.
  • Advantages: Simple, inexpensive.
  • Disadvantages: Pressure loss, sensitive to fluid properties.
• Variable Area Flowmeters (Rotameters):
  • Measure flow rate by the position of a float in a tapered tube.
  • Advantages: Simple, direct reading.
  • Disadvantages: Limited accuracy, vertical installation required.
• Magnetic Flowmeters:
  • Measure flow rate of conductive fluids by inducing a voltage as the fluid passes through a magnetic field.
  • Advantages: Linear output, unaffected by viscosity.
  • Disadvantages: Only for conductive fluids, expensive.
• Ultrasonic Flowmeters:
  • Measure flow rate by measuring the velocity of sound waves in the fluid.
  • Types: Transit-time, Doppler.
  • Advantages: Non-intrusive, can measure a wide range of fluids.
  • Disadvantages: Affected by fluid properties.
• Coriolis Flowmeters:
  • Measure mass flow rate by measuring the Coriolis force exerted on a vibrating tube.
  • Advantages: Accurate, measures mass flow directly.
  • Disadvantages: Expensive.

4. Level Measurement

• Level measurement determines the height of a liquid or solid in a container.
• Direct Level Measurement:
  • Dipstick: Simple, manual measurement.
  • Sight Glass: Allows visual observation of the liquid level.
  • Float: A float moves with the liquid surface.
  • Advantages: Simple, direct indication.
  • Disadvantages: Manual or limited to visual indication.
• Hydrostatic Pressure Measurement:
  • Measures level by measuring the pressure exerted by the liquid column.
  • Types: Pressure transmitter, bubbler.
  • Advantages: Simple, reliable.
  • Disadvantages: Affected by fluid density.
• Displacement Measurement:
  • Measures level by measuring the buoyant force on a displacer.
  • Advantages: Accurate.
  • Disadvantages: Complex.
• Capacitance Measurement:
  • Measures level by the change in capacitance between a probe and the liquid.
  • Advantages: Can measure various liquids.
  • Disadvantages: Affected by fluid conductivity.
• Ultrasonic Level Measurement:
  • Measures level by measuring the time it takes for an ultrasonic pulse to travel to the liquid surface and back.
  • Advantages: Non-contact.
  • Disadvantages: Affected by foam and turbulence.
• Radar Level Measurement:
  • Similar to ultrasonic, but uses electromagnetic waves.
  • Advantages: Unaffected by vacuum, pressure, temperature.
  • Disadvantages: More expensive.

This part provides a comprehensive overview of the main types of instruments used in instrumentation. Each type has its own principles of operation, advantages, and disadvantages, making it suitable for specific applications.