Temperature Sensing Devices: Thermocouples and Thermistors

Unit 1. Temperature Sensing Devices

Thermocouple

A thermocouple is a device made by joining two dissimilar metals that produces a voltage (Seebeck effect), which is a function of the temperature difference between one end called the "hot junction" and another called the "cold junction" or reference junction. In industrial instrumentation, thermocouples are widely used as temperature sensors. They are inexpensive, interchangeable, have standard connectors, and are capable of measuring a wide range of temperatures.

Selection of wires is based on:

• Resistance to corrosion, oxidation, and reduction.
• Crystallization stability.
• Relatively high EMF stability.
• Lower cost.
• Low electrical resistance.
• EMF rise parallels the rise of temperature ($\text{T}^{\circ}$).

Thermistor

A thermistor is a semiconductor device with a negative temperature coefficient. With increasing temperature ($\text{T}^{\circ}$), its resistance lowers, presenting large and rapid variations for small changes in $\text{T}^{\circ}$. They are small; their response time depends on the thermal capacity and mass of the thermistor.

Thermoresistance

Thermoresistance is based on the change in the electrical resistance value of a metallic conductor depending on the temperature ($\text{T}$):

$$R(t) = R_0 (1 + \alpha t)$$ (Where $\alpha$ is the temperature coefficient.)

They are of two types: metal and semiconductor.

Radiation Pyrometers

Radiation pyrometers are used to measure high temperatures ($\text{T}^{\circ}$), capturing all or most of the radiation emitted by the body to be analyzed. They are based on the Stefan-Boltzmann law, which states that the intensity of radiant energy emitted by the surface of a black body increases proportionally to the fourth power of the absolute temperature of the body.

Dilation Thermometers

Thermometer Bulb and Capillary

These consist of a bulb containing a fluid connected through a capillary tube. When the temperature of the bulb changes, the fluid in the bulb expands, causing the fluid column in the capillary to move, indicating the variation of temperature ($\text{T}^{\circ}$) in the bulb.

Glass Thermometer

It consists of a glass vessel containing a fluid that expands when heated and rises through the capillary tube. Its use is limited depending on the freezing point and boiling point characteristics of the liquid used for dilation.

Bimetal Thermometer

Formed by two blades welded together, each having a different coefficient of expansion. As the metals expand differently due to heat, the entire assembly tends to bend. Therefore, these thermometers do not need gears and have an unlimited duration.