3. MEASUREMENT OF NON-ELECTRIC QUANTITIES
Measuring non-electric quantities such as temperature, pressure, speed, vibration, and flow is crucial in various industrial and laboratory applications. Different devices and principles are used for each type of measurement, often converting non-electrical quantities into electrical signals that can be easily processed and analyzed.
3.1 Temperature Measurement
Temperature measurement is a critical aspect of industrial processes, HVAC systems, and scientific experiments. There are various devices used to measure temperature, such as RTDs, thermistors, and thermocouples.
3.1.1 RTD (Resistance Temperature Detector)
- Construction: An RTD consists of a resistive element made from a material like platinum. This element is usually formed as a thin wire or a film wound on a ceramic or glass base.
- Working Principle: RTDs operate on the principle that the resistance of certain metals (like platinum) increases linearly with temperature. The RTD changes its resistance with temperature, and this change can be measured and converted to a temperature reading.
- Technical Specifications:
- Accuracy: High accuracy, typically ±0.1°C to ±0.5°C.
- Temperature Range: Common RTDs have a temperature range from -200°C to 850°C.
- Resistance: Typically, 100 ohms at 0°C for platinum RTDs.
- Applications: RTDs are used in industries like chemical processing, oil & gas, HVAC, and food processing, where precise temperature measurement is critical.
3.1.2 Thermistor
- Construction: A thermistor is a type of resistor whose resistance varies significantly with temperature. It is made from ceramic materials that are sensitive to temperature changes.
- Working Principle: The resistance of a thermistor decreases with an increase in temperature for Negative Temperature Coefficient (NTC) thermistors or increases for Positive Temperature Coefficient (PTC) thermistors.
- Technical Specifications:
- Accuracy: Generally ±0.5°C to ±2°C.
- Temperature Range: Typically from -50°C to 150°C.
- Resistance: The resistance of thermistors can vary from a few ohms to several megaohms depending on the type and temperature.
- Applications: Thermistors are commonly used in household appliances (like thermostats), automotive temperature sensors, and battery management systems.
3.1.3 Thermocouple
- Construction: A thermocouple consists of two dissimilar metal wires (e.g., copper and iron, or nickel and chromium) joined at one end to form a junction.
- Working Principle: When the junction of the two metals is heated or cooled, a voltage (Seebeck effect) is generated. The voltage is related to the temperature difference between the junction and the other ends of the wires.
- Technical Specifications:
- Accuracy: Varies based on type; common ranges are ±1°C to ±5°C.
- Temperature Range: Typically from -200°C to 2500°C depending on the metal pair.
- Voltage Output: The output voltage is usually in the range of microvolts per degree of temperature change.
- Applications: Thermocouples are widely used in high-temperature applications, such as furnaces, engines, and metal processing.
3.2 Pressure Measurement
Pressure measurement is essential in industries like oil & gas, chemical processing, and manufacturing. Various devices are used to measure pressure, including Bourdon tubes, bellows, and strain gauges.
3.2.1 Bourdon Tube
- Construction: A Bourdon tube is a hollow, curved tube that is typically made of metal. It has a C-shape or helical shape, and one end is sealed while the other is connected to the pressure source.
- Working Principle: When pressure is applied, the Bourdon tube tends to straighten or deform, and this mechanical deformation is transferred to a pointer or dial, indicating the pressure.
- Applications: Commonly used in mechanical pressure gauges for measuring gas or liquid pressure in pipelines and tanks.
3.2.2 Bellow Diaphragm
- Construction: A bellows is a flexible, accordion-like structure made of metal, while a diaphragm is a flexible, thin membrane.
- Working Principle: When pressure is applied to the bellows or diaphragm, it causes the bellow to expand or the diaphragm to deflect. This deflection is then measured to indicate the pressure.
- Applications: Used in pressure transmitters, manometers, and other pressure-measuring devices where flexibility is needed for accurate readings.
3.2.3 Strain Gauge
- Construction: A strain gauge is a small electrical resistor whose resistance changes when it is subjected to mechanical strain.
- Working Principle: Strain gauges work on the principle that when the gauge is deformed (due to pressure-induced strain), its resistance changes. This change is proportional to the amount of deformation and can be measured electrically.
- Applications: Often used in pressure sensors, load cells, and other devices where precise measurement of force or pressure-induced strain is required.
