Temperature Sensors: A Comprehensive Overview

A temperature sensor is a device that measures temperature variations. These sensors fall into several main categories, each with its unique characteristics and applications.

Types of Temperature Sensors

• Resistance Temperature Detectors (RTD)
• Thermocouples
• Thermistors
• Ultrasonic transducers

Resistance Temperature Detectors (RTD)

RTDs commonly use materials like platinum and nickel. Their resistance changes with temperature, offering high accuracy. They come in various configurations suitable for immersion and surface mounting.

The relationship between temperature and resistance in conductors is defined by the following equation:

$R = R_0 * (1 + \alpha\Delta T)$

Where:

• $R$ = Resistance of conductor at temperature T
• $R_0$ = Resistance at reference temperature (usually 23°C)
• $\alpha$ = Temperature coefficient of resistance
• $\Delta T$ = Difference between operating and reference temperature

Thermocouples

Thermocouples are a popular choice, consisting of two wires made of different metals joined at one end. A voltage is generated between the wires based on the temperature difference between the joined end and the other ends.

The voltage’s magnitude depends on the wire materials and the temperature difference. The electromotive force (EMF) of a thermocouple can be expressed as:

$E = c *(T_1 - T_2) + K *(T_1^2 - T_2^2)$

Where:

• $c$ and $K$ are constants specific to the thermocouple materials
• $T_1$ = Temperature of the hot junction
• $T_2$ = Temperature of the cold junction (reference junction)

Thermocouple Types and Characteristics

Thermistors

A thermistor is a semiconductor made from sintered mixtures of metallic oxides like nickel, manganese, copper, cobalt, and uranium oxides. Thermistors exhibit either a negative temperature coefficient (NTC) or a positive temperature coefficient (PTC).

• NTC Thermistors: Resistance decreases as temperature increases.
• PTC Thermistors: Resistance increases as temperature increases.

Thermistors are available in various forms:

• Disc: 1 to 1 MΩ
• Washer: 1 to 50 KΩ
• Rod: High resistance

Resistive Position Transducer

The principle is based on physical variable under measurement causes a resistance change in the sensing element. A common requirement in industrial measurement and control work is to be able to sense the position of an object or distance it has moved. The core principle relies on a change in resistance caused by the physical variable being measured. A common scenario is to sense the position of an object or the distance it has moved.

$R = \rho * \frac{L}{A}$

This equation represents a simple potentiometer, where:

• $R$ is the change in resistance
• $\rho$ is the resistivity
• $L$ is the length
• $A$ is the cross-sectional area