*Current Status and State-of-Art Developments in Temperature Sensor Technology DOI: http://dx.doi.org/10.5772/intechopen.112877*

standardizations include Type S, Type T, and Type K, which are known for their reliability and versatility in different measurement scenarios. Specific thermocouple types are shown in **Table 1**. It also demonstrates the effective measurement range and accuracy for the rank II thermocouples. With advances in measurement accuracy and remote signal transmission, thermocouples have become widely used for temperature measurement from −40°C to 1700°C. Given their widespread use and versatility, researchers have also explored various improvements to basic thermocouples, such as multi-probe type thermocouples and thin film thermocouples (TFTCs), as shown in **Figure 2**, to expand their functionality and precision in a wide range of applications.

## **2.3 Resistance temperature detector**

A resistance temperature detector (RTD) operates on the principle that resistance varies with temperature. This type of temperature sensor provides an analog signal of resistance value when an external constant excitation current is applied. It uses the RTD's excellent linear thermal response to characterize the measured temperature value. Platinum (Pt) is the preferred material for RTD temperature sensors due to its


#### **Table 1.**

*Thermocouple types [1].*

**Figure 2.** *Thin film thermocouple [2].*


#### **Table 2.**

*Platinum resistance temperature detector [3].*

excellent analog and digital linearity. Pt series RTDs have a temperature measurement range from −196°C to 600°C, and their measurement accuracy is much higher than that of thermocouples. For different RTDs within different wire-wound element and thin film element types, specific information is shown in **Table 2**. However, due to the small change in resistance with temperature, a high-precision reference and high-resolution analog-to-digital conversion (ADC) circuit are required. Therefore, platinum resistance is divided into three circuit wiring methods: two-wire, three-wire, and four-wire measurement methods. The four-wire circuit form can completely eliminate the error caused by lead resistance, as shown in **Figure 3**. It is mainly used for high-precision temperature measurement.

## **2.4 Thermistor**

A thermistor is an inexpensive temperature measurement element that exhibits a change in resistance magnitude with temperature. Unlike an RTD, its degree of linear thermal response is low, but it has high-temperature sensitivity and is made from materials that include metals, alloys, and semiconductors. Depending on the type of resistance change with temperature, thermistors are classified as either NTC (negative temperature coefficient, exhibiting a nonlinear relationship) or PTC (positive temperature coefficient). Due to their simple structure, thermistors are also widely used in the electronics industry, such as **Figure 4**.

**Figure 3.** *4 leg measurement for RTD.*

*Current Status and State-of-Art Developments in Temperature Sensor Technology DOI: http://dx.doi.org/10.5772/intechopen.112877*

**Figure 4.** *Thermistor schematic.*

#### **2.5 Summary**

Traditional temperature sensors have been widely used in daily life and industry, but they have gradually revealed certain limitations during their use. For example, thermal expansion-based sensors, such as the mercury thermometer, are prone to high-temperature failures and have poor universality in industrial applications. Additionally, these sensors are not suitable for dynamic measurements. Although the thermocouple, RTD, and thermistor measurement methods have overcome some of these limitations, they are still contact monitoring methods and have certain shortcomings such as point contact measurement, complex wiring systems, measurement errors caused by equipment self-heating during the process, and unsuitability for high-temperature extreme operating conditions.