3.3 Speed Measurement
Speed measurement is crucial in monitoring rotational speed in motors, turbines, and other machinery. Different types of tachometers and sensors are used for this purpose.
3.3.1 Contacting and Non-contact Type – DC Tachometer
- Contacting Tachometer: A contacting tachometer uses a physical connection with the rotating object (usually through a probe or wheel) to measure its speed.
- Non-contacting Tachometer: A non-contacting tachometer measures the speed of a rotating object using infrared light or laser technology without physically touching the object.
- Working Principle: Both types measure the rotational speed, but contacting tachometers rely on friction or physical engagement, while non-contacting types use optical sensors or magnets.
- Applications: Used in motors, fans, and turbines to monitor their speed.
3.3.2 Photoelectric Tachometer
- Construction: A photoelectric tachometer consists of a light source (like an LED) and a photodetector. It uses the reflection or interruption of light from a rotating object to measure speed.
- Working Principle: The rotating object, marked with a reflective or opaque patch, passes through the light beam, causing interruptions or reflections. The rate of these interruptions corresponds to the speed.
- Applications: Common in automotive, industrial machinery, and research applications where high accuracy is required.
3.3.3 Toothed Rotor Tachometer Generator
- Construction: A toothed rotor tachometer has a rotor with a series of teeth or slots. A magnetic field or optical sensor detects the passing of each tooth.
- Working Principle: The generator produces a frequency proportional to the rotational speed of the rotor. The more teeth or slots, the more pulses are generated, allowing for precise speed measurement.
- Applications: Used in turbines, motors, and engines for speed monitoring.
3.3.4 Magnetic Pickup
- Construction: A magnetic pickup consists of a coil of wire and a magnetic core. It is positioned near a rotating metallic object.
- Working Principle: As the metallic object moves, it induces a change in the magnetic field, producing an electrical signal. The frequency of the signal corresponds to the rotational speed.
- Applications: Common in automotive applications to measure the speed of engine components.
3.3.5 Stroboscope
- Construction: A stroboscope consists of a flashing light source and a synchronization mechanism.
- Working Principle: The stroboscope flashes light at a frequency that matches the speed of the rotating object. By adjusting the flash frequency, the rotating object appears to freeze or move in slow motion, allowing its speed to be determined.
- Applications: Used for visual inspection of rotating machinery or to measure the speed of small motors and moving parts.
3.4 Vibration Measurement by Accelerometer
Vibration measurement is important in diagnosing machinery and structural integrity. Accelerometers are the most common instruments used for this purpose.
3.4.1 LVDT Accelerometer
- Construction: An LVDT (Linear Variable Differential Transformer) accelerometer has a moving core inside a coil structure, and the core's motion is sensed by the LVDT.
- Working Principle: When the object to which the accelerometer is attached experiences acceleration, the core moves. This movement changes the inductance, and the LVDT converts this change into an electrical signal.
- Applications: Used in industries for measuring acceleration, vibration, and shock.
3.4.2 Piezoelectric Type
- Construction: A piezoelectric accelerometer contains crystals (such as quartz) that generate an electrical charge when subjected to mechanical stress.
- Working Principle: When the accelerometer experiences vibration or acceleration, the piezoelectric crystals deform, generating a charge that is proportional to the acceleration.
- Applications: Widely used in industrial applications to monitor machinery vibrations and detect faults.
3.5 Flow Measurement
Flow measurement is essential in industries like water treatment, oil & gas, and food processing. Various technologies are employed for flow measurement.
3.5.1 Electromagnetic Flow Meter
- Construction: An electromagnetic flow meter uses a magnetic field and electrodes to measure the flow of conductive fluids.
- Working Principle: As the conductive fluid flows through a magnetic field, a voltage is induced in the fluid (according to Faraday’s Law). This voltage is measured by electrodes, and the flow rate is determined from the voltage.
- Applications: Used for measuring the flow of water, slurries, and chemicals in industries where the fluid is conductive.
3.5.2 Turbine Flow Meter
- Construction: A turbine flow meter consists of a rotor or turbine wheel positioned in the flow path. The fluid flow causes the rotor to spin.
- Working Principle: The rotational speed of the turbine is directly proportional to the velocity of the fluid passing through it. The rotor's speed is converted into an electrical signal to indicate the flow rate.
- Applications: Common in the oil, gas, and water industries for measuring the flow of clean liquids and gases.
0 Comments