Sensor Types: Temperature, Pressure, Flow, Weight, and pH
Sensor Types and Their Applications
Temperature Sensors
Temperature sensors are essential for various applications. Common types include:
- Bulb
- RTDs (Resistance Temperature Detectors)
- Thermistors
- Thermocouples
RTDs (Resistance Temperature Detectors)
Measurement Principle: Based on the change in electrical conductivity of metals with temperature variations.
- Platinum: -200°C to 900°C (Accuracy: ±0.01°C)
- Nickel: -150°C to 120°C (Accuracy: ±0.5°C)
- Copper: -200°C to 120°C (Accuracy: ±0.1°C)
Advantages: High accuracy, rapid response, good stability.
Disadvantages: More expensive than thermocouples and thermistors, fragile, self-heating can be problematic.
Thermocouples
Measurement Principle: Based on the Seebeck effect, where current flows in a closed circuit of two different metals with junctions at different temperatures.
Applications: Gas-electric industrial furnaces, boilers, moving grates.
Ranges: -200°C to 1700°C (depending on the alloy)
Advantages: Small, inexpensive, fast response, easy assembly.
Disadvantages: Cold junction compensation required. Simplest and most commonly used for remote temperature indication and multi-point measurements. Cost-effective. Common material types: J, K, T, R, O, S.
Pressure Sensors
- Bourdon Tube
- Strain Gauges/Load Cells
- Capacitive Transducers
- Piezoelectric Transducers
Bourdon Tube
Operating Principle: Based on the deflection of flexible bodies under pressure.
Applications: Liquids and vapors (steam mixed with chips)
Ranges:
- C-Type: 0.5 to 6,000 bar (Accuracy: ±0.5% to ±1%)
- Spiral: 0.5 to 2,500 bar (Accuracy: ±0.5% to ±1%)
- Helical: 0.5 to 5,000 bar (Accuracy: ±0.5% to ±1%)
Advantages: Wide availability, helical and spiral types offer greater movement range (ideal for recorders), small size, long length.
Disadvantages: Higher construction cost depending on the material.
Piezoelectric Transducers
Operating Principle: Based on the generation of an EMF when certain crystals deform under pressure.
Ranges: 0.1 to 600 kg/cm² (Accuracy: ±1%)
Advantages: High-frequency response, self-generating, small size, robust, linear, low sensitivity to vibration.
Disadvantages: Sensitive to temperature changes, high output impedance, low output, poor stability, zero shift after extreme shock.
Flow Sensors
- Orifice Plate
- Rotameter
- Turbine
- Ultrasound
- Magnetic
Orifice Plate
Operating Principle: Flow through a restricted pipe creates a pressure differential.
Applications: Gases, liquids, vapors, solids, and combinations.
Advantages: Various hole types for different fluids, simple, relatively low cost, adaptable to various pipe sizes.
Disadvantages: High-pressure drop, unsuitable for viscous or dirty fluids, non-linear (quadratic) response, unsuitable for small flows.
Types of Holes: Concentric, eccentric, segmental.
Weight Sensors
- Hydraulic Load Cells
- Strain Gauges
- Pneumatic Load Cells
Strain Gauges
Operating Principle: Strain gauges installed on platform ends deform under pressure, changing electrical resistance, measured using a Wheatstone bridge.
Applications: Belt conveyors (weightometers)
Ranges: 20 kg – 150 tons (Accuracy: ±0.2%)
Advantages: Corrosion-protected, non-contact with material.
Disadvantages: Temperature sensitive, relatively high cost.
Hydraulic Load Cells
Operating Principle: Piston exerts pressure on hydraulic fluid.
Ranges: 40 kg – 90 tons (Accuracy: ±0.2%)
Advantages: Simple installation, fast response, vibration resistant, supports overload (up to 40%), explosion-proof, remote indicators.
Disadvantages: Relatively high cost.
Pneumatic Load Cells
Operating Principle: Weight on the platform is compared to the force exerted by a diaphragm with adjustable pressure.
Applications: Generally for static weighing.
Ranges: 10 kg – 10 tons (Accuracy: ±0.2%)
Advantages: Suitable for pneumatic remote indication.
Disadvantages: Requires compressed air instrumentation.
pH Sensors
Principle of Operation: Sealed glass tube with a membrane sensitive to hydrogen ions.
Applications: Acidic and neutral solutions
Ranges: 0-14 (Accuracy: ±0.25% to ±1%)
Advantages: Simple installation, quick response.
Disadvantages: Requires regular maintenance, temperature variations affect measurements.
Temperature Conversion Formulas
- °F = (5/9 * °C) + 32
- °C = 9/5 * (°F – 32)
- K = °C + 273