Temperature Sensors and Thermistors Quiz

Sensors and Transducers

• Sensors detect changes in physical stimuli, converting them into measurable signals.
• Transducers convert energy from one form to another. Many convert non-electrical stimuli (like temperature, sound, or light) into electrical signals.

Transducer Classification

• Transducers are classified by their application, based on the physical quantity measured.
• Passive transducers require external power; their output is a measure of some variation (e.g., resistance, capacitance).
  • Example: Condenser microphone
• Self-generating transducers don't need external power; they generate analog voltage or current when stimulated by physical energy.
  • Example: Thermocouple

Temperature Transducers

• Thermocouples: Two dissimilar wires joined at both ends. Heating one junction creates a small electric current proportional to the temperature difference.
• The relationship between temperature and EMF (electromotive force) is generally linear within operating ranges. The specific metals used in the wires determine the characteristics. Standard types are used for consistent readings.
• Thermocouples come in various forms: insulated wires, insulated wire pairs in tubes with mineral insulation or metal enclosures (e.g., stainless steel).
• Thermocouples can measure temperatures from -270°C up to 2700°C.
• The EMF is related to the temperature difference between the hot and cold junctions and to constants of the materials. The formula is: E = c(T1 – T2) + k(T1² – T2²) where c and k are constants.

Resistance Type Sensors

• Resistance changes with temperature for conductive materials.
• A constant voltage applied results in current changes that vary with temperature.
• Resistance type sensors' resistance increases as the conductor gets hotter.
  • Example: Platinum resistance thermometer
• A basic temperature sensor uses a thin resistance wire wound into a small head, the resistance directly representing the temperature.
• This type is unaffected by the gauge end temperature, unlike thermocouples.
• Platinum is frequently used.
• Thermistors: Semiconductor materials designed with a strong temperature-resistance relationship; available in negative temperature coefficient- (NTC) or positive temperature coefficient-(PTC) types.
  • NTC thermistors: resistance decreases as temperature increases
  • PTC thermistors: resistance increases as temperature increases
• Typical range of NTC thermistors is -200°C to 1000°C.
• Thermistors are typically suitable for use in daily-use temperature measurements, in an acceptable range (-20°C to 120°C).

Liquid Expansion and Vapour Pressure Sensors

• These measure temperature by monitoring liquid expansion or evaporation.
• The liquid-filled sensor head and connecting tube are completely filled.
• An increase in temperature results in expansion or evaporation of the liquid creating pressure.
• The pressure is measured using a pressure gauge.
• These sensors are robust and can operate over a wide range.
• Often used as thermometers, or for controlling temperature using thermostats and alarms.
• Example uses: mercury thermometers, evaporating liquids used in refrigerators.

Bimetallic Sensors

• This relies on differing expansion rates of two rigidly-joined metals.
• When heated, one metal expands more than the other.
• The difference causes the bimetal strip to bend. This bend is linked to a pointer on a dial, or limit switches to set off alarms or act as a thermostat.

Pressure Transducers

• These measure pressure changes.
• Bourdon Tube: A hollow tube with an elliptical cross-section. Pressure difference causes the tube to straighten, moving a needle on a dial or a secondary device, such as an air nozzle (commonly used in gauges to measure pressure)
• Piston Type: Pressure acts directly on a piston, compressing a spring. The piston's position relates directly to the pressure; a window in the outer case indicates the pressure with a pressure gauge. (Found commonly in hydraulic instruments)
• Bellows: Made from flattened metal capsules which expand when pressurized. The movement is proportional to the difference between the inside and outside pressure; useful for precise measurement of low pressures

Speed Transducers

• Optical Types: Light beam sensors that either reflect or interrupt a light beam, counting pulses over a fixed time. Electronic processing calculates speed.
• Magnetic Pickups: Inductive coil placed near a rotating body with a small magnet on it generating a pulse with each coil passage. Discontinuities in magnetic materials also trigger pulses.
• Tachometers: These are commonly built into electric motors to measure their rotational speed. The voltage frequency is proportionate to the rotational speed.

Flow Meters

• These quantify liquid, gas or vapor flow, using a variety of methods; based on types of displacement, inferential, variable area or differential pressure methods.
• Positive Displacement Types: Mechanical elements that rotate once for a precise amount of fluid; the flow rate depends on shaft rotations and speed, measurable mechanically or electronically. Examples: meshing motor.
• Inferential Types: Calculate flow by an effect of the flow produced - rotation of rotors is detected (mechanically or electronically) for turbine and rotating vane types.
• Variable Area Types: Utilizing a float inside a tapered tube with a flow rate affecting the pressure; examples: float type, tapered plug.
• Differential Pressure Types: Measures the flow rate by determining differential pressure (using orifice, Venturi or nozzle methods or pitot tubes). The working principle relies on changes in fluid velocity and corresponding changes in pressure. The volumetric flow rate is related to the differential pressure by the formula Q = √kΔP.

Force Sensors

• Mechanical Types: Based on the deflection of a spring proportional to the force applied; the deflection (movement) is measured on a scale.
• Hydraulic Types: Capsules filled with liquid that become pressurized when squeezed; the pressure represents the force and can be measured using a pressure gauge.
• Electrical Strain Gauge Types: Employing metal cylinders with strain gauges. Stretched or compressed cylinders change the resistance, thus enabling force to be measured in voltage or resistance changes.

Position Sensors

• Resistive Types: Using a potentiometer that is made up of a resistor tracks (made from carbon or resistive material). The movement of a slider along the track picks off the voltage depending on its position or angle.
• Optical Types: Employing light emitted onto photoelectric cells along lines engraved on a transparent strip or disc, to calculate position or angle.
• Inductive Types: A primary coil and two secondary coils used to generate voltage changes proportional to the position of a movable magnetic core.

Depth Gauges

• Instruments used to measure the liquid or powdered depth of tanks.
• Various methods are employed, often involving the use of electricity or air pressure for measuring depth based on fluid properties.
  • Example: Ultrasound sensors use sound waves to determine the depth.