Thermocouple Temperature Measurement

Temperature Measurement

• Thermocouples consist of two dissimilar metals (A, B) and extension leads (C, D) with internal reference junctions (A', B') for temperature compensation.
• Thermocouples can be configured with dual reference junctions.

Thermoresistive Sensors

• Resistance of the sensing element changes with temperature.
• There are two basic types of thermoresistive sensors: RTDs (Resistance Temperature Devices) and Thermistors.

Resistance Temperature Devices (RTD)

• The resistance of the device usually increases with temperature (PTCs).
• RTDs are constructed of metallic wire wound around a ceramic or glass core.
• In some cases, the TCR (Temperature Coefficient of Resistance) is not a constant, and relations have been established based on actual measurements.
• Coefficients are specified by standards measurements.

Thermistors

• Thermistors are semiconductor metal oxide devices where resistance changes exponentially with temperature.
• Usually, resistance decreases with increasing temperature (NTC thermistors).
• There are different types of thermistors, including bead thermistors, chip thermistors, and deposited thermistors on ceramic substrate.
• PTC thermistors are also available, depending on the type of material.
• Thermistors are more sensitive than RTDs, but have a lower operating range.

Position, Displacement, and Speed Measurement

Linear Potentiometer

• Linear potentiometers are used for rectilinear motion measurements, while rotary potentiometers are used for angular motion measurements.
• Linear potentiometers are electromechanical devices containing a fixed resistive element and a conductive wiper that slides against the element.
• The output is given by vo = kx, where x is the displacement, but the actual relationship is non-linear due to finite loading impedance at output terminals.

Proximity Sensor

• Proximity sensors generally consist of an element that changes its state when an object is close by, but not actually touching it.

Pressure Sensors

• Most common pressure sensing element types include Bourdon tube, diaphragm, capsule, and bellows types.

Flow Measurement

• Different types of techniques used to estimate the flow rate of fluids include Coriolis meters, electromagnetic principle, ultrasonic flow meters, vortex flow meters, and orifice/nozzle/venturi type flow meters.
• Traditional method is to measure using an orifice plate, which causes a change in pressure.

Temperature Measurement

• Most commonly based on contraction or expansion of solid, liquid, or gas, and change in the electric resistance of a material.
• Common sensors and techniques include thermocouples, RTD (Resistive temperature devices), pyroelectric devices, thermal imaging, and bimetallic thermometers.